March Test

Question 1

What did Arm design the Cortex-M series to be?

Whats the M stand for?

Answer 1

Low-cost and energy efficient

Microcontroller

Question 2

What are the M4 and M0 designed for?

Whats special about the M0+?

Answer 2

M4: high performance, general use
M0: low-cost, simplicity
M0+: most energy efficient

Question 3

Whats the F in M4F stand for?

Whats it good at doing? Two things

Answer 3

Built-in floating-point unit (FPU)
Designed to do well with floating-point numbers
Does them in one clock cycle
Has a DSP so good at digital signal processing

Question 4

Why is power loss small in CMOS?

Answer 4

There isn’t a static current path from Vdd to Vss so static power consumption is low

Question 5

What can cause power loss in CMOS?

Three things

Answer 5

When output switches, it will either charge or discharge the gate capacitence

During output transition a short circuit can occur between Vdd and Vss

Leakage current while transistor is off

Question 6

4 ways to reduce power consumption in a CMOS circuit

Answer 6

Lower frequency-however this is a a trade-off between power and performance

Lower amount of gates

Lower activity-i.e. when circuit not used it is powered down

Lower supply voltage. K46F can work down to 1.7V

Question 7

What is a task?

Two types of tasks

Answer 7

An activity that is required to be performed-read a sensor, update a display etc
Event-triggered tasks: i.e. light an LED when a button is pressed
Time-triggered tasks: i.e. reading a sensor every 1 second

Question 8

Responding to event triggers: polling
Edge-triggered interrupts
What to remember about the ISR

Answer 8

Repeatedly checking the status of an input

When voltage change on input is detected, the program goes from sleep to execute a interrupt service routine (ISR) then back to sleep
Keep code inside the ISR as short as possible

Question 9

Hybrid approach to event triggers

Why do this?

Answer 9

Set a global flag variable in the ISR. When the program comes out of sleep it check the flag and then does the code
ISR can still be short

Question 10

Time-triggered task: two ways to do it

Sleep modes: two types

Answer 10

Periodically: ticker
One-off event: timeout

Sleep: clock stopped until reset or interrupt occurs. Can be woken by internal or external interrupt
Deepsleep: can only wake up with external interrupt

Question 11

5 things that make up a finite state machine

Answer 11

States that the system can exist in
Finite set of inputs
Finite set of outputs
State transitions (what happens when we are in a state and get a certain input)
How outputs are determined

Question 12

How to define a state? (what to write inside the circle)

Answer 12

Name (often a number)
Output state
Wait time for how long it will be in state

Question 13

Using transition tables and structs with FSM’s

Answer 13

Revise

Question 14

I^2C: whats it stand for
How’s it work
Two types of wires, what is required with them

Answer 14

Inter-integrated circuit
MCU is master, sensors are slaves
Multiple slaves can be connected to same bus
Data (SDA), clock (SCL)
Lines are open-collector meaning that pull-up resistors are needed

Question 15

I^2C: how many pull-up resistors are needed?
How many slaves can be on the same bus in theory?
And in practise

Answer 15

One for each line
Each slave has a 7-bit address so 127 slaves (address 0 is a general call address to all slaves)
In practise numbe of devices are limited by capacitive loading of lines

Question 16

I^2C: how is start and stop defined using both lines?
How is data sent
What is sent after the data

Answer 16

Starts when SDA goes low while SCL is high
Stops when SDA goes high while SCL is high
Data is sent serially on SDA, one byte at a time on a clock edge (on SCL)
A acknowledgement bit

Question 17

I^2C: what is sent on the first transaction to a slave?

Answer 17

The 8th bit is a read/write bit
If 0, master writes to slave
If 1, master reads from slave

Question 18

Whats the bit shift operator?
Setting a bit using bit shift
Clearing a bit

Answer 18

>> for right, << for left
Use a mask - char mask = (1 << 2);
This will stick a 1 in bit 2
To set a bit without overwriting, use OR
a = a | (1<<2); will keep a but set bit 2 to 1
Use AND with a NOT 1
a &amp;= ~(1<<7); clears bit 7

Question 19

Reading single values from sensors

What phrase to look for for reading multiple values and how it works

Answer 19

Master: tells slave which register it wants to read using write 
-Slave acks by holding SDA low
-Master sends register address
-Slave acks again
Master sends read address
-Slave sends bits using SDA

Reading ‘burst’ of values: after getting a read request the slave can increment address

Question 20

Writing values to a sensor: what 3 bytes need to be sent

Which type of register are you writing to?

Answer 20

Slave write address
Register address
Value to store in register

Configuration register

Question 21

Creating an I^2C object

Setting the frequency

Answer 21

I2C name(I2C_SDA, I2C_SCL);
name.frequency(40000);

Question 22

C++ code rough: send a single byte to a register
Read a single byte
How to read multiple bits

Answer 22

Set an int to a sensor.write value. Returns 0 if ack received and 1 if not

Write to it and check for ack
Use a sensor.read and check for ack
Return in variable (rx)

Store in an array (function is void and array is passed by reference)

Question 23

MBED error: where is the error message printed, what happens

Answer 23

Prints to stderr (USB serial ports), flashs LED’s

Question 24

FXOS8700CQ: what two things make it up and how many axis can they use
What is the overall package made from?

Answer 24

Accelerometer and compass
3 axis
NXP Semiconductors

Question 25

FXOS8700CQ initialisation process: 5 steps

Answer 25

1. Check if one of those is on the I^2C bus using WHO_AM_I register
2. Stick it in standby mode by sending 0x00
3. Set it to 200Hz sampling rate
4. Configure it to +/- 4 g sensitivity
5. Take it out of standby

Question 26

Public vs private in classes

Answer 26

Public: member/method can be accessed by users
Private: hidden from users

Question 27

What does a header guard prevent

Answer 27

A header being included more than once

Question 28

Number of pixels on the Nokia 5110 LCD
How many lines does SPI have and what are they called?
What does a master do if it wants to talk to slaves

Answer 28

84*48
Serial clock line (SCLK)
Master-out, slave-in (MOSI)
Master-in, slave-out (MISO)
Slave-select or slave-chip-enable (SS or SCE)
Makes the relevent SCE line low, then starts clock signal

Question 29

Whats the LCD's highest clock frequency?
LCD C++ commands for:
Initilising
Setting contrast
Clearing the screen
Printing strings and chars

Answer 29

4 MHz

lcd. init();
lcd. setContract(0.5);
lcd. clear();
lcd. printString(“Hello”,0,0);
lcd. printChar(‘X’,5,3);

Question 30

LCD C++ commands for:
Setting and clearing pixels
Check status of pixel
Drawing lines
Drawing circles
Drawing rectangles
Refresh buffer

Answer 30

lcd.setPixel(2,5,true/false);
if (lcd.getPixel(2,5) ;
lcd.drawLine(x0,y0,x1,y1,type);
lcd.draw(x,y,radius,type);
lcd.drawRect(x,y,width,height,type);
lcd.refresh();

Question 31

What issues can occur with the joystick and why?
How to fix this
What does it make more sense to do with the coordinates

Answer 31

If you point it to a corner you get a magnitude of root 2 instead of 1 despite being moved the same amount
Map all points to points inside a unit circle
Use magnitude and angle instead
Can use get direction instead