For information about how to run the program, please visit the COMP2300 course website.

microbit-light-show

An engaging musical light show on the BBC micro:bit v2 (ANU COMP2300 assignment).

Design

Program

We develop an ARM Assembly program that produces a two-part light show with sound on micro:bit.

• Pt.1
  • Light
    • Small dots move over the board and transform into a crosshair.
    • Crosshair aims around and expands into spurts, transforming into a windmill.
    • The windmill spins clockwise and then anti-clockwise, ends as a beating heart.
  • Sound
    • Matching the blink of lights, James Bond Theme is played.
• Pt.2
  • Light
    • The Matrix-style falling strips replace the Bond-style blood drip scene.
    • Stripe lengths are generated randomly to make the light show ever-changing.
  • Sound
    • A specific byte beat music is played continuously.

Light

An LED can be enlightened by changing the value of its corresponding pins for the row and the column using the load-twiddle-store pattern [1].

Sound

• Pt.1 music consists of various music notes with durations, and each music note corresponds to a specific frequency value [2]. Such frequency values can be used with constant positive/negative amplitude values to produce audible square waves as music.

• Pt.2 byte beat music derives from arithmetic operations with an incrementing number in an infinite loop [3]. Essentially, it produces an audible sawtooth wave as music.

Challenges

• There is no built-in function for a random number generator in ARM Assembly.
• The RNG peripheral on the micro:bit is constrained in a range of 0-255.

Mathematics

The goal can still be achieved using the linear congruential generator (LCG); the equation is as below [4]

$$ a \cdot seed + c \mod m $$

$$ a = 75, c = 74, m = 2^{16} + 1 $$

Note that the result value generated by LCG is deterministic if the seed is defined.

We first produce a random value using RNG and then seed that value to LCG to create a truly random number that is not constrained to RNG's range.

Implementation

Project structure

src
 ┣ main.S      
 ┣ music.S     
 ┣ part-1.S     
 ┣ part-2.S    
 ┗ rand.S      

Control flow

The main function first initialises the micro:bit audio function by calling audio_init, then plays Pt.1 and Pt.2 of the light show in sequence by calling Pt_1 and Pt_2.

• In Pt_1, we pass the input data from memory to frame_n_music, where we scan an LED matrix column by column repeatedly and enlighten the rows that should be on by calling a set of set/clear-DIR/OUT functions. The duration of this process is adjusted roughly the same as the current music note duration. When it ends, we play the current music note by calling music_note.
• In Pt_2, we call generate_next_frame to generate the next frame of an image by shifting down the current frame and leaving the header row to randomness. Hence, when we call frame_n_bytebeat, the strips are random in length and look like falling, while exciting byte beat music is played.

Analysis

Validity

Our program imitates classic movies to implement an ever-changing light show with exciting sound effects and some randomness on the micro:bit, which lives up to our goal of an engaging light show.

Decisions

• We use highly modularised functions with high cohesion and low coupling; hence, they are easy to modify and understand.
• The LED status matrix and music frequency-duration table data are all stored in the .data section on memory, which saves lots of register usage.

Future work

• When playing music notes in Pt.1, LED lights are cut off. This can be improved by merging the loops for both light and sound.
• Although the falling strips in Pt.2 are generative, the byte beat is always in the same loop. The sound can be randomised too.

References

[1]"Week 4: blinky", Computer Organisation and Program Execution, 2022. [Online]. Available: https://comp.anu.edu.au/courses/comp2300/labs/04-blinky/. [Accessed: 03- Apr- 2022].

[2]"Frequencies of Musical Notes, A4 = 440 Hz", Pages.mtu.edu, 2022. [Online]. Available: https://pages.mtu.edu/~suits/notefreqs.html. [Accessed: 03- Apr- 2022].

[3]V. Heikkilä, "Discovering novel computer music techniques by exploring the space of short computer programs", arXiv, 2011. Available: https://arxiv.org/abs/1112.1368. [Accessed 3 April 2022].

[4]"Linear congruential generator - Wikipedia", En.wikipedia.org, 2022. [Online]. Available: https://en.wikipedia.org/wiki/Linear_congruential_generator#Parameters_in_common_use. [Accessed: 03- Apr- 2022].