SCC.369 Coursework 1: Working with GPIO

Aim

Instructions for all subtasks

We will be looking at and testing your code with the help of some automated scripts, so it’s super important that you follow the following guidelines. If you do not, you will lose marks:

For each subtask, 20-30% of the marks will depend on code quality. The kinds of things we will be looking for include:

Remember to have fun . Use the labs to ask about anything you don’t understand!

Subtask 1, 20%: Display a binary number that counts up at 5Hz

Function prototype: void displayBinary(uint8_t value);

Set the bit pattern of a row of LEDs on the micro:bit to match the least significant 5 bits of the unsigned 8-bit value provided as a parameter. The least significant bit should be on the right when looking at the micro:bit with the USB cable pointing up. A ‘1’ in a bit position should turn the corresponding LED on, a ‘0’ should turn the LED off. You can use any row of LEDs on the micro:bit to show this 5 bit number, but only use one row – the LEDs on the other rows should not light up.

The first time displayBinary() is called it will need to initialise GPIOs. It’s good practice not to repeatedly re-initialise registers with the same value, so you could use a local static variable to record the first time displayBinary() is called so that subsequent calls don’t repeatedly initialise.

Write a function that causes the number on the row you chose above to count up in binary, one count at a time, starting at the value passed in. You should call your displayBinary() function from above. After reaching a displayed count of 0b11111 the counter should ‘keep going’, i.e. wrap around to 0b00000. The frequency of counting should be 5Hz, i.e. 5 counts per second or 200ms per count. Think about how you can test the frequency of counting; the stretch goal is to see if you can adjust it to be within 5% of the target. 

Subtask 2, 20%: Display a binary number that counts down/up with buttons A/B

Function prototype: void countWithButtonsBinary(uint8_t initialValue);

This function displays the initial count value passed in, using the displayBinary() function from ,Subtask 1, and updates the display with a new value when a micro:bit button is pressed. Button A should decrement the value by one count, and button B should increment it by one. To make this work well you will need to debounce the button inputs. The count should wrap around to 0b11111 when decremented below zero, and vice-versa. The count should only change on a button press, not on a button release, and it should not keep incrementing while a button is held down. ,Remember to use the relevant PIN_CNF[x] register to access all the settings you need.

Subtask 3, 25%: Measure and display an analogue voltage

For this subtask you will configure the GPIO connected to micro:bit pin P0 as an analogue input and read the voltage present on that pin. To test this you will need to apply a variable analogue voltage to that pin. You’ll need a breadboard, a micro:bit breakout adapter, a variable resistor and some jumper wires.

Wire up the ends of the variable resistor to power and ground, and connect the slider to P0. For this subtask, in addition to your code please submit a photo of your working ,micro:bit/breadboard setup in .jpg format for some easy marks! Please name it ST3.jpg.

Write a function to measure the magnitude of the analogue voltage on the large P0 pin of the micro:bit edge connector. There are many ways to configure the analogue-to-digital converter (ADC) on the nRF, but the important thing is that this function returns an 8-bit unsigned value where 0 represents an input of 0V and 255 represents an input of 3V (that the MCU is being powered from). Wire the variable resistor so that fully anticlockwise produces 0V on the wiper and fully clockwise 3V.

Function prototype: void displayVoltageBinary(void);

Write a function to repeatedly display in binary the magnitude of the analogue voltage measured on the large P0 pin. Use your displayBinary() function from Subtask 1 and make sure to display the five most significant bits of the sampled voltage so that the display reaches 0b00000 when the variable resistor is turned fully anticlockwise and 0b11111 when it’s turned fully clockwise.

Subtask 4, 25%: Drive an RGB LED

NB the week 3 lecture will explain aspects of this Subtask.

For this subtask you will connect an RGB LED to P1 (red), P8 (blue) and P9 (green) on the micro:bit edge connector, each via a current-limiting resistor. Use a 220R resistor for red and 100R for blue and green. The LED we are using is a common anode type.

Function prototype: void driveRGB(void);

For this subtask, in addition to your code please submit a photo of your working micro:bit/breadboard setup in .jpg format for some easy marks! Please name it ST4.jpg.

NB the week 3 lecture will explain aspects of this Subtask.

The final subtask has a lower weighting but is here to stretch you!

It’s like Subtask 2 but the display should count up by one count when you touch the golden micro:bit “face” logo above the LEDs. No need to worry about counting down for this subtask though.

Function prototype: void countWithTouchesBinary(uint8_t initialValue);

Mark Scheme