4. The cortex M3 microcontroller

Question 1

What is high performance 32 bit RISC CPU?

Answer 1

Uses a reduced instruction set computer (RISC) processor with most instructions executing in a single CPU cycle.

Question 2

What is the opposite of RISC? and what is most common in embedded systems, why?

Answer 2

CISC - complex instruction set computer. RISC processors tend to be simper, consume less power and be faster. RISC is dominant in embedded systems.

Question 3

What is an example of the use of CISC?

Answer 3

In order to maintain x86 compatibility, intel’s processors (eg Atom) are CISC.

Question 4

What does 32 bit mean?

Answer 4

32 bit means the microcontroller can carry out operations on 32 bit wide packets of data. Instructions aren’t necessarily 32 bit wide.

Question 5

What type of bugging does the Cortex M3 use?

Answer 5

Advanced debugging with coresight - the cortex m3 allows a low-cost debugger to step through the program slowly, set break points and view the content of memory on the fly (without halting the program).

Question 6

How much memory can the cortex m3 address?

Answer 6

Up to 4G of memory. The SAM3X8E has 2-256 Kbytes of flash memory, which is mainly used to store the program, though it can also be used to store data. It also has 96 Kbytes of SRAM, organised as 64 Kbytes SRAM0 and 32 Kbytes SRAM1.

Question 7

What is interrupt handling?

Answer 7

An ‘interrupt’ causes the mC to stop what it is doing, carry out an urgent task, then return to its original operation. For example, do this while the button is not pressed, if the button gets pressed do this and then return to what you were doing before.

Question 8

What are the different clocks used by the mC?

Answer 8

Fast - used for the system in normal operation (84 MHz)
Intermediate - 4-12MHz, used for ast start-up mode, and generating the fast clock signal
Slow - used in backup mode (very low power). Exactly 32.768KHz if using a crystal

Question 9

What are the types of oscillators that can be used to create the clocks?

Answer 9

External quartz crystal - used when accuracy is important e.g. a real clock.
Internal oscillator - cheap (free!) and saves 2 pins, accuracy about 3%

Question 10

What is power saving back-up mode?

Answer 10

Many systems spend most of their time doing nothing (e.g. a remote control) so it is useful if they can ‘go to sleep’. The purpose of back up mode is to achieve the lowest power consumption possible in a system which is performing periodic wake-ups to perform tasks but not requiring fast start up time (

Question 11

What are WAKE and SLEEP modes?

Answer 11

these use more power than backup (current >= 15 microAmps) but which start up much more quickly (=10 microSecs)

Question 12

What is the voltage supply tolerance in the SAM3X/A series?

Answer 12

Wide voltage supply tolerance - supports a 1.62-3.6 V single supply mode.

Question 13

What is an ADC?

Answer 13

An analog to digital converter - converts an external voltage into a digital value, which the mC can process. There are 16 channels (different inputs) and the offset and gain of each can be varied.

Question 14

What is a DAC?

Answer 14

Digital to analog converter, this is the other way around and has 2 channels

Question 15

what is PWM?

Answer 15

Pulse width modulation - 8 channels with 16 bit accuracy. Useful for controlling analog devices (eg motors, lights)

Question 16

How many pins?

Answer 16

The package has 144 pins. Each can be selected to be either an input or an output (I/O) and this can be changed while the device is operating.

Question 17

What are general purpose I/O lines?

Answer 17

these are managed by parallel input/output controllers (PIO) and have a number of useful modes.

Question 18

What are the modes of GPIO?

Answer 18

(general purpose I/O lines)

1. Input schmitt triggers
2. Debouncing
3. Built in pull-up resistors

Question 19

What are input schmitt triggers?

Answer 19

many events are triggered by a rising or falling signal, eg from an analog sensor. If you are not careful the system can oscillate. The schmitt trigger can be used to clean up noisy signals: DRAW DIAGRAM.

Question 20

What is debouncing?

Answer 20

An input from a switch is often used (eg keyboard), but the signal is generally noisy and the switch may appear to change several times. The mC will automatically clean this up if requested.

Question 21

What forms of external connectivity are supported for the mC?

Answer 21

USB, ethernet, SPI, CAN, I2C

Question 22

How many timers does the mC have?

Answer 22

9

Question 23

Examples of uses of timers as inputs and outputs?

Answer 23

Timing inputs - when are 2 clicks of a mouse a doubleclick?
Frequency of inputs - a speedometer might count the frequency of pulses produced when a wheel rotates
Timing processes - the length of a wash in a washing machine
Generating given frequencies - different tones for tuning a guitar
Generating regular waveforms - control power to a heater or lamp using pulse width modulation

Question 24

What is the 3 stage pipeline for the SAM3X?

Answer 24

1. Fetch - the next instruction is read from memory
2. Decode - turns the instruction into a set of signals that drive the ALU
3. Execute - actually carry out instruction
   Each happens once per clock tick

Question 25

what are CPU registers?

Answer 25

Registers used by the CPU for any kind of data processing such as addition or multiplication

Question 26

How is a register used?

Answer 26

Data has to be loaded into the register from the main memory, then the command is executed leaving the result in the register. Finally the result is moved back to main memory.

Question 27

Name/describe some special registers?

Answer 27

MSP: Main Stack Pointer - points to region of memory used for temporary data
LR: Link Register - stores return address on subroutine call
PC: program counter - address of next instruction
PSR: program status register - details later

Question 28

How many bits in the PSR? How are various status flags stored within the PSR?

Answer 28

32 bit, not all of which are currently used. SEE DIAGRAM

Question 29

What do N,Z,C and V stand for in program status register?

Answer 29

N - negative or less than flag: 0=operation results was positive, zero, greater than or equal
1= operation result was negative or less than
Z - zero flag: 0=operation result was not zero, 1=operation result was zero
C - Carry or borrow flag:
did add or subtract result in a carry bit?
V - overflow flag: did the operation result in an overflow?

Question 30

How is the memory space in the cortex m3 divided?

Answer 30

SEE DIAGRAM

Question 31

What is a binary digit?

Answer 31

a bit which hold the value 0 or 1

Question 32

How many bits in a byte?

Answer 32

8 bits in a byte

Question 33

What is the range of a byte?

Answer 33

from 00000000 to 11111111. In decimal this is the range of 0 to 255.

Question 34

How else can the value of a byte be interpreted?

Answer 34

to mean -128 … 127.

Question 35

What is the smallest unit that can be processed in most computers?

Answer 35

a byte

Question 36

How can a byte be divided?

Answer 36

Into 2 units of 4 bits, known as nibbles.. a nibble can range from 0000 to 1111 = 0 to 15 (decimal)

Question 37

How does hexadecimal (hex) notation work?

Answer 37

Each digit has a range of 16, values 0…15 in decimal, written 0-9 with A-F for 10-15. A prefix 0x is often used to indicate hex. One hex digit holds the value in a nibble 0000..1111 = 0x0…0xF. Thus the contents of one byte lie in the range 0x00…0xFF

Question 38

Max an min values of a 32bit number (4 bytes) as an 8digit hex number:

Answer 38

Min value: 0x00000000= 0(decimal)

Max value: 0xFFFFFFFF= 2^(32)-1 = 4G.

Question 39

What is memory mapped input and output viewed from the outside?

Answer 39

Digital input and output takes place through the pins that form ‘ports’ A to D. (number of pins: 30, 32, 32, 11)

Question 40

What is memory mapped input and output?

Answer 40

The input and output ports look like ordinary memory addresses to a program, with addresses like any other memory location Thus the ports can be accessed by reading and writing to the correct memory locations.

Question 41

What does reading a pin do?

Answer 41

Reading a pin returns the value on the pin of the mC, not a number that has been stored in memory

Question 42

What does writing do?

Answer 42

Writing or storing a bit in one of these locations puts the values on the pins of the mC if they are configured as outputs:
- a bit of 0 causes the mC to drive the pin to ground, VSS=0V
- a bit of 1 causes the mC to drive the pin to VDD (positive eg +3V)
SEE DIAGRAM

Question 43

What is an RISC?Compare with CISC?

Answer 43

RISC (reduced instruction set computer) is a microprocessor that is designed to perform a smaller number of types of computer instructions so that it can operate at a higher speed. Since each instruction type that a computer must perform requires additional transistors and circuitry, a larger list or set of computer instructions tends to make the microprocessor more complicated and slower in operation.

Question 44

Can you compare a 32 bit processor with an 8 bit processor?

Answer 44

A 32 bit processor can deal with 4* as much data in a single clock cycle than an 8 bit processor. For example, an 8 bit processor can deal with integers >255 but this will require 2 or more bytes to hold the data and several clock cycles to manipulate such data, since only 8 bits can be handled in a single step. This means that the 32 bit processor is faster than the 8 bit processor. Also the code is shorter and simpler for a 32 bit processor. Floating point numbers require at least 32 bits and so can be handled much more simply and quickly using a 32 bit processor.