Language Reference Flashcards

Question

\>

Answer 1

x \> y (x is greater than y)

Answer 2

x \<= y (x is less than or equal to y)

Answer 3

x \>= y (x is greater than or equal to y)

Answer 4

&& (logical and) True only if both operands are true, e.g. if (digitalRead(2) == HIGH && digitalRead(3) == HIGH) { // read two switches // ... } is true only if both inputs are high.

Answer 5

|| (logical or) True if either operand is true, e.g. if (x \> 0 || y \> 0) { // ... } is true if either x or y is greater than 0.

Answer 6

! (not) True if the operand is false, e.g. if (!x) { // ... } is true if x is false (i.e. if x equals 0).

Answer 7

Bitwise AND (&) The bitwise operators perform their calculations at the bit level of variables. They help solve a wide range of common programming problems. Much of the material below is from an excellent tutorial on bitwise math wihch may be found[here.](http://www.arduino.cc/playground/Code/BitMath) Description and Syntax Below are descriptions and syntax for all of the operators. Further details may be found in the referenced tutorial. Bitwise AND (&) The bitwise AND operator in C++ is a single ampersand, &, used between two other integer expressions. Bitwise AND operates on each bit position of the surrounding expressions independently, according to this rule: if both input bits are 1, the resulting output is 1, otherwise the output is 0. Another way of expressing this is: ``` 0 0 1 1 operand1 0 1 0 1 operand2 ---------- 0 0 0 1 (operand1 & operand2) - returned result ``` In Arduino, the type int is a 16-bit value, so using & between two int expressions causes 16 simultaneous AND operations to occur. In a code fragment like: ``` int a = 92; // in binary: 0000000001011100 int b = 101; // in binary: 0000000001100101 int c = a & b; // result: 0000000001000100, or 68 in decimal. ``` Each of the 16 bits in a and b are processed by using the bitwise AND, and all 16 resulting bits are stored in c, resulting in the value 01000100 in binary, which is 68 in decimal. One of the most common uses of bitwise AND is to select a particular bit (or bits) from an integer value, often called masking. See below for an example

Answer 8

Bitwise OR (|) The bitwise OR operator in C++ is the vertical bar symbol, |. Like the & operator, | operates independently each bit in its two surrounding integer expressions, but what it does is different (of course). The bitwise OR of two bits is 1 if either or both of the input bits is 1, otherwise it is 0. In other words: ``` 0 0 1 1 operand1 0 1 0 1 operand2 ---------- 0 1 1 1 (operand1 | operand2) - returned result ``` Here is an example of the bitwise OR used in a snippet of C++ code: ``` int a = 92; // in binary: 0000000001011100 int b = 101; // in binary: 0000000001100101 int c = a | b; // result: 0000000001111101, or 125 in decimal. ``` Example Program for Arduino Uno A common job for the bitwise AND and OR operators is what programmers call Read-Modify-Write on a port. On microcontrollers, a port is an 8 bit number that represents something about the condition of the pins. Writing to a port controls all of the pins at once. PORTD is a built-in constant that refers to the output states of digital pins 0,1,2,3,4,5,6,7. If there is 1 in an bit position, then that pin is HIGH. (The pins already need to be set to outputs with the pinMode() command.) So if we write`PORTD = B00110001;` we have made pins 0,4 & 5 HIGH. One slight hitch here is that we *may* also have changeed the state of Pins 0 & 1, which are used by the Arduino for serial communications so we may have interfered with serial communication. ``` Our algorithm for the program is: ``` * Get PORTD and clear out only the bits corresponding to the pins we wish to control (with bitwise AND). * Combine the modified PORTD value with the new value for the pins under control (with biwise OR). ``` int i; // counter variable int j; void setup(){ DDRD = DDRD | B11111100; // set direction bits for pins 2 to 7, leave 0 and 1 untouched (xx | 00 == xx) // same as pinMode(pin, OUTPUT) for pins 2 to 7 Serial.begin(9600); } void loop(){ for (i=0; i\<64; i++){ PORTD = PORTD & B00000011; // clear out bits 2 - 7, leave pins 0 and 1 untouched (xx & 11 == xx) j = (i \<\< 2); // shift variable up to pins 2 - 7 - to avoid pins 0 and 1 PORTD = PORTD | j; // combine the port information with the new information for LED pins Serial.println(PORTD, BIN); // debug to show masking delay(100); } } ```

Answer 9

Bitwise XOR (^) There is a somewhat unusual operator in C++ called bitwise EXCLUSIVE OR, also known as bitwise XOR. (In English this is usually pronounced "eks-or".) The bitwise XOR operator is written using the caret symbol ^. This operator is very similar to the bitwise OR operator |, only it evaluates to 0 for a given bit position when both of the input bits for that position are 1: ``` 0 0 1 1 operand1 0 1 0 1 operand2 ---------- 0 1 1 0 (operand1 ^ operand2) - returned result ``` Another way to look at bitwise XOR is that each bit in the result is a 1 if the input bits are different, or 0 if they are the same. Here is a simple code example: ``` int x = 12; // binary: 1100 int y = 10; // binary: 1010 int z = x ^ y; // binary: 0110, or decimal 6 ``` The ^ operator is often used to toggle (i.e. change from 0 to 1, or 1 to 0) some of the bits in an integer expression. In a bitwise OR operation if there is a 1 in the mask bit, that bit is inverted; if there is a 0, the bit is not inverted and stays the same. Below is a program to blink digital pin 5. ``` // Blink\_Pin\_5 // demo for Exclusive OR void setup(){ DDRD = DDRD | B00100000; // set digital pin five as OUTPUT Serial.begin(9600); } void loop(){ PORTD = PORTD ^ B00100000; // invert bit 5 (digital pin 5), leave others untouched delay(100); } ```

Answer 10

Bitwise NOT (~) The bitwise NOT operator in C++ is the tilde character ~. Unlike & and |, the bitwise NOT operator is applied to a single operand to its right. Bitwise NOT changes each bit to its opposite: 0 becomes 1, and 1 becomes 0. For example: ``` 0 1 operand1 ``` ``` ---------- 1 0 ~ operand1 ``` ``` int a = 103; // binary: 0000000001100111 int b = ~a; // binary: 1111111110011000 = -104 ``` You might be surprised to see a negative number like -104 as the result of this operation. This is because the highest bit in an int variable is the so-called sign bit. If the highest bit is 1, the number is interpreted as negative. This encoding of positive and negative numbers is referred to as two's complement. For more information, see the Wikipedia article on[two's complement.](http://en.wikipedia.org/wiki/Twos_complement) As an aside, it is interesting to note that for any integer x, ~x is the same as -x-1. At times, the sign bit in a signed integer expression can cause some unwanted surprises.

Answer 11

bitshift left (\<\<), bitshift right (\>\>) Description From *The Bitmath Tutorial* in The Playground There are two bit shift operators in C++: the left shift operator \<\< and the right shift operator \>\>. These operators cause the bits in the left operand to be shifted left or right by the number of positions specified by the right operand. More on bitwise math may be found [here.](http://www.arduino.cc/playground/Code/BitMath) Syntax variable \<\< number\_of\_bits variable \>\> number\_of\_bits Parameters variable - (byte, int, long) number\_of\_bits integer \<= 32 Example: ``` int a = 5; // binary: 0000000000000101 int b = a \<\< 3; // binary: 0000000000101000, or 40 in decimal int c = b \>\> 3; // binary: 0000000000000101, or back to 5 like we started with ``` When you shift a value x by y bits (x \<\< y), the leftmost y bits in x are lost, literally shifted out of existence: ``` int a = 5; // binary: 0000000000000101 int b = a \<\< 14; // binary: 0100000000000000 - the first 1 in 101 was discarded ``` If you are certain that none of the ones in a value are being shifted into oblivion, a simple way to think of the left-shift operator is that it multiplies the left operand by 2 raised to the right operand power. For example, to generate powers of 2, the following expressions can be employed: ``` 1 \<\< 0 == 1 1 \<\< 1 == 2 1 \<\< 2 == 4 1 \<\< 3 == 8 ... 1 \<\< 8 == 256 1 \<\< 9 == 512 1 \<\< 10 == 1024 ... ``` When you shift x right by y bits (x \>\> y), and the highest bit in x is a 1, the behavior depends on the exact data type of x. If x is of type int, the highest bit is the sign bit, determining whether x is negative or not, as we have discussed above. In that case, the sign bit is copied into lower bits, for esoteric historical reasons: ``` int x = -16; // binary: 1111111111110000 int y = x \>\> 3; // binary: 1111111111111110 ``` This behavior, called sign extension, is often not the behavior you want. Instead, you may wish zeros to be shifted in from the left. It turns out that the right shift rules are different for unsigned int expressions, so you can use a typecast to suppress ones being copied from the left: ``` int x = -16; // binary: 1111111111110000 int y = (unsigned int)x \>\> 3; // binary: 0001111111111110 ``` If you are careful to avoid sign extension, you can use the right-shift operator \>\> as a way to divide by powers of 2. For example: ``` int x = 1000; int y = x \>\> 3; // integer division of 1000 by 8, causing y = 125. ```

Answer 12

++ (increment) / -- (decrement) Description Increment or decrement a variable Syntax ``` x++; // increment x by one and returns the old value of x ++x; // increment x by one and returns the new value of x x-- ; // decrement x by one and returns the old value of x --x ; // decrement x by one and returns the new value of x ``` Parameters x: an integer or long (possibly unsigned) Returns The original or newly incremented / decremented value of the variable. Examples ``` x = 2; y = ++x; // x now contains 3, y contains 3 y = x--; // x contains 2 again, y still contains 3 ```

Answer 13

x += y; // equivalent to the expression x = x + y;

Answer 14

x -= y; // equivalent to the expression x = x - y;

Answer 15

x \*= y; // equivalent to the expression x = x \* y;

Answer 16

``` x /= y; // equivalent to the expression x = x / y; ```

Answer 17

x %= y; // equivalent to the expression x = x % y;

Answer 18

compound bitwise AND (&=) Description The compound bitwise AND operator (&=) is often used with a variable and a constant to force particular bits in a variable to the LOW state (to 0). This is often referred to in programming guides as "clearing" or "resetting" bits. Syntax: ``` x &= y; // equivalent to x = x & y; ``` Parameters x: a char, int or long variable y: an integer constant or char, int, or long Example: First, a review of the Bitwise AND (&) operator ``` 0 0 1 1 operand1 0 1 0 1 operand2 ---------- 0 0 0 1 (operand1 & operand2) - returned result ``` Bits that are "bitwise ANDed" with 0 are cleared to 0 so, if myByte is a byte variable, `myByte & B00000000 = 0;` Bits that are "bitwise ANDed" with 1 are unchanged so, `myByte & B11111111 = myByte;` Note: because we are dealing with bits in a bitwise operator - it is convenient to use the binary formatter with[constants.](https://www.arduino.cc/en/Reference/IntegerConstants) The numbers are still the same value in other representations, they are just not as easy to understand. Also, B00000000 is shown for clarity, but zero in any number format is zero (hmmm something philosophical there?) Consequently - to clear (set to zero) bits 0 & 1 of a variable, while leaving the rest of the variable unchanged, use the compound bitwise AND operator (&=) with the constant B11111100 ``` 1 0 1 0 1 0 1 0 variable 1 1 1 1 1 1 0 0 mask ---------------------- 1 0 1 0 1 0 0 0 variable unchanged bits cleared ``` Here is the same representation with the variable's bits replaced with the symbol x ``` x x x x x x x x variable 1 1 1 1 1 1 0 0 mask ---------------------- x x x x x x 0 0 variable unchanged bits cleared ``` So if: ``` myByte = B10101010; myByte &= B11111100 == B10101000; ```

Answer 19

compound bitwise OR (|=) Description The compound bitwise OR operator (|=) is often used with a variable and a constant to "set" (set to 1) particular bits in a variable. Syntax: ``` x |= y; // equivalent to x = x | y; ``` Parameters x: a char, int or long variable y: an integer constant or char, int, or long Example: First, a review of the Bitwise OR (|) operator ``` 0 0 1 1 operand1 0 1 0 1 operand2 ---------- 0 1 1 1 (operand1 | operand2) - returned result ``` Bits that are "bitwise ORed" with 0 are unchanged, so if myByte is a byte variable, myByte | B00000000 = myByte; Bits that are "bitwise ORed" with 1 are set to 1 so: myByte | B11111111 = B11111111; Consequently - to set bits 0 & 1 of a variable, while leaving the rest of the variable unchanged, use the compound bitwise OR operator (|=) with the constant B00000011 ``` 1 0 1 0 1 0 1 0 variable 0 0 0 0 0 0 1 1 mask ---------------------- 1 0 1 0 1 0 1 1 variable unchanged bits set ``` Here is the same representation with the variables bits replaced with the symbol x ``` x x x x x x x x variable 0 0 0 0 0 0 1 1 mask ---------------------- x x x x x x 1 1 variable unchanged bits set ``` So if: ``` myByte = B10101010; myByte |= B00000011 == B10101011; ```

Answer 20

Defining Logical Levels: true and false (Boolean Constants) There are two constants used to represent truth and falsity in the Arduino language: true, and false. false `false` is the easier of the two to define. `false` is defined as 0 (zero). true `true` is often said to be defined as 1, which is correct, but `true` has a wider definition. Any integer which is non-zero is `true`, in a Boolean sense. So -1, 2 and -200 are all defined as `true`, too, in a Boolean sense.

Answer 21

Defining Pin Levels: HIGH and LOW When reading or writing to a digital pin there are only two possible values a pin can take/be-set-to: HIGH and LOW. HIGH The meaning of `HIGH` (in reference to a pin) is somewhat different depending on whether a pin is set to an `INPUT` or`OUTPUT`. When a pin is configured as an `INPUT` with `pinMode()`, and read with `digitalRead()`, the Arduino (Atmega) will report `HIGH` if: * a voltage greater than 3 volts is present at the pin (5V boards); * a voltage greater than 2 volts is present at the pin (3.3V boards); A pin may also be configured as an `INPUT` with `pinMode()`, and subsequently made `HIGH` with `digitalWrite()`. This will enable the internal 20K pullup resistors, which will *pull up* the input pin to a `HIGH` reading unless it is pulled`LOW` by external circuitry. This is how `INPUT_PULLUP` works and is described below in more detail. When a pin is configured to `OUTPUT` with `pinMode()`, and set to `HIGH` with `digitalWrite()`, the pin is at: * 5 volts (5V boards); * 3.3 volts (3.3V boards); In this state it can *source* current, e.g. light an LED that is connected through a series resistor to ground. LOW The meaning of `LOW` also has a different meaning depending on whether a pin is set to `INPUT` or `OUTPUT`. When a pin is configured as an `INPUT` with `pinMode()`, and read with `digitalRead()`, the Arduino (Atmega) will report`LOW` if: * a voltage less than 3 volts is present at the pin (5V boards); * a voltage less than 2 volts is present at the pin (3.3V boards); When a pin is configured to `OUTPUT` with `pinMode()`, and set to `LOW` with `digitalWrite()`, the pin is at 0 volts (both 5V and 3.3V boards). In this state it can *sink* current, e.g. light an LED that is connected through a series resistor to +5 volts (or +3.3 volts).

Answer 22

Pins Configured as INPUT Arduino (Atmega) pins configured as INPUT with `pinMode()` are said to be in a high-impedance state. Pins configured as `INPUT` make extremely small demands on the circuit that they are sampling, equivalent to a series resistor of 100 Megohms in front of the pin. This makes them useful for reading a sensor. If you have your pin configured as an `INPUT`, and are reading a switch, when the switch is in the open state the input pin will be "floating", resulting in unpredictable results. In order to assure a proper reading when the switch is open, a pull-up or pull-down resistor must be used. The purpose of this resistor is to pull the pin to a known state when the switch is open. A 10 K ohm resistor is usually chosen, as it is a low enough value to reliably prevent a floating input, and at the same time a high enough value to not not draw too much current when the switch is closed. See the [Digital Read Serial](https://www.arduino.cc/en/Tutorial/DigitalReadSerial) tutorial for more information. If a pull-down resistor is used, the input pin will be `LOW` when the switch is open and `HIGH` when the switch is closed. If a pull-up resistor is used, the input pin will be `HIGH` when the switch is open and `LOW` when the switch is closed.

Answer 23

Pins Configured as INPUT\_PULLUP The Atmega microcontroller on the Arduino has internal pull-up resistors (resistors that connect to power internally) that you can access. If you prefer to use these instead of external pull-up resistors, you can use the INPUT\_PULLUPargument in `pinMode()`. See the [Input Pullup Serial](https://www.arduino.cc/en/Tutorial/InputPullupSerial) tutorial for an example of this in use. Pins configured as inputs with either `INPUT` or `INPUT_PULLUP` can be damaged or destroyed if they are connected to voltages below ground (negative voltages) or above the positive power rail (5V or 3V).

Answer 24

Pins Configured as Outputs Pins configured as OUTPUT with `pinMode()` are said to be in a low-impedance state. This means that they can provide a substantial amount of current to other circuits. Atmega pins can source (provide current) or sink (absorb current) up to 40 mA (milliamps) of current to other devices/circuits. This makes them useful for powering LEDsbecause LEDs typically use less than 40 mA. Loads greater than 40 mA (e.g. motors) will require a transistor or other interface circuitry. Pins configured as outputs can be damaged or destroyed if they are connected to either the ground or positive power rails.

Answer 25

Defining built-ins: LED\_BUILTIN Most Arduino boards have a pin connected to an on-board LED in series with a resistor. The constant `LED_BUILTIN` is the number of the pin to which the on-board LED is connected. Most boards have this LED connected to digital pin 13.

Answer 26

Integer Constants Integer constants are numbers used directly in a sketch, like `123`. By default, these numbers are treated as [int](https://www.arduino.cc/en/Reference/Int)'s but you can change this with the U and L modifiers (see below). Normally, integer constants are treated as base 10 (decimal) integers, but special notation (formatters) may be used to enter numbers in other bases. ``` Base Example Formatter Comment 10 (decimal) 123 none 2 (binary) B1111011 leading 'B' only works with 8 bit values (0 to 255) characters 0-1 valid 8 (octal) 0173 leading "0" characters 0-7 valid 16 (hexadecimal) 0x7B leading "0x" characters 0-9, A-F, a-f valid ``` Decimal is base 10. This is the common-sense math with which you are acquainted. Constants without other prefixes are assumed to be in decimal format. Example: ``` 101 // same as 101 decimal ((1 \* 10^2) + (0 \* 10^1) + 1) ``` Binary is base two. Only characters 0 and 1 are valid. Example: ``` B101 // same as 5 decimal ((1 \* 2^2) + (0 \* 2^1) + 1) ``` The binary formatter only works on bytes (8 bits) between 0 (B0) and 255 (B11111111). If it is convenient to input an int (16 bits) in binary form you can do it a two-step procedure such as: ``` myInt = (B11001100 \* 256) + B10101010; // B11001100 is the high byte ``` Octal is base eight. Only characters 0 through 7 are valid. Octal values are indicated by the prefix "0" Example: ``` 0101 // same as 65 decimal ((1 \* 8^2) + (0 \* 8^1) + 1) ``` Warning It is possible to generate a hard-to-find bug by (unintentionally) including a leading zero before a constant and having the compiler unintentionally interpret your constant as octal. Hexadecimal (or hex) is base sixteen. Valid characters are 0 through 9 and letters A through F; A has the value 10, B is 11, up to F, which is 15. Hex values are indicated by the prefix "0x". Note that A-F may be syted in upper or lower case (a-f). Example: ``` 0x101 // same as 257 decimal ((1 \* 16^2) + (0 \* 16^1) + 1) ``` U & L formatters By default, an integer constant is treated as an [int](https://www.arduino.cc/en/Reference/Int) with the attendant limitations in values. To specify an integer constant with another data type, follow it with: * a 'u' or 'U' to force the constant into an unsigned data format. Example: `33u` * a 'l' or 'L' to force the constant into a long data format. Example: `100000L` * a 'ul' or 'UL' to force the constant into an unsigned long constant. Example: `32767ul`

Answer 27

floating point constants Similar to integer constants, floating point constants are used to make code more readable. Floating point constants are swapped at compile time for the value to which the expression evaluates. Examples: `n = .005;` Floating point constants can also be expressed in a variety of scientific notation. 'E' and 'e' are both accepted as valid exponent indicators. ``` floating-point evaluates to: also evaluates to: constant 10.0 10 2.34E5 2.34 \* 10^5 234000 67e-12 67.0 \* 10^-12 .000000000067 ```

Answer 28

void The void keyword is used only in function declarations. It indicates that the function is expected to return no information to the function from which it was called. Example: ``` // actions are performed in the functions "setup" and "loop" // but no information is reported to the larger program void setup() { // ... } void loop() { // ... } ```

Answer 29

boolean A boolean holds one of two values, [true](https://www.arduino.cc/en/Reference/Constants) or [false](https://www.arduino.cc/en/Reference/Constants). (Each boolean variable occupies one byte of memory.) Example ``` int LEDpin = 5; // LED on pin 5 int switchPin = 13; // momentary switch on 13, other side connected to ground boolean running = false; void setup() { pinMode(LEDpin, OUTPUT); pinMode(switchPin, INPUT); digitalWrite(switchPin, HIGH); // turn on pullup resistor } void loop() { if (digitalRead(switchPin) == LOW) { // switch is pressed - pullup keeps pin high normally delay(100); // delay to debounce switch running = !running; // toggle running variable digitalWrite(LEDpin, running) // indicate via LED } } ```

Answer 30

char Description A data type that takes up 1 byte of memory that stores a character value. Character literals are written in single quotes, like this: 'A' (for multiple characters - strings - use double quotes: "ABC"). Characters are stored as numbers however. You can see the specific encoding in the [ASCII chart](https://www.arduino.cc/en/Reference/ASCIIchart). This means that it is possible to do arithmetic on characters, in which the ASCII value of the character is used (e.g. 'A' + 1 has the value 66, since the ASCII value of the capital letter A is 65). See [Serial.println](https://www.arduino.cc/en/Serial/Println) reference for more on how characters are translated to numbers. The char datatype is a signed type, meaning that it encodes numbers from -128 to 127. For an unsigned, one-byte (8 bit) data type, use the *byte* data type. Example ``` char myChar = 'A'; char myChar = 65; // both are equivalent ```

Answer 31

unsigned char Description An unsigned data type that occupies 1 byte of memory. Same as the [byte](https://www.arduino.cc/en/Reference/Byte) datatype. The unsigned char datatype encodes numbers from 0 to 255. For consistency of Arduino programming style, the *byte* data type is to be preferred. Example ``` unsigned char myChar = 240; ```

Answer 32

byte Description A byte stores an 8-bit unsigned number, from 0 to 255. Example ``` byte b = B10010; // "B" is the binary formatter (B10010 = 18 decimal) ```

Answer 33

int Description Integers are your primary data-type for number storage. On the Arduino Uno (and other ATMega based boards) an `int` stores a 16-bit (2-byte) value. This yields a range of -32,768 to 32,767 (minimum value of -2^15 and a maximum value of (2^15) - 1). On the Arduino Due, an `int` stores a 32-bit (4-byte) value. This yields a range of -2,147,483,648 to 2,147,483,647 (minimum value of -2^31 and a maximum value of (2^31) - 1). `int`'s store negative numbers with a technique called [2's complement math.](http://en.wikipedia.org/wiki/2%27s_complement) The highest bit, sometimes referred to as the "sign" bit, flags the number as a negative number. The rest of the bits are inverted and 1 is added. The Arduino takes care of dealing with negative numbers for you, so that arithmetic operations work transparently in the expected manner. There can be an unexpected complication in dealing with the [bitshift right operator (\>\>)](https://www.arduino.cc/en/Reference/Bitshift) however. Example ``` int ledPin = 13; ``` Syntax ``` int var = val; ``` * var - your int variable name * val - the value you assign to that variable Coding Tip When variables are made to exceed their maximum capacity they "roll over" back to their minimum capacity, note that this happens in both directions. Example for a 16-bit int: ``` int x; x = -32768; x = x - 1; // x now contains 32,767 - rolls over in neg. direction x = 32767; x = x + 1; // x now contains -32,768 - rolls over ```

Answer 34

unsigned int Description On the Uno and other ATMEGA based boards, unsigned ints (unsigned integers) are the same as ints in that they store a 2 byte value. Instead of storing negative numbers however they only store positive values, yielding a useful range of 0 to 65,535 (2^16) - 1). The Due stores a 4 byte (32-bit) value, ranging from 0 to 4,294,967,295 (2^32 - 1). The difference between unsigned ints and (signed) ints, lies in the way the highest bit, sometimes refered to as the "sign" bit, is interpreted. In the Arduino int type (which is signed), if the high bit is a "1", the number is interpreted as a negative number, and the other 15 bits are interpreted with [2's complement math.](http://en.wikipedia.org/wiki/2%27s_complement) Example ``` unsigned int ledPin = 13; ``` Syntax ``` unsigned int var = val; ``` * var - your unsigned int variable name * val - the value you assign to that variable Coding Tip When variables are made to exceed their maximum capacity they "roll over" back to their minimum capacitiy, note that this happens in both directions ``` unsigned int x x = 0; x = x - 1; // x now contains 65535 - rolls over in neg direction x = x + 1; // x now contains 0 - rolls over ```

Answer 35

word Description On the Uno and other ATMEGA based boards, a word stores a 16-bit unsigned number. On the Due and Zero instead it stores a 32-bit unsigned number. Example ``` word w = 10000; ```

Answer 36

long Description Long variables are extended size variables for number storage, and store 32 bits (4 bytes), from -2,147,483,648 to 2,147,483,647. If doing math with integers, at least one of the numbers must be followed by an L, forcing it to be a `long`. See the[Integer Constants](https://www.arduino.cc/en/Reference/IntegerConstants) page for details. Example ``` long speedOfLight = 186000L; // see the Integer Constants page for explanation of the 'L' ``` Syntax ``` long var = val; ``` * var - the long variable name * val - the value assigned to the variable

Answer 37

unsigned long Description Unsigned long variables are extended size variables for number storage, and store 32 bits (4 bytes). Unlike standard longs unsigned longs won't store negative numbers, making their range from 0 to 4,294,967,295 (2^32 - 1). Example ``` unsigned long time; void setup() { Serial.begin(9600); } void loop() { Serial.print("Time: "); time = millis(); //prints time since program started Serial.println(time); // wait a second so as not to send massive amounts of data delay(1000); } ``` Syntax ``` unsigned long var = val; ``` * var - your long variable name * val - the value you assign to that variable

Answer 38

short Description A `short` is a 16-bit data-type. On all Arduinos (ATMega and ARM based) a `short` stores a 16-bit (2-byte) value. This yields a range of -32,768 to 32,767 (minimum value of -2^15 and a maximum value of (2^15) - 1). Example `short ledPin = 13;` Syntax `short var = val;` * var - your short variable name * val - the value you assign to that variable

Answer 39

float Description Datatype for floating-point numbers, a number that has a decimal point. Floating-point numbers are often used to approximate analog and continuous values because they have greater resolution than integers. Floating-point numbers can be as large as 3.4028235E+38 and as low as -3.4028235E+38. They are stored as 32 bits (4 bytes) of information. Floats have only 6-7 decimal digits of precision. That means the total number of digits, not the number to the right of the decimal point. Unlike other platforms, where you can get more precision by using a double (e.g. up to 15 digits), on the Arduino, double is the same size as float. Floating point numbers are not exact, and may yield strange results when compared. For example `6.0 / 3.0` may not equal `2.0`. You should instead check that the absolute value of the difference between the numbers is less than some small number. Floating point math is also much slower than integer math in performing calculations, so should be avoided if, for example, a loop has to run at top speed for a critical timing function. Programmers often go to some lengths to convert floating point calculations to integer math to increase speed. If doing math with floats, you need to add a decimal point, otherwise it will be treated as an `int`. See the [Floating point constants](https://www.arduino.cc/en/Reference/Fpconstants) page for details. Examples ``` float myfloat; float sensorCalbrate = 1.117; ``` Syntax ``` float var = val; ``` * var - your float variable name * val - the value you assign to that variable Example Code ``` int x; int y; float z; x = 1; y = x / 2; // y now contains 0, ints can't hold fractions z = (float)x / 2.0; // z now contains .5 (you have to use 2.0, not 2) ```

Answer 40

double Description Double precision floating point number. On the Uno and other ATMEGA based boards, this occupies 4 bytes. That is, the double implementation is exactly the same as the float, with no gain in precision. On the Arduino Due, doubles have 8-byte (64 bit) precision. Tip Users who borrow code from other sources that includes double variables may wish to examine the code to see if the implied precision is different from that actually achieved on ATMEGA based Arduinos.

Answer 41

string Description Text strings can be represented in two ways. you can use the String data type, which is part of the core as of version 0019, or you can make a string out of an array of type char and null-terminate it. This page described the latter method. For more details on the String object, which gives you more functionality at the cost of more memory, see the String object page. Examples All of the following are valid declarations for strings. char Str1[15]; char Str2[8] = {'a', 'r', 'd', 'u', 'i', 'n', 'o'}; char Str3[8] = {'a', 'r', 'd', 'u', 'i', 'n', 'o', '\0'}; char Str4[] = "arduino"; char Str5[8] = "arduino"; char Str6[15] = "arduino"; Possibilities for declaring strings Declare an array of chars without initializing it as in Str1 Declare an array of chars (with one extra char) and the compiler will add the required null character, as in Str2 Explicitly add the null character, Str3 Initialize with a string constant in quotation marks; the compiler will size the array to fit the string constant and a terminating null character, Str4 Initialize the array with an explicit size and string constant, Str5 Initialize the array, leaving extra space for a larger string, Str6 Null termination Generally, strings are terminated with a null character (ASCII code 0). This allows functions (like Serial.print()) to tell where the end of a string is. Otherwise, they would continue reading subsequent bytes of memory that aren't actually part of the string. This means that your string needs to have space for one more character than the text you want it to contain. That is why Str2 and Str5 need to be eight characters, even though "arduino" is only seven - the last position is automatically filled with a null character. Str4 will be automatically sized to eight characters, one for the extra null. In Str3, we've explicitly included the null character (written '\0') ourselves. Note that it's possible to have a string without a final null character (e.g. if you had specified the length of Str2 as seven instead of eight). This will break most functions that use strings, so you shouldn't do it intentionally. If you notice something behaving strangely (operating on characters not in the string), however, this could be the problem. Single quotes or double quotes? Strings are always defined inside double quotes ("Abc") and characters are always defined inside single quotes('A'). Wrapping long strings You can wrap long strings like this: char myString[] = "This is the first line" " this is the second line" " etcetera"; Arrays of strings It is often convenient, when working with large amounts of text, such as a project with an LCD display, to setup an array of strings. Because strings themselves are arrays, this is in actually an example of a two-dimensional array. In the code below, the asterisk after the datatype char "char\*" indicates that this is an array of "pointers". All array names are actually pointers, so this is required to make an array of arrays. Pointers are one of the more esoteric parts of C for beginners to understand, but it isn't necessary to understand pointers in detail to use them effectively here. Example char\* myStrings[]={"This is string 1", "This is string 2", "This is string 3", "This is string 4", "This is string 5","This is string 6"}; void setup(){ Serial.begin(9600); } void loop(){ for (int i = 0; i \< 6; i++){ Serial.println(myStrings[i]); delay(500); } }

Answer 42

String Description The String class, part of the core as of version 0019, allows you to use and manipulate strings of text in more complex ways than [character arrays](https://www.arduino.cc/en/Reference/String) do. You can concatenate Strings, append to them, search for and replace substrings, and more. It takes more memory than a simple character array, but it is also more useful. For reference, character arrays are referred to as strings with a small s, and instances of the String class are referred to as Strings with a capital S. Note that constant strings, specified in "double quotes" are treated as char arrays, not instances of the String class. Examples * [StringConstructors](https://www.arduino.cc/en/Tutorial/StringConstructors) * [StringAdditionOperator](https://www.arduino.cc/en/Tutorial/StringAdditionOperator) * [StringIndexOf](https://www.arduino.cc/en/Tutorial/StringIndexOf) * [StringAppendOperator](https://www.arduino.cc/en/Tutorial/StringAppendOperator) * [StringLengthTrim](https://www.arduino.cc/en/Tutorial/StringLengthTrim) * [StringCaseChanges](https://www.arduino.cc/en/Tutorial/StringCaseChanges) * [StringReplace](https://www.arduino.cc/en/Tutorial/StringReplace) * [StringRemove](https://www.arduino.cc/en/Tutorial/StringRemove) * [StringCharacters](https://www.arduino.cc/en/Tutorial/StringCharacters) * [StringStartsWithEndsWith](https://www.arduino.cc/en/Tutorial/StringStartsWithEndsWith) * [StringComparisonOperators](https://www.arduino.cc/en/Tutorial/StringComparisonOperators) * [StringSubstring](https://www.arduino.cc/en/Tutorial/StringSubstring)

Answer 43

Arrays An array is a collection of variables that are accessed with an index number. Arrays in the C programming language, on which Arduino is based, can be complicated, but using simple arrays is relatively straightforward. Creating (Declaring) an Array All of the methods below are valid ways to create (declare) an array. ``` int myInts[6]; int myPins[] = {2, 4, 8, 3, 6}; int mySensVals[6] = {2, 4, -8, 3, 2}; char message[6] = "hello"; ``` You can declare an array without initializing it as in myInts. In myPins we declare an array without explicitly choosing a size. The compiler counts the elements and creates an array of the appropriate size. Finally you can both initialize and size your array, as in mySensVals. Note that when declaring an array of type char, one more element than your initialization is required, to hold the required null character. Accessing an Array Arrays are zero indexed, that is, referring to the array initialization above, the first element of the array is at index 0, hence `mySensVals[0] == 2, mySensVals[1] == 4,` and so forth. It also means that in an array with ten elements, index nine is the last element. Hence: ``` int myArray[10]={9,3,2,4,3,2,7,8,9,11}; // myArray[9] contains 11 // myArray[10] is invalid and contains random information (other memory address) ``` For this reason you should be careful in accessing arrays. Accessing past the end of an array (using an index number greater than your declared array size - 1) is reading from memory that is in use for other purposes. Reading from these locations is probably not going to do much except yield invalid data. Writing to random memory locations is definitely a bad idea and can often lead to unhappy results such as crashes or program malfunction. This can also be a difficult bug to track down. Unlike BASIC or JAVA, the C compiler does no checking to see if array access is within legal bounds of the array size that you have declared. To assign a value to an array: ``` mySensVals[0] = 10; ``` To retrieve a value from an array: `x = mySensVals[4]`; Arrays and FOR Loops Arrays are often manipulated inside for loops, where the loop counter is used as the index for each array element. For example, to print the elements of an array over the serial port, you could do something like this: ``` int i; for (i = 0; i \< 5; i = i + 1) { Serial.println(myPins[i]); } ``` Example For a complete program that demonstrates the use of arrays, see the [Knight Rider example](http://www.arduino.cc/en/Tutorial/KnightRider) from the [Tutorials](http://www.arduino.cc/en/Main/LearnArduino).

Answer 44

char() Description Converts a value to the [char](https://www.arduino.cc/en/Reference/Char) data type. Syntax char(x) Parameters x: a value of any type Returns char

Answer 45

byte() Description Converts a value to the [byte](https://www.arduino.cc/en/Reference/Byte) data type. Syntax byte(x) Parameters x: a value of any type Returns byte

Answer 46

int() Description Converts a value to the [int](https://www.arduino.cc/en/Reference/Int) data type. Syntax int(x) Parameters x: a value of any type Returns int

Answer 47

word() Description Convert a value to the [word](https://www.arduino.cc/en/Reference/Word) data type or create a word from two bytes. Syntax word(x) word(h, l) Parameters x: a value of any type h: the high-order (leftmost) byte of the word l: the low-order (rightmost) byte of the word Returns word

Answer 48

long() Description Converts a value to the [long](https://www.arduino.cc/en/Reference/Long) data type. Syntax long(x) Parameters x: a value of any type Returns long

Answer 49

float() Description Converts a value to the [float](https://www.arduino.cc/en/Reference/Float) data type. Syntax float(x) Parameters x: a value of any type Returns float

Answer 50

Variable Scope Variables in the C programming language, which Arduino uses, have a property called *scope*. This is in contrast to early versions of languages such as BASIC where every variable is a *global* variable. A global variable is one that can be *seen* by every function in a program. Local variables are only visible to the function in which they are declared. In the Arduino environment, any variable declared outside of a function (e.g. setup(), loop(), etc. ), is a global variable. When programs start to get larger and more complex, local variables are a useful way to insure that only one function has access to its own variables. This prevents programming errors when one function inadvertently modifies variables used by another function. It is also sometimes handy to declare and initialize a variable inside a *for* loop. This creates a variable that can only be accessed from inside the for-loop brackets. Example: ``` int gPWMval; // any function will see this variable void setup() { // ... } void loop() { int i; // "i" is only "visible" inside of "loop" float f; // "f" is only "visible" inside of "loop" // ... for (int j = 0; j \<100; j++){ // variable j can only be accessed inside the for-loop brackets } } ```

Answer 51

Static The static keyword is used to create variables that are visible to only one function. However unlike local variables that get created and destroyed every time a function is called, static variables persist beyond the function call, preserving their data between function calls. Variables declared as static will only be created and initialized the first time a function is called. Example ``` /\* RandomWalk \* Paul Badger 2007 \* RandomWalk wanders up and down randomly between two \* endpoints. The maximum move in one loop is governed by \* the parameter "stepsize". \* A static variable is moved up and down a random amount. \* This technique is also known as "pink noise" and "drunken walk". \*/ #define randomWalkLowRange -20 #define randomWalkHighRange 20 int stepsize; int thisTime; int total; void setup() { Serial.begin(9600); } void loop() { // tetst randomWalk function stepsize = 5; thisTime = randomWalk(stepsize); Serial.println(thisTime); delay(10); } int randomWalk(int moveSize){ static int place; // variable to store value in random walk - declared static so that it stores // values in between function calls, but no other functions can change its value place = place + (random(-moveSize, moveSize + 1)); if (place \< randomWalkLowRange){ // check lower and upper limits place = place + (randomWalkLowRange - place); // reflect number back in positive direction } else if(place \> randomWalkHighRange){ place = place - (place - randomWalkHighRange); // reflect number back in negative direction } return place; } ```

Answer 52

volatile keyword volatile is a keyword known as a variable *qualifier*, it is usually used before the datatype of a variable, to modify the way in which the compiler and subsequent program treats the variable. Declaring a variable volatile is a directive to the compiler. The compiler is software which translates your C/C++ code into the machine code, which are the real instructions for the Atmega chip in the Arduino. Specifically, it directs the compiler to load the variable from RAM and not from a storage register, which is a temporary memory location where program variables are stored and manipulated. Under certain conditions, the value for a variable stored in registers can be inaccurate. A variable should be declared volatile whenever its value can be changed by something beyond the control of the code section in which it appears, such as a concurrently executing thread. In the Arduino, the only place that this is likely to occur is in sections of code associated with interrupts, called an interrupt service routine. Example ``` // toggles LED when interrupt pin changes state int pin = 13; volatile int state = LOW; void setup() { pinMode(pin, OUTPUT); attachInterrupt(0, blink, CHANGE); } void loop() { digitalWrite(pin, state); } void blink() { state = !state; } ```

Answer 53

const keyword The const keyword stands for constant. It is a variable *qualifier* that modifies the behavior of the variable, making a variable "*read-only*". This means that the variable can be used just as any other variable of its type, but its value cannot be changed. You will get a compiler error if you try to assign a value to a const variable. Constants defined with the *const* keyword obey the rules of *[variable scoping](https://www.arduino.cc/en/Reference/Scope)* that govern other variables. This, and the pitfalls of using*#define*, makes the *const* keyword a superior method for defining constants and is preferred over using*[#define](https://www.arduino.cc/en/Reference/Define)*. Example ``` const float pi = 3.14; float x; // .... x = pi \* 2; // it's fine to use const's in math pi = 7; // illegal - you can't write to (modify) a constant ``` #define or const You can use either const or #define for creating numeric or string constants. For [arrays](https://www.arduino.cc/en/Reference/Array), you will need to use const. In general *const* is preferred over *#define* for defining constants.

Answer 54

sizeof Description The sizeof operator returns the number of bytes in a variable type, or the number of bytes occupied by an array. Syntax sizeof(variable) Parameters variable: any variable type or array (e.g. int, float, byte) Example code The sizeof operator is useful for dealing with arrays (such as strings) where it is convenient to be able to change the size of the array without breaking other parts of the program. This program prints out a text string one character at a time. Try changing the text phrase. char myStr[] = "this is a test"; int i; ``` void setup(){ Serial.begin(9600); } ``` ``` void loop() { for (i = 0; i \< sizeof(myStr) - 1; i++){ Serial.print(i, DEC); Serial.print(" = "); Serial.write(myStr[i]); Serial.println(); } delay(5000); // slow down the program } ``` [[Get Code]](https://www.arduino.cc/en/Reference/Sizeof?action=sourceblock&num=1) Note that sizeof returns the total number of bytes. So for larger variable types such as ints, the for loop would look something like this. Note also that a properly formatted string ends with the NULL symbol, which has ASCII value 0. ``` for (i = 0; i \< (sizeof(myInts)/sizeof(int)) - 1; i++) { // do something with myInts[i] } ```

Answer 55

PROGMEM Store data in flash (program) memory instead of SRAM. There's a description of the various [types of memory](http://www.arduino.cc/playground/Learning/Memory) available on an Arduino board. The PROGMEM keyword is a variable modifier, it should be used only with the datatypes defined in pgmspace.h. It tells the compiler "put this information into flash memory", instead of into SRAM, where it would normally go. PROGMEM is part of the [pgmspace.h](http://www.nongnu.org/avr-libc/user-manual/group__avr__pgmspace.html) library that is available in the AVR architecture only. So you first need to include the library at the top your sketch, like this: #include [[Get Code]](https://www.arduino.cc/en/Reference/PROGMEM?action=sourceblock&num=1) Syntax const dataType variableName[] PROGMEM = {data0, data1, data3...}; [[Get Code]](https://www.arduino.cc/en/Reference/PROGMEM?action=sourceblock&num=2) * dataType - any variable type * variableName - the name for your array of data Note that because PROGMEM is a variable modifier, there is no hard and fast rule about where it should go, so the Arduino compiler accepts all of the definitions below, which are also synonymous. However experiments have indicated that, in various versions of Arduino (having to do with GCC version), PROGMEM may work in one location and not in another. The "string table" example below has been tested to work with Arduino 13. Earlier versions of the IDE may work better if PROGMEM is included after the variable name. const dataType variableName[] PROGMEM = {}; // use this form const PROGMEM dataType variableName[] = {}; // or this form const dataType PROGMEM variableName[] = {}; // not this one [[Get Code]](https://www.arduino.cc/en/Reference/PROGMEM?action=sourceblock&num=3) While PROGMEM could be used on a single variable, it is really only worth the fuss if you have a larger block of data that needs to be stored, which is usually easiest in an array, (or another C data structure beyond our present discussion). Using PROGMEM is also a two-step procedure. After getting the data into Flash memory, it requires special methods (functions), also defined in the [pgmspace.h](http://www.nongnu.org/avr-libc/user-manual/group__avr__pgmspace.html) library, to read the data from program memory back into SRAM, so we can do something useful with it. Example The following code fragments illustrate how to read and write chars (bytes) and ints (2 bytes) to PROGMEM. #include

// save some unsigned ints
const PROGMEM uint16_t charSet[] = { 65000, 32796, 16843, 10, 11234};

// save some chars
const char signMessage[] PROGMEM = {"I AM PREDATOR, UNSEEN COMBATANT. CREATED BY THE UNITED STATES DEPART"};

unsigned int displayInt;
int k; // counter variable
char myChar;

void setup() {
Serial.begin(9600);
while (!Serial);

// put your setup code here, to run once:
// read back a 2-byte int
for (k = 0; k < 5; k++)
{
displayInt = pgm_read_word_near(charSet + k);
Serial.println(displayInt);
}
Serial.println();

// read back a char
int len = strlen_P(signMessage);
for (k = 0; k < len; k++)
{
myChar = pgm_read_byte_near(signMessage + k);
Serial.print(myChar);
}

Serial.println();
}

void loop() {
// put your main code here, to run repeatedly:

} [[Get Code]](https://www.arduino.cc/en/Reference/PROGMEM?action=sourceblock&num=4) Arrays of strings It is often convenient when working with large amounts of text, such as a project with an LCD display, to setup an array of strings. Because strings themselves are arrays, this is in actually an example of a two-dimensional array. These tend to be large structures so putting them into program memory is often desirable. The code below illustrates the idea. /\* PROGMEM string demo How to store a table of strings in program memory (flash), and retrieve them. Information summarized from: http://www.nongnu.org/avr-libc/user-manual/pgmspace.html Setting up a table (array) of strings in program memory is slightly complicated, but here is a good template to follow. Setting up the strings is a two-step process. First define the strings. \*/ #include
const char string_0[] PROGMEM = "String 0"; // "String 0" etc are strings to store - change to suit.
const char string_1[] PROGMEM = "String 1";
const char string_2[] PROGMEM = "String 2";
const char string_3[] PROGMEM = "String 3";
const char string_4[] PROGMEM = "String 4";
const char string_5[] PROGMEM = "String 5";

// Then set up a table to refer to your strings.

const char* const string_table[] PROGMEM = {string_0, string_1, string_2, string_3, string_4, string_5};

char buffer[30]; // make sure this is large enough for the largest string it must hold

void setup()
{
Serial.begin(9600);
while(!Serial);
Serial.println("OK");
}

void loop()
{
/* Using the string table in program memory requires the use of special functions to retrieve the data.
The strcpy_P function copies a string from program space to a string in RAM ("buffer").
Make sure your receiving string in RAM is large enough to hold whatever
you are retrieving from program space. */

for (int i = 0; i < 6; i++)
{
strcpy_P(buffer, (char*)pgm_read_word(&(string_table[i]))); // Necessary casts and dereferencing, just copy.
Serial.println(buffer);
delay( 500 );
}
}

Answer 56

pinMode() Description Configures the specified pin to behave either as an input or an output. See the description of [digital pins](https://www.arduino.cc/en/Tutorial/DigitalPins) for details on the functionality of the pins. As of Arduino 1.0.1, it is possible to enable the internal pullup resistors with the mode INPUT\_PULLUP. Additionally, the INPUT mode explicitly disables the internal pullups. Syntax pinMode(pin, mode) Parameters pin: the number of the pin whose mode you wish to set mode: [INPUT](https://www.arduino.cc/en/Reference/Constants), [OUTPUT](https://www.arduino.cc/en/Reference/Constants), or [INPUT\_PULLUP](https://www.arduino.cc/en/Reference/Constants). (see the [digital pins](https://www.arduino.cc/en/Tutorial/DigitalPins) page for a more complete description of the functionality.) Returns None Example int ledPin = 13; // LED connected to digital pin 13 ``` void setup() { pinMode(ledPin, OUTPUT); // sets the digital pin as output } ``` void loop() { digitalWrite(ledPin, HIGH); // sets the LED on delay(1000); // waits for a second digitalWrite(ledPin, LOW); // sets the LED off delay(1000); // waits for a second }

Answer 57

digitalWrite() Description Write a [HIGH](https://www.arduino.cc/en/Reference/Constants) or a [LOW](https://www.arduino.cc/en/Reference/Constants) value to a digital pin. If the pin has been configured as an OUTPUT with [pinMode](https://www.arduino.cc/en/Reference/PinMode)(), its voltage will be set to the corresponding value: 5V (or 3.3V on 3.3V boards) for HIGH, 0V (ground) for LOW. If the pin is configured as an INPUT, digitalWrite() will enable (HIGH) or disable (LOW) the internal pullup on the input pin. It is recommended to set the [pinMode](https://www.arduino.cc/en/Reference/PinMode)() to [INPUT\_PULLUP](https://www.arduino.cc/en/Reference/Constants) to enable the internal pull-up resistor. See the [digital pins tutorial](https://www.arduino.cc/en/Tutorial/DigitalPins) for more information. NOTE: If you do not set the pinMode() to OUTPUT, and connect an LED to a pin, when calling digitalWrite(HIGH), the LED may appear dim. Without explicitly setting pinMode(), digitalWrite() will have enabled the internal pull-up resistor, which acts like a large current-limiting resistor. Syntax digitalWrite(pin, value) Parameters pin: the pin number value: [HIGH](https://www.arduino.cc/en/Reference/Constants) or [LOW](https://www.arduino.cc/en/Reference/Constants) Returns none Example ``` int ledPin = 13; // LED connected to digital pin 13 void setup() { pinMode(ledPin, OUTPUT); // sets the digital pin as output } void loop() { digitalWrite(ledPin, HIGH); // sets the LED on delay(1000); // waits for a second digitalWrite(ledPin, LOW); // sets the LED off delay(1000); // waits for a second } ``` Sets pin 13 to HIGH, makes a one-second-long delay, and sets the pin back to LOW.

Answer 58

digitalRead() Description Reads the value from a specified digital pin, either [HIGH](https://www.arduino.cc/en/Reference/Constants) or [LOW](https://www.arduino.cc/en/Reference/Constants). Syntax digitalRead(pin) Parameters pin: the number of the digital pin you want to read (*int*) Returns [HIGH](https://www.arduino.cc/en/Reference/Constants) or [LOW](https://www.arduino.cc/en/Reference/Constants) Example Sets pin 13 to the same value as pin 7, declared as an input. int ledPin = 13; // LED connected to digital pin 13 int inPin = 7; // pushbutton connected to digital pin 7 int val = 0; // variable to store the read value void setup() { pinMode(ledPin, OUTPUT); // sets the digital pin 13 as output pinMode(inPin, INPUT); // sets the digital pin 7 as input } ``` void loop() { val = digitalRead(inPin); // read the input pin digitalWrite(ledPin, val); // sets the LED to the button's value ```

Answer 59

analogReference() Description Configures the reference voltage used for analog input (i.e. the value used as the top of the input range). The options are: * DEFAULT: the default analog reference of 5 volts (on 5V Arduino boards) or 3.3 volts (on 3.3V Arduino boards) * INTERNAL: an built-in reference, equal to 1.1 volts on the ATmega168 or ATmega328 and 2.56 volts on the ATmega8(*not available on the Arduino Mega*) * INTERNAL1V1: a built-in 1.1V reference (*Arduino Mega only*) * INTERNAL2V56: a built-in 2.56V reference (*Arduino Mega only*) * EXTERNAL: the voltage applied to the AREF pin (0 to 5V only) is used as the reference. Syntax analogReference(type) Parameters type: which type of reference to use (DEFAULT, INTERNAL, INTERNAL1V1, INTERNAL2V56, or EXTERNAL). Returns None. Note After changing the analog reference, the first few readings from analogRead() may not be accurate. Warning Don't use anything less than 0V or more than 5V for external reference voltage on the AREF pin! If you're using an external reference on the AREF pin, you must set the analog reference to EXTERNAL before calling analogRead().Otherwise, you will short together the active reference voltage (internally generated) and the AREF pin, possibly damaging the microcontroller on your Arduino board. Alternatively, you can connect the external reference voltage to the AREF pin through a 5K resistor, allowing you to switch between external and internal reference voltages. Note that the resistor will alter the voltage that gets used as the reference because there is an internal 32K resistor on the AREF pin. The two act as a voltage divider, so, for example, 2.5V applied through the resistor will yield 2.5 \* 32 / (32 + 5) = ~2.2V at the AREF pin.

Answer 60

analogRead() Description Reads the value from the specified analog pin. The Arduino board contains a 6 channel (8 channels on the Mini and Nano, 16 on the Mega), 10-bit analog to digital converter. This means that it will map input voltages between 0 and 5 volts into integer values between 0 and 1023. This yields a resolution between readings of: 5 volts / 1024 units or, .0049 volts (4.9 mV) per unit. The input range and resolution can be changed using [analogReference](https://www.arduino.cc/en/Reference/AnalogReference)(). It takes about 100 microseconds (0.0001 s) to read an analog input, so the maximum reading rate is about 10,000 times a second. Syntax analogRead(pin) Parameters pin: the number of the analog input pin to read from (0 to 5 on most boards, 0 to 7 on the Mini and Nano, 0 to 15 on the Mega) Returns int (0 to 1023) Note If the analog input pin is not connected to anything, the value returned by analogRead() will fluctuate based on a number of factors (e.g. the values of the other analog inputs, how close your hand is to the board, etc.). Example ``` ``` ``` int analogPin = 3; // potentiometer wiper (middle terminal) connected to analog pin 3 // outside leads to ground and +5V int val = 0; // variable to store the value read ``` ``` void setup() { Serial.begin(9600); // setup serial } ``` ``` void loop() { val = analogRead(analogPin); // read the input pin Serial.println(val); // debug value } ```

Answer 61

analogWrite() Description Writes an analog value ([PWM wave](https://www.arduino.cc/en/Tutorial/PWM)) to a pin. Can be used to light a LED at varying brightnesses or drive a motor at various speeds. After a call to analogWrite(), the pin will generate a steady square wave of the specified duty cycle until the next call to analogWrite() (or a call to digitalRead() or digitalWrite() on the same pin). The frequency of the PWM signal on most pins is approximately 490 Hz. On the Uno and similar boards, pins 5 and 6 have a frequency of approximately 980 Hz. Pins 3 and 11 on the Leonardo also run at 980 Hz. On most Arduino boards (those with the ATmega168 or ATmega328), this function works on pins 3, 5, 6, 9, 10, and 11. On the Arduino Mega, it works on pins 2 - 13 and 44 - 46. Older Arduino boards with an ATmega8 only support analogWrite() on pins 9, 10, and 11. The Arduino Due supports analogWrite() on pins 2 through 13, plus pins DAC0 and DAC1. Unlike the PWM pins, DAC0 andDAC1 are Digital to Analog converters, and act as true analog outputs. You do not need to call pinMode() to set the pin as an output before calling analogWrite(). The *analogWrite* function has nothing to do with the analog pins or the *analogRead* function. Syntax analogWrite(pin, value) Parameters pin: the pin to write to. value: the duty cycle: between 0 (always off) and 255 (always on). Returns nothing Notes and Known Issues The PWM outputs generated on pins 5 and 6 will have higher-than-expected duty cycles. This is because of interactions with the millis() and delay() functions, which share the same internal timer used to generate those PWM outputs. This will be noticed mostly on low duty-cycle settings (e.g 0 - 10) and may result in a value of 0 not fully turning off the output on pins 5 and 6. Example Sets the output to the LED proportional to the value read from the potentiometer. ``` ``` int ledPin = 9; // LED connected to digital pin 9 int analogPin = 3; // potentiometer connected to analog pin 3 int val = 0; // variable to store the read value ``` void setup() { pinMode(ledPin, OUTPUT); // sets the pin as output } ``` ``` void loop() { val = analogRead(analogPin); // read the input pin analogWrite(ledPin, val / 4); // analogRead values go from 0 to 1023, analogWrite values from 0 to 255 } ```

Answer 62

analogReadResolution() Description analogReadResolution() is an extension of the Analog API for the Arduino Due and Zero. Sets the size (in bits) of the value returned by analogRead(). It defaults to 10 bits (returns values between 0-1023) for backward compatibility with AVR based boards. The Due and the Zero have 12-bit ADC capabilities that can be accessed by changing the resolution to 12. This will return values from analogRead() between 0 and 4095. Syntax analogReadResolution(bits) Parameters bits: determines the resolution (in bits) of the value returned by analogRead() function. You can set this 1 and 32. You can set resolutions higher than 12 but values returned by analogRead() will suffer approximation. See the note below for details. Returns None. Note If you set the analogReadResolution() value to a value higher than your board's capabilities, the Arduino will only report back at its highest resolution padding the extra bits with zeros. For example: using the Due or the Zero with analogReadResolution(16) will give you an approximated 16-bit number with the first 12 bits containing the real ADC reading and the last 4 bits padded with zeros. If you set the analogReadResolution() value to a value lower than your board's capabilities, the extra least significant bits read from the ADC will be discarded. Using a 16 bit resolution (or any resolution higher than actual hardware capabilities) allows you to write sketches that automatically handle devices with a higher resolution ADC when these become available on future boards without changing a line of code. Example ``` void setup() { // open a serial connection Serial.begin(9600); } ``` ``` void loop() { // read the input on A0 at default resolution (10 bits) // and send it out the serial connection analogReadResolution(10); Serial.print("ADC 10-bit (default) : "); Serial.print(analogRead(A0)); ``` ``` // change the resolution to 12 bits and read A0 analogReadResolution(12); Serial.print(", 12-bit : "); Serial.print(analogRead(A0)); ``` ``` // change the resolution to 16 bits and read A0 analogReadResolution(16); Serial.print(", 16-bit : "); Serial.print(analogRead(A0)); ``` ``` // change the resolution to 8 bits and read A0 analogReadResolution(8); Serial.print(", 8-bit : "); Serial.println(analogRead(A0)); ``` ``` // a little delay to not hog serial monitor delay(100); } ```

Answer 63

analogWriteResolution() Description analogWriteResolution() is an extension of the Analog API for the Arduino Due and Zero. analogWriteResolution() sets the resolution of the analogWrite() function. It defaults to 8 bits (values between 0-255) for backward compatibility with AVR based boards. The Due has the following hardare capabilities: * 12 pins which default to 8-bit PWM, like the AVR-based boards. These can be changed to 12-bit resolution. * 2 pins with 12-bit DAC (Digital-to-Analog Converter) By setting the write resolution to 12, you can use analogWrite() with values between 0 and 4095 to exploit the full DAC resolution or to set the PWM signal without rolling over. The Zero has the following hardare capabilities: * 10 pins which default to 8-bit PWM, like the AVR-based boards. These can be changed to 12-bit resolution. * 1 pin with 10-bit DAC (Digital-to-Analog Converter). By setting the write resolution to 10, you can use analogWrite() with values between 0 and 1023 to exploit the full DAC resolution Syntax analogWriteResolution(bits) Parameters bits: determines the resolution (in bits) of the values used in the analogWrite() function. The value can range from 1 to 32. If you choose a resolution higher or lower than your board's hardware capabilities, the value used in analogWrite() will be either truncated if it's too high or padded with zeros if it's too low. See the note below for details. Returns None. Note If you set the analogWriteResolution() value to a value higher than your board's capabilities, the Arduino will discard the extra bits. For example: using the Due with analogWriteResolution(16) on a 12-bit DAC pin, only the first 12 bits of the values passed to analogWrite() will be used and the last 4 bits will be discarded. If you set the analogWriteResolution() value to a value lower than your board's capabilities, the missing bits will bepadded with zeros to fill the hardware required size. For example: using the Due with analogWriteResolution(8) on a 12-bit DAC pin, the Arduino will add 4 zero bits to the 8-bit value used in analogWrite() to obtain the 12 bits required. Example void setup(){ // open a serial connection Serial.begin(9600); // make our digital pin an output pinMode(11, OUTPUT); pinMode(12, OUTPUT); pinMode(13, OUTPUT); } ``` void loop(){ // read the input on A0 and map it to a PWM pin // with an attached LED int sensorVal = analogRead(A0); Serial.print("Analog Read) : "); Serial.print(sensorVal); ``` ``` // the default PWM resolution analogWriteResolution(8); analogWrite(11, map(sensorVal, 0, 1023, 0 ,255)); Serial.print(" , 8-bit PWM value : "); Serial.print(map(sensorVal, 0, 1023, 0 ,255)); ``` ``` // change the PWM resolution to 12 bits // the full 12 bit resolution is only supported // on the Due analogWriteResolution(12); analogWrite(12, map(sensorVal, 0, 1023, 0, 4095)); Serial.print(" , 12-bit PWM value : "); Serial.print(map(sensorVal, 0, 1023, 0, 4095)); ``` ``` // change the PWM resolution to 4 bits analogWriteResolution(4); analogWrite(13, map(sensorVal, 0, 1023, 0, 127)); Serial.print(", 4-bit PWM value : "); Serial.println(map(sensorVal, 0, 1023, 0, 127)); ``` delay(5); }

Answer 64

tone() Description Generates a square wave of the specified frequency (and 50% duty cycle) on a pin. A duration can be specified, otherwise the wave continues until a call to [noTone](https://www.arduino.cc/en/Reference/NoTone)(). The pin can be connected to a piezo buzzer or other speaker to play tones. Only one tone can be generated at a time. If a tone is already playing on a different pin, the call to tone() will have no effect. If the tone is playing on the same pin, the call will set its frequency. Use of the tone() function will interfere with PWM output on pins 3 and 11 (on boards other than the Mega). Board Min frequency (Hz) Max frequency (Hz) Uno, Mega, Leonardo and other AVR boards 31 65535 Gemma Not implemented Not implemented Zero 41 275000 Due Not implemented Not implemented For technical details, see [Brett Hagman's notes](https://code.google.com/p/rogue-code/wiki/ToneLibraryDocumentation#Ugly_Details). NOTE: if you want to play different pitches on multiple pins, you need to call noTone() on one pin before calling tone() on the next pin. Syntax tone(pin, frequency) tone(pin, frequency, duration) Parameters pin: the pin on which to generate the tone frequency: the frequency of the tone in hertz - *unsigned int* duration: the duration of the tone in milliseconds (optional) - *unsigned long* Returns nothing

Answer 65

noTone() Description Stops the generation of a square wave triggered by [tone](https://www.arduino.cc/en/Reference/Tone)(). Has no effect if no tone is being generated. NOTE: if you want to play different pitches on multiple pins, you need to call noTone() on one pin before calling tone() on the next pin. Syntax noTone(pin) Parameters pin: the pin on which to stop generating the tone Returns nothing

Answer 66

shiftOut() Description Shifts out a byte of data one bit at a time. Starts from either the most (i.e. the leftmost) or least (rightmost) significant bit. Each bit is written in turn to a data pin, after which a clock pin is pulsed (taken high, then low) to indicate that the bit is available. Note: if you're interfacing with a device that's clocked by rising edges, you'll need to make sure that the clock pin is low before the call to shiftOut(), e.g. with a call to digitalWrite(clockPin, LOW). This is a software implementation; see also the [SPI library](https://www.arduino.cc/en/Reference/SPI), which provides a hardware implementation that is faster but works only on specific pins. Syntax shiftOut(dataPin, clockPin, bitOrder, value) Parameters dataPin: the pin on which to output each bit (*int*) clockPin: the pin to toggle once the dataPin has been set to the correct value (*int*) bitOrder: which order to shift out the bits; either MSBFIRST or LSBFIRST. (Most Significant Bit First, or, Least Significant Bit First) value: the data to shift out. (*byte*) Returns None Note The dataPin and clockPin must already be configured as outputs by a call to [pinMode](https://www.arduino.cc/en/Reference/PinMode)(). shiftOut is currently written to output 1 byte (8 bits) so it requires a two step operation to output values larger than 255. // Do this for MSBFIRST serial int data = 500; // shift out highbyte shiftOut(dataPin, clock, MSBFIRST, (data \>\> 8)); // shift out lowbyte shiftOut(dataPin, clock, MSBFIRST, data); // Or do this for LSBFIRST serial data = 500; // shift out lowbyte shiftOut(dataPin, clock, LSBFIRST, data); // shift out highbyte shiftOut(dataPin, clock, LSBFIRST, (data \>\> 8)); [[Get Code]](https://www.arduino.cc/en/Reference/ShiftOut?action=sourceblock&num=1) Example *For accompanying circuit, see the [tutorial on controlling a 74HC595 shift register](https://www.arduino.cc/en/Tutorial/ShiftOut).* ``` //\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*// // Name : shiftOutCode, Hello World // // Author : Carlyn Maw,Tom Igoe // // Date : 25 Oct, 2006 // // Version : 1.0 // // Notes : Code for using a 74HC595 Shift Register // // : to count from 0 to 255 // //\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\* ``` ``` //Pin connected to ST\_CP of 74HC595 int latchPin = 8; //Pin connected to SH\_CP of 74HC595 int clockPin = 12; ////Pin connected to DS of 74HC595 int dataPin = 11; ``` void setup() { //set pins to output because they are addressed in the main loop pinMode(latchPin, OUTPUT); pinMode(clockPin, OUTPUT); pinMode(dataPin, OUTPUT); } ``` void loop() { //count up routine for (int j = 0; j \< 256; j++) { //ground latchPin and hold low for as long as you are transmitting digitalWrite(latchPin, LOW); shiftOut(dataPin, clockPin, LSBFIRST, j); //return the latch pin high to signal chip that it //no longer needs to listen for information digitalWrite(latchPin, HIGH); delay(1000); } } ```

Answer 67

shiftIn() Description Shifts in a byte of data one bit at a time. Starts from either the most (i.e. the leftmost) or least (rightmost) significant bit. For each bit, the clock pin is pulled high, the next bit is read from the data line, and then the clock pin is taken low. If you're interfacing with a device that's clocked by rising edges, you'll need to make sure that the clock pin is low before the first call to shiftIn(), e.g. with a call to digitalWrite(clockPin, LOW). Note: this is a software implementation; Arduino also provides an [SPI library](https://www.arduino.cc/en/Reference/SPI) that uses the hardware implementation, which is faster but only works on specific pins. Syntax ``` byte incoming = shiftIn(dataPin, clockPin, bitOrder) ``` Parameters dataPin: the pin on which to input each bit (*int*) clockPin: the pin to toggle to signal a read from dataPin bitOrder: which order to shift in the bits; either MSBFIRST or LSBFIRST. (Most Significant Bit First, or, Least Significant Bit First) Returns the value read (*byte*)

Answer 68

pulseIn() Description Reads a pulse (either HIGH or LOW) on a pin. For example, if value is HIGH, pulseIn() waits for the pin to go HIGH, starts timing, then waits for the pin to go LOW and stops timing. Returns the length of the pulse in microseconds or 0 if no complete pulse was received within the timeout. The timing of this function has been determined empirically and will probably show errors in shorter pulses. Works on pulses from 10 microseconds to 3 minutes in length. Please also note that if the pin is already high when the function is called, it will wait for the pin to go LOW and then HIGH before it starts counting. This routine can be used only if interrupts are activated. Furthermore the highest resolution is obtained with short intervals. Syntax pulseIn(pin, value) pulseIn(pin, value, timeout) Parameters pin: the number of the pin on which you want to read the pulse. (*int*) value: type of pulse to read: either [HIGH](https://www.arduino.cc/en/Reference/Constants) or [LOW](https://www.arduino.cc/en/Reference/Constants). (*int*) timeout (optional): the number of microseconds to wait for the pulse to be completed: the function returns 0 if no complete pulse was received within the timeout. Default is one second (*unsigned long*). Returns the length of the pulse (in microseconds) or 0 if no pulse is completed before the timeout (*unsigned long*) Example ``` ``` int pin = 7; unsigned long duration; ``` void setup() { pinMode(pin, INPUT); } ``` ``` void loop() { duration = pulseIn(pin, HIGH); } ```

Answer 69

millis() Description Returns the number of milliseconds since the Arduino board began running the current program. This number will overflow (go back to zero), after approximately 50 days. Parameters None Returns Number of milliseconds since the program started (*unsigned long*) Note: Please note that the return value for millis() is an unsigned long, logic errors may occur if a programmer tries to do arithmetic with smaller data types such as int's. Even signed long may encounter errors as its maximum value is half that of its unsigned counterpart. Example unsigned long time; ``` void setup(){ Serial.begin(9600); } void loop(){ Serial.print("Time: "); time = millis(); //prints time since program started Serial.println(time); // wait a second so as not to send massive amounts of data delay(1000); } ```

Answer 70

micros() Description Returns the number of microseconds since the Arduino board began running the current program. This number will overflow (go back to zero), after approximately 70 minutes. On 16 MHz Arduino boards (e.g. Duemilanove and Nano), this function has a resolution of four microseconds (i.e. the value returned is always a multiple of four). On 8 MHzArduino boards (e.g. the LilyPad), this function has a resolution of eight microseconds. *Note*: there are 1,000 microseconds in a millisecond and 1,000,000 microseconds in a second. Parameters None Returns Number of microseconds since the program started (*unsigned long*) Example ``` unsigned long time; void setup(){ Serial.begin(9600); } void loop(){ Serial.print("Time: "); time = micros(); //prints time since program started Serial.println(time); // wait a second so as not to send massive amounts of data delay(1000); } ```

Answer 71

delay() Description Pauses the program for the amount of time (in miliseconds) specified as parameter. (There are 1000 milliseconds in a second.) Syntax delay(ms) Parameters ms: the number of milliseconds to pause (*unsigned long*) Returns nothing Example int ledPin = 13; // LED connected to digital pin 13 ``` void setup() { pinMode(ledPin, OUTPUT); // sets the digital pin as output } ``` void loop() { digitalWrite(ledPin, HIGH); // sets the LED on delay(1000); // waits for a second digitalWrite(ledPin, LOW); // sets the LED off delay(1000); // waits for a second } [[Get Code]](https://www.arduino.cc/en/Reference/Delay?action=sourceblock&num=1) Caveat While it is easy to create a blinking LED with the delay() function, and many sketches use short delays for such tasks as switch debouncing, the use of delay() in a sketch has significant drawbacks. No other reading of sensors, mathematical calculations, or pin manipulation can go on during the delay function, so in effect, it brings most other activity to a halt. For alternative approaches to controlling timing see the [millis()](https://www.arduino.cc/en/Reference/Millis) function and the sketch sited below. More knowledgeable programmers usually avoid the use of delay() for timing of events longer than 10's of milliseconds unless the Arduino sketch is very simple. Certain things *do* go on while the delay() function is controlling the Atmega chip however, because the delay function does not disable interrupts. Serial communication that appears at the RX pin is recorded, PWM ([analogWrite](https://www.arduino.cc/en/Reference/AnalogWrite)) values and pin states are maintained, and [interrupts](https://www.arduino.cc/en/Reference/AttachInterrupt) will work as they should.

Answer 72

delayMicroseconds() Description Pauses the program for the amount of time (in microseconds) specified as parameter. There are a thousand microseconds in a millisecond, and a million microseconds in a second. Currently, the largest value that will produce an accurate delay is 16383. This could change in future Arduino releases. For delays longer than a few thousand microseconds, you should use delay() instead. Syntax delayMicroseconds(us) Parameters us: the number of microseconds to pause (*unsigned int*) Returns None Example int outPin = 8; // digital pin 8 ``` void setup() { pinMode(outPin, OUTPUT); // sets the digital pin as output } ``` void loop() { digitalWrite(outPin, HIGH); // sets the pin on delayMicroseconds(50); // pauses for 50 microseconds digitalWrite(outPin, LOW); // sets the pin off delayMicroseconds(50); // pauses for 50 microseconds } [[Get Code]](https://www.arduino.cc/en/Reference/DelayMicroseconds?action=sourceblock&num=1) configures pin number 8 to work as an output pin. It sends a train of pulses of approximately 100 microseconds period. The approximation is due to execution of the other instructions in the code. Caveats and Known Issues This function works very accurately in the range 3 microseconds and up. We cannot assure that delayMicroseconds will perform precisely for smaller delay-times. As of Arduino 0018, delayMicroseconds() no longer disables interrupts.

Answer 73

min(x, y) Description Calculates the minimum of two numbers. Parameters x: the first number, any data type y: the second number, any data type Returns The smaller of the two numbers. Examples ``` sensVal = min(sensVal, 100); // assigns sensVal to the smaller of sensVal or 100 // ensuring that it never gets above 100. ``` [[Get Code]](https://www.arduino.cc/en/Reference/Min?action=sourceblock&num=1) Note Perhaps counter-intuitively, max() is often used to constrain the lower end of a variable's range, while min() is used to constrain the upper end of the range. Warning Because of the way the min() function is implemented, avoid using other functions inside the brackets, it may lead to incorrect results min(a++, 100); // avoid this - yields incorrect results min(a, 100); a++; // use this instead - keep other math outside the function

Answer 74

max(x, y) Description Calculates the maximum of two numbers. Parameters x: the first number, any data type y: the second number, any data type Returns The larger of the two parameter values. Example ``` sensVal = max(senVal, 20); // assigns sensVal to the larger of sensVal or 20 // (effectively ensuring that it is at least 20) ``` [[Get Code]](https://www.arduino.cc/en/Reference/Max?action=sourceblock&num=1) Note Perhaps counter-intuitively, max() is often used to constrain the lower end of a variable's range, while min() is used to constrain the upper end of the range. Warning Because of the way the max() function is implemented, avoid using other functions inside the brackets, it may lead to incorrect results max(a--, 0); // avoid this - yields incorrect results max(a, 0); a--; //use this instead - keep other math outside the function

Answer 75

abs(x) Description Computes the absolute value of a number. Parameters x: the number Returns x: if x is greater than or equal to 0. - x: if x is less than 0. Warning Because of the way the abs() function is implemented, avoid using other functions inside the brackets, it may lead to incorrect results. abs(a++); // avoid this - yields incorrect results abs(a); a++; // use this instead - keep other math outside the function

Answer 76

constrain(x, a, b) Description Constrains a number to be within a range. Parameters x: the number to constrain, all data types a: the lower end of the range, all data types b: the upper end of the range, all data types Returns x: if x is between a and b a: if x is less than a b: if x is greater than b Example ``` sensVal = constrain(sensVal, 10, 150); // limits range of sensor values to between 10 and 150 ```

Answer 77

map(value, fromLow, fromHigh, toLow, toHigh) Description Re-maps a number from one range to another. That is, a value of fromLow would get mapped to toLow, a value offromHigh to toHigh, values in-between to values in-between, etc. Does not constrain values to within the range, because out-of-range values are sometimes intended and useful. The constrain() function may be used either before or after this function, if limits to the ranges are desired. Note that the "lower bounds" of either range may be larger or smaller than the "upper bounds" so the map() function may be used to reverse a range of numbers, for example `y = map(x, 1, 50, 50, 1);` The function also handles negative numbers well, so that this example `y = map(x, 1, 50, 50, -100);` is also valid and works well. The map() function uses integer math so will not generate fractions, when the math might indicate that it should do so. Fractional remainders are truncated, and are not rounded or averaged. Parameters value: the number to map fromLow: the lower bound of the value's current range fromHigh: the upper bound of the value's current range toLow: the lower bound of the value's target range toHigh: the upper bound of the value's target range Returns The mapped value. Example ``` /\* Map an analog value to 8 bits (0 to 255) \*/ void setup() {} ``` ``` void loop() { int val = analogRead(0); val = map(val, 0, 1023, 0, 255); analogWrite(9, val); } ``` [[Get Code]](https://www.arduino.cc/en/Reference/Map?action=sourceblock&num=1) Appendix For the mathematically inclined, here's the whole function ``` long map(long x, long in\_min, long in\_max, long out\_min, long out\_max) { return (x - in\_min) \* (out\_max - out\_min) / (in\_max - in\_min) + out\_min; } ```

Answer 78

pow(base, exponent) Description Calculates the value of a number raised to a power. Pow() can be used to raise a number to a fractional power. This is useful for generating exponential mapping of values or curves. Parameters base: the number (*float*) exponent: the power to which the base is raised (*float*) Returns The result of the exponentiation (*double*) Example See the [fscale](http://arduino.cc/playground/Main/Fscale) function in the code library.

Answer 79

sqrt(x) Description Calculates the square root of a number. Parameters x: the number, any data type Returns double, the number's square root

Answer 80

sin(rad) Description Calculates the sine of an angle (in radians). The result will be between -1 and 1. Parameters rad: the angle in radians (*float*) Returns the sine of the angle (*double*)

Answer 81

cos(rad) Description Calculates the cos of an angle (in radians). The result will be between -1 and 1. Parameters rad: the angle in radians (*float*) Returns The cos of the angle ("double")

Answer 82

isAlphaNumeric(thisChar) Description Analyse if a char is alphanumeric. Parameters thisChar: the char to be analysed. Returns True or false.

Answer 83

isAlpha(thisChar) Description Analyse if a char is is alpha. Parameters thisChar: the char to be analysed Returns True or false.

Answer 84

isAscii(thisChar) Description Analyse if a char is ASCII. Parameters thisChar: the char to be analysed Returns True or false.

Answer 85

isWhitespace(thisChar) Description Analyse if a char is a white space. Parameters thisChar: the char to be analysed Returns True or false.

Answer 86

isControl(thisChar) Description Analyse if a char is a control character. Parameters thisChar: the char to be analysed Returns True or false.

Answer 87

isDigit(thisChar) Description Analyse if a char is a digit. Parameters thisChar: the char to be analysed Returns True or false.

Answer 88

isGraph(thisChar) Description Analyse if a char is a printable character. Parameters thisChar: the char to be analysed Returns True or false.

Answer 89

isLowerCase(thisChar) Description Analyse if a char is a lower case character. Parameters thisChar: the char to be analysed Returns True or false.

Answer 90

isPrintable(thisChar) Description Analyse if a char is a printable character. Parameters thisChar: the char to be analysed Returns True or false.

Answer 91

isPunct(thisChar) Description Analyse if a char is punctuation character. Parameters thisChar: the char to be analysed Returns True or false.

Answer 92

isSpace(thisChar) Description Analyse if a char is a space character. Parameters thisChar: the char to be analysed Returns True or false.

Answer 93

isUpperCase(thisChar) Description Analyse if a char is an upper case character. Parameters thisChar: the char to be analysed Returns True or false.

Answer 94

isHexadecimalDigit(thisChar) Description Analyse if a char is a valid hexadecimal digit. Parameters thisChar: the char to be analysed Returns True or false.

Answer 95

randomSeed(seed) Description randomSeed() initializes the pseudo-random number generator, causing it to start at an arbitrary point in its random sequence. This sequence, while very long, and random, is always the same. If it is important for a sequence of values generated by random() to differ, on subsequent executions of a sketch, use randomSeed() to initialize the random number generator with a fairly random input, such as analogRead() on an unconnected pin. Conversely, it can occasionally be useful to use pseudo-random sequences that repeat exactly. This can be accomplished by calling randomSeed() with a fixed number, before starting the random sequence. Parameters long, int - pass a number to generate the seed. Returns no returns Example ``` long randNumber; void setup(){ Serial.begin(9600); randomSeed(analogRead(0)); } void loop(){ randNumber = random(300); Serial.println(randNumber); delay(50); } ```

Answer 96

random() Description The random function generates pseudo-random numbers. Syntax random(max) random(min, max) Parameters min - lower bound of the random value, inclusive *(optional)* max - upper bound of the random value, exclusive Returns a random number between min and max-1 (*long*) Note: If it is important for a sequence of values generated by random() to differ, on subsequent executions of a sketch, use randomSeed() to initialize the random number generator with a fairly random input, such as analogRead() on an unconnected pin. Conversely, it can occasionally be useful to use pseudo-random sequences that repeat exactly. This can be accomplished by calling randomSeed() with a fixed number, before starting the random sequence. Example ``` long randNumber; void setup(){ Serial.begin(9600); // if analog input pin 0 is unconnected, random analog // noise will cause the call to randomSeed() to generate // different seed numbers each time the sketch runs. // randomSeed() will then shuffle the random function. randomSeed(analogRead(0)); } void loop() { // print a random number from 0 to 299 randNumber = random(300); Serial.println(randNumber); // print a random number from 10 to 19 randNumber = random(10, 20); Serial.println(randNumber); delay(50); } ```

Answer 97

lowByte() Description Extracts the low-order (rightmost) byte of a variable (e.g. a word). Syntax lowByte(x) Parameters x: a value of any type Returns byte

Answer 98

highByte() Description Extracts the high-order (leftmost) byte of a word (or the second lowest byte of a larger data type). Syntax highByte(x) Parameters x: a value of any type Returns byte

Answer 99

bitRead() Description Reads a bit of a number. Syntax bitRead(x, n) Parameters x: the number from which to read n: which bit to read, starting at 0 for the least-significant (rightmost) bit Returns the value of the bit (0 or 1).

Answer 100

bitWrite() Description Writes a bit of a numeric variable. Syntax bitWrite(x, n, b) Parameters x: the numeric variable to which to write n: which bit of the number to write, starting at 0 for the least-significant (rightmost) bit b: the value to write to the bit (0 or 1) Returns none

Answer 101

bitSet() Description Sets (writes a 1 to) a bit of a numeric variable. Syntax bitSet(x, n) Parameters x: the numeric variable whose bit to set n: which bit to set, starting at 0 for the least-significant (rightmost) bit Returns none

Answer 102

bitClear() Description Clears (writes a 0 to) a bit of a numeric variable. Syntax bitClear(x, n) Parameters x: the numeric variable whose bit to clear n: which bit to clear, starting at 0 for the least-significant (rightmost) bit Returns none

Answer 103

bit() Description Computes the value of the specified bit (bit 0 is 1, bit 1 is 2, bit 2 is 4, etc.). Syntax bit(n) Parameters n: the bit whose value to compute Returns the value of the bit

Answer 104

attachInterrupt() Description Digital Pins With Interrupts The first parameter to attachInterrupt is an interrupt number. Normally you should use digitalPinToInterrupt(pin) to translate the actual digital pin to the specific interrupt number. For example, if you connect to pin 3, use digitalPinToInterrupt(3) as the first parameter to attachInterrupt. ``` Board Digital Pins Usable For Interrupts Uno, Nano, Mini, other 328-based 2, 3 Mega, Mega2560, MegaADK 2, 3, 18, 19, 20, 21 Micro, Leonardo, other 32u4-based 0, 1, 2, 3, 7 Zero all digital pins, except 4 Due all digital pins #### Note ``` *Inside the attached function, delay() won't work and the value returned by millis() will not increment. Serial data received while in the function may be lost. You should declare as volatile any variables that you modify within the attached function. See the section on ISRs below for more information.* Using Interrupts Interrupts are useful for making things happen automatically in microcontroller programs, and can help solve timing problems. Good tasks for using an interrupt may include reading a rotary encoder, or monitoring user input. If you wanted to insure that a program always caught the pulses from a rotary encoder, so that it never misses a pulse, it would make it very tricky to write a program to do anything else, because the program would need to constantly poll the sensor lines for the encoder, in order to catch pulses when they occurred. Other sensors have a similar interface dynamic too, such as trying to read a sound sensor that is trying to catch a click, or an infrared slot sensor (photo-interrupter) trying to catch a coin drop. In all of these situations, using an interrupt can free the microcontroller to get some other work done while not missing the input. About Interrupt Service Routines ISRs are special kinds of functions that have some unique limitations most other functions do not have. An ISR cannot have any parameters, and they shouldn't return anything. Generally, an ISR should be as short and fast as possible. If your sketch uses multiple ISRs, only one can run at a time, other interrupts will be executed after the current one finishes in an order that depends on the priority they have. millis() relies on interrupts to count, so it will never increment inside an ISR. Since delay() requires interrupts to work, it will not work if called inside an ISR. micros() works initially, but will start behaving erratically after 1-2 ms. delayMicroseconds() does not use any counter, so it will work as normal. Typically global variables are used to pass data between an ISR and the main program. To make sure variables shared between an ISR and the main program are updated correctly, declare them as `volatile`. For more information on interrupts, see [Nick Gammon's notes](http://gammon.com.au/interrupts). ``` Syntax attachInterrupt(digitalPinToInterrupt(pin), ISR, mode); (*recommended*) attachInterrupt(interrupt, ISR, mode); (*not recommended*) attachInterrupt(pin, ISR, mode) ; (*not recommended Arduino Due, Zero only*) #### Parameters interrupt: the number of the interrupt (*int*) pin: the pin number (*Arduino Due, Zero only*) ISR: the ISR to call when the interrupt occurs; this function must take no parameters and return nothing. This function is sometimes referred to as an*interrupt service routine.* mode: ``` defines when the interrupt should be triggered. Four contstants are predefined as valid values: * LOW to trigger the interrupt whenever the pin is low, * CHANGE to trigger the interrupt whenever the pin changes value * RISING to trigger when the pin goes from low to high, * FALLING for when the pin goes from high to low. The Due board allows also: ``` * HIGH to trigger the interrupt whenever the pin is high. (*Arduino Due, Zero only*) #### Returns ``` none Example int pin = 13; volatile int state = LOW; void setup() { pinMode(pin, OUTPUT); attachInterrupt(digitalPinToInterrupt(pin), blink, CHANGE); } ``` void loop() { digitalWrite(pin, state); } ``` ``` void blink() { state = !state; } ``` [[Get Code]](https://www.arduino.cc/en/Reference/AttachInterrupt?action=sourceblock&num=1) Interrupt numbers Normally you should use digitalPinToInterrupt(pin), rather than place interrupt an number directly into your sketch. The specific pins with interrupts, and their mapping to interrupt number varies on each type of board. Direct use of interrupt numbers may seem simple, but it can cause compatibility trouble when your sketch is run on a different board. However, older sketches often have direct interrupt numbers. Often number 0 (for digital pin 2) or number 1 (for digital pin 3) were used. The table below shows the available interrupt pins on various boards. Board int.0 int.1 int.2 int.3 int.4 int.5 Uno, Ethernet 2 3 Mega2560 2 3 21 20 19 18 32u4 based (e.g Leonardo, Micro) 3 2 0 1 7 Due, Zero (see below) The Arduino Due board has powerful interrupt capabilities that allows you to attach an interrupt function on all available pins. You can directly specify the pin number in attachInterrupt(). The Arduino Zero board allows you to attach an interrupt function on all available pins except for pin 4. You can directly specify the pin number in attachInterrupt().

Answer 105

detachInterrupt() Description Turns off the given interrupt. ``` Syntax detachInterrupt(*interrupt*) detachInterrupt(digitalPinToInterrupt(pin)); detachInterrupt(*pin*) (*Arduino Due, Zero only*) #### Parameters ``` * *interrupt*: the number of the interrupt to disable (see [attachInterrupt](https://www.arduino.cc/en/Reference/AttachInterrupt)() for more details). * *pin*: the pin number of the interrupt to disable (*Arduino Due only*)

Answer 106

interrupts() Description Re-enables interrupts (after they've been disabled by [noInterrupts](https://www.arduino.cc/en/Reference/NoInterrupts)()). Interrupts allow certain important tasks to happen in the background and are enabled by default. Some functions will not work while interrupts are disabled, and incoming communication may be ignored. Interrupts can slightly disrupt the timing of code, however, and may be disabled for particularly critical sections of code. Parameters None Returns None Example ``` void setup() {} void loop() { noInterrupts(); // critical, time-sensitive code here interrupts(); // other code here } ```

Answer 107

noInterrupts() Description Disables interrupts (you can re-enable them with interrupts()). Interrupts allow certain important tasks to happen in the background and are enabled by default. Some functions will not work while interrupts are disabled, and incoming communication may be ignored. Interrupts can slightly disrupt the timing of code, however, and may be disabled for particularly critical sections of code. Parameters None. Returns None. Example ``` void setup() {} void loop() { noInterrupts(); // critical, time-sensitive code here interrupts(); // other code here } ```

Answer 108

Serial Used for communication between the Arduino board and a computer or other devices. All Arduino boards have at least one serial port (also known as a UART or USART): Serial. It communicates on digital pins 0 (RX) and 1 (TX) as well as with the computer via USB. Thus, if you use these functions, you cannot also use pins 0 and 1 for digital input or output. You can use the Arduino environment's built-in serial monitor to communicate with an Arduino board. Click the serial monitor button in the toolbar and select the same baud rate used in the call to begin(). The [Arduino Mega](https://www.arduino.cc/en/Main/ArduinoBoardMega2560) has three additional serial ports: Serial1 on pins 19 (RX) and 18 (TX), Serial2 on pins 17 (RX) and 16 (TX), Serial3 on pins 15 (RX) and 14 (TX). To use these pins to communicate with your personal computer, you will need an additional USB-to-serial adaptor, as they are not connected to the Mega's USB-to-serial adaptor. To use them to communicate with an external TTL serial device, connect the TX pin to your device's RX pin, the RX to your device's TX pin, and the ground of your Mega to your device's ground. (Don't connect these pins directly to an RS232 serial port; they operate at +/- 12V and can damage your Arduino board.) The [Arduino Due](https://www.arduino.cc/en/Main/ArduinoBoardDue) has three additional 3.3V TTL serial ports: Serial1 on pins 19 (RX) and 18 (TX); Serial2 on pins 17 (RX) and 16 (TX), Serial3 on pins 15 (RX) and 14 (TX). Pins 0 and 1 are also connected to the corresponding pins of the ATmega16U2 USB-to-TTL Serial chip, which is connected to the USB debug port. Additionally, there is a native USB-serial port on the SAM3X chip, *SerialUSB*'. The Arduino Leonardo board uses Serial1 to communicate via TTL (5V) serial on pins 0 (RX) and 1 (TX). Serial is reserved for USB CDC communication. For more information, refer to the Leonardo [getting started](https://www.arduino.cc/en/Guide/ArduinoLeonardo) page and [hardware page](https://www.arduino.cc/en/Main/ArduinoBoardLeonardo).

Answer 109

Stream Stream is the base class for character and binary based streams. It is not called directly, but invoked whenever you use a function that relies on it. Stream defines the reading functions in Arduino. When using any core functionality that uses a read() or similar method, you can safely assume it calls on the Stream class. For functions like print(), Stream inherits from the Print class. Some of the libraries that rely on Stream include : * [Serial](https://www.arduino.cc/en/Reference/Serial) * [Wire](https://www.arduino.cc/en/Reference/Wire) * [Ethernet Client](https://www.arduino.cc/en/Reference/Ethernet) * [Ethernet Server](https://www.arduino.cc/en/Reference/Ethernet) * [SD](https://www.arduino.cc/en/Reference/SD) Functions * [available](https://www.arduino.cc/en/Reference/StreamAvailable)() * [read](https://www.arduino.cc/en/Reference/StreamRead)() * [flush](https://www.arduino.cc/en/Reference/StreamFlush)() * [find](https://www.arduino.cc/en/Reference/StreamFind)() * [findUntil](https://www.arduino.cc/en/Reference/StreamFindUntil)() * [peek](https://www.arduino.cc/en/Reference/StreamPeek)() * [readBytes](https://www.arduino.cc/en/Reference/StreamReadBytes)() * [readBytesUntil](https://www.arduino.cc/en/Reference/StreamReadBytesUntil)() * [readString](https://www.arduino.cc/en/Reference/StreamReadString)() * [readStringUntil](https://www.arduino.cc/en/Reference/StreamReadStringUntil)() * [parseInt](https://www.arduino.cc/en/Reference/StreamParseInt)() * [parsefloat](https://www.arduino.cc/en/Reference/StreamParseFloat)() * [setTimeout](https://www.arduino.cc/en/Reference/StreamSetTimeout)()

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