Twenty Three Hundred Learn Computer Systems by Hacking Hardware
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Week 9: inputs and outputs

This lab includes in-lab assessment for the Assignment 2 Pre-Submission task. Read about it how that works on the assignment presubmission page (link).

Introduction

Last week you worked on understanding the fundamentals of interrupts and using the two key types of interrupt you need in COMP2300: timer and GPIO interrupts.

This week you will combine this knowledge with what you learned in previous labs and assessments to create a real interactive application—now things are really getting interesting!

If you haven’t finished the tasks in the previous lab, you should probably look at these before you attempt this week’s tasks.

The tasks this week are directly related to assignment 2. Make sure you try them and get help from your tutors if you are stuck!

Task 1: QuickClick

In the first task, your job is to take your new knowledge of interrupts and make a game called QuickClick. It’s a simple game:

  1. you blink an LED on your microbit for a short time every 5 seconds

  2. the player’s goal is then to press a button when the LED is on

  3. if you get the timing right (i.e. the LED is on when the button is pressed) a different LED comes on.

  4. each time you get it right, the LED blink duration gets shorter (so that it’s harder to get the timing right for the next round).

For this task, you can take advantage of your ability to enable & disable different interrupts in software to make it easy to implement the “is the LED on? if so, then clicking the button will turn on the other LED” logic:

  1. perform all the configurations steps necessary (including defining the handler function) to use button A as an input device

  2. in your SysTick_Handler:

    • enable the GPIOTE interrupt by setting the bit in the interrupt set enable register NVIC_ISER0 (address: 0xE000E100) The bit you are looking to set can be found in the ID column of the interrupt vector table in section 4.2.4 “Instantiation” of the microbit reference manual

    • You may also want to clear both GPIOTE and NVIC interrupt pending bits before enabling the GPIOTE interrupt. Otherwise the pending interrupt will trigger when you enable it.

    • blink the red LED in a blocking fashion (i.e. use a delay, so that the red LED goes on and then off again before the SysTick_Handler exits)

    • before SysTick_Handler exits, disable the GPIOTE interrupt by setting the bit in the interrupt clear enable register NVIC_ICER0 (address: 0xE000E180)

There’s one more “gotcha” to be aware of when dealing with the clear enable and clear pending NVIC control registers (e.g. NVIC_ICER0 or NVIC_ICPR0). As described above, to disable an interrupt (ICER) or clear a pending interrupt (ICPR) you write a 1 to the corresponding bit (e.g. to disable the interrupt in position 6 of the NVIC you write a 1 to the 7th bit from the right in NVIC_ICER0).

However, you might have noticed something if you were reading Sections B3.4.5 (p684) and B3.4.7 (p685) the ARM reference manual really closely. In the description for those registers it says:

1: On reads, interrupt enabled

which means that if an interrupt is enabled, then a read from that (memory-mapped) register will show the corresponding bit as 1.

This is a problem for the load-twiddle-store pattern, because the point of the load twiddle store is to leave all the bits unchanged except for the one you’re interested in. However, this means that all of the currently enabled interrupts (whose bits will read as 1 in the load phase) will be disabled when you write the bits back in the store phase—which (almost certainly) isn’t what you want!

Again, here’s an example: say there are 3 interrupts currently enabled, then a load from the corresponding register would have 1s in those three positions, and 0s elsewhere. If you load/twiddle/store the value, then all three of those interrupts would be cleared by the store operation.

This means that for the clear enable/pending registers you should just write a 1 for the particular interrupt you’re interested in, and a 0 in all the other bits.

Can you see how you can use this technique to temporarily enable the button interrupt in the SysTick interrupt handler so that the button will only work when the LED is on?

Implement the QuickClick game following the steps above (you can use as much of the startup code provided earlier as you like). Commit and push your program to GitLab.

Task 2: Quick Click Data Structure

So far, the quick click game doesn’t show you much about your current “score”.

Use your knowledge of data structures to create a way to record the player’s current score and display it on the LED screen somehow.

For example, you could use a record to store the player’s current number of successful clicks, and their reaction time. After each successful or unsuccessful click, the player’s score (out of 5) could be displayed as a number of LEDs lit in one column.

There are lots of ways to do this, but the important thing is to find a way to incorporate buttons (inputs), LEDs (outputs), and memory in an interactive game.

Task 3: Click vs Click

The microbit has two buttons, so can you make a two player version of quick click?

A simple way to do this would be to have two independent games runing on either side of the microbit. You’ll have to be careful about using different channels in the GPIOTE module, one for each button, and enabling, checking, and clearing them independently.

A more complex (and maybe more fun) option would be to arrange so that only one player can win each click (the first one to have their button press registered).

You could use your score storage more engagingly by removing a point from a player that loses each round, as well as adding a point to the winner.

Extra Tasks

No Interrupt Click

Can you re-implement the QuickClick game without interrupts?

Random Number Generator QuickClick

Can you turn on the microbit’s random number generator (RNG) and use it so that LEDs blink randomly, rather than at regular intervals? Hint: Section 6.19 of the nRF52833 Product Specification is the place to find the configuration steps required to get the RNG working—it’s not too difficult.

QuickClick Byte Beats

Can you add sound to the quickclick game? You could start by playing a small “victory” sound when a player wins a round, or a starting squeak when the LED lights up for each game.

The easiest “sounds” to make are the little byte beats we tried out in week 5, but you’ll have to experiment with how to make these sounds start and stop

Hint: you can’t just not send new samples and expect the sound to be silent, you will have to send “silent samples” to turn off the sound.