109

Question 1

What is a counter?

Answer 1

> They increase in value by 1

Question 2

How are counters built?

Answer 2

> With flip-flops

> A 4 bit counter requires 4 flip-flops

Question 3

How is a timer made? How are they applied?

Answer 3

> Using counters

> Increase on every clock cycle

Question 4

How can a timer be used to calculate the time passed?

Answer 4

> If we know the clock frequency then we can measure how many clocks have passed to work out time

Question 5

What is a JK Flip-flop?

Answer 5

They are like a flip flop except they toggle the current value being stored

Question 6

What are the inputs and outputs of a JK flip-flop?

Answer 6

> J
> K
> Clk
> Q
> Q(NOT)

Question 7

What is the truth table for a JK Flip-flop

Answer 7

CLK J K Q
   ^   0 0 Q(t-1)
   ^   0 1  0
   ^   1  0  1
   ^   1  1  [NOT] Q(t-1) It flips the value stored

Question 8

What is a Synchronous counter and how is it implemented?

Answer 8

> This is a binary counter which increased based off the clock signal.
Made using JK Flip-flops

Question 9

In a binary counter, when is the next bit flipped?

Answer 9

When all the previous bits are 1

Question 10

How is the least significant bit clocked?

Answer 10

With the clock cycle input

Question 11

How is a Synchronous counter made from JK flip-flops and logic gates (for a 4 bit counter)?

Answer 11

> There are 4 JK flip-flops in a line
Each has the same clock signal as its input
JK1 has both its J and K inputs set at 1
The output of the JK1 goes into the J and K input for JK2
An AND gate is placed beside the JK2 with one of its input from the output of JK1 and the output of JK2.
The result of the AND gate goes into both the J and K of JK3
Another AND gate is placed beside JK3 with one of its inputs being the result of the first AND gate and the other from the output of JK3.
The result fo this second AND gate goes into both the J and K inputs of JK4
The outputs of each JK represent the bits of the counter

Question 12

What is a good application of a counter?

Answer 12

> Frequency divider

Question 13

What is a frequency divider?

Answer 13

> This divides the frequency by 2^x

> This is good because we can make very accurate clocks using quartz crystals.

Question 14

How are quartz crystals divided with a frequency divider?

Answer 14

> They vibrate at 32,768Hz

> A frequency divider with 15 stages can produce a very accurate clock with a frequency of 1 Hz

Question 15

How is the clock frequency increased?

Answer 15

> An internal phase-lock loop is used to multiply the base frequency

Question 16

What is the issue with a high frequency clock counter?

Answer 16

A 32 bit counter is not enough for even a 10 MHz clock frequency and will not last long so a larger counter is needed

Question 17

What is a watchdog timer?

Answer 17

> This prevents a computer from becoming stuck and recover from a malfunction
This is a timer which counts clock cycles
When it overflows it resets the computer.
To stop it always resetting the computer program needs to continuously reset the clock to 0 (referred to as petting the dog)

Question 18

What is an important situation for a watchdog timer?

Answer 18

For flight essential computers in space probes

Question 19

What is a UART?

Answer 19

> Universal Asynchronous receiver transmitter

> Used to send data between two devices 1 bit at a time

Question 20

How is a UART synchronised and why?

Answer 20

> There is no clock speed between them
So both devices need to know the rate at which data is being transferred
Timers are used to send data between the devices at the correct times

Question 21

What is pulse width modulation?

Answer 21

> Turning on and off very quickly to control analogue devices
Used to digitally control the speed of motors

Question 22

What is the duty cycle?

Answer 22

The ratio of the time on and off for a PWM signal

Question 23

How is a duty cycle for PWM implemented?

Answer 23

> When the signal is high, a counter is used to count up to a number and turn off the signal.
Then it repeats over a standard amount of time

Question 24

What is used to control the times of a PWM signal?

Answer 24

The Match register

Question 25

What are the different times for a match register?

Answer 25

MR0 - How often the signal repeats

MR1…MRn - How long the signal is on for

Question 26

What is an RTC? What does it stand for?

Answer 26

> A real time clock

> A clock used to count the real time and date

Question 27

How is the RTC powered?

Answer 27

> It has a constant power supply even when the device is turned off
Uses a quartz crystal

Question 28

What is UNIX time?

Answer 28

> It is the number of seconds passed since the 1st January 1970
Uses a 32 bit counter

Question 29

What is the problem with UNIX time with a 32 bit counter?

Answer 29

> It will overflow in 2038 and so will not count from 1/01/1970
It is possible a 64 bit counter will be used instead