Project 4: PIO Assembly for Custom Protocol (Raspberry Pi Pico)

Use the RP2040’s unique PIO (Programmable I/O) state machines to implement the WS2812B (NeoPixel) LED protocol in PIO assembly—a completely different assembly language than ARM!

Quick Reference

Attribute	Value
Primary Language	See main guide
Alternative Languages	N/A
Difficulty	Level 4: Expert
Time Estimate	1-2 weeks
Knowledge Area	Embedded Systems / Timing
Tooling	Raspberry Pi Pico (PIO)
Prerequisites	Projects 1-2 completed, basic understanding of timing diagrams

What You Will Build

Use the RP2040’s unique PIO (Programmable I/O) state machines to implement the WS2812B (NeoPixel) LED protocol in PIO assembly—a completely different assembly language than ARM!

Why It Matters

This project builds core skills that appear repeatedly in real-world systems and tooling.

Core Challenges

Learning PIO assembly’s unique instruction set (just 9 instructions!)
Understanding side-set and delay for precise timing
Writing ARM assembly to load PIO programs and configure state machines
Generating precisely-timed signals (WS2812B needs 800kHz timing)
Coordinating between ARM code and PIO execution

Key Concepts

PIO Architecture: RP2040 Datasheet Chapter 3 (PIO) - Raspberry Pi Foundation
WS2812B Protocol Timing: “RP2040 Assembly Language Programming” Ch. 13-15 - Stephen Smith
State Machine Concepts: DigiKey - How to Use PIO
ARM/PIO Integration: Raspberry Pi Pico Assembly Programming - Codalogic

Real-World Outcome

   ; Simplified (not complete):
   .wrap_target
       out x, 1        ; Get one bit from OSR into x
       jmp !x, send_0  ; If bit is 0, jump to send_0
   send_1:
       set pins, 1 [T1-1]  ; High for T1 cycles
       set pins, 0 [T2-1]  ; Low for T2 cycles
       jmp .wrap_target
   send_0:
       set pins, 1 [T3-1]  ; High for T3 cycles
       set pins, 0 [T4-1]  ; Low for T4 cycles
   .wrap

Implementation Guide

Reproduce the simplest happy-path scenario.
Build the smallest working version of the core feature.
Add input validation and error handling.
Add instrumentation/logging to confirm behavior.
Refactor into clean modules with tests.

Milestones

Milestone 1: Minimal working program that runs end-to-end.
Milestone 2: Correct outputs for typical inputs.
Milestone 3: Robust handling of edge cases.
Milestone 4: Clean structure and documented usage.

Validation Checklist

Output matches the real-world outcome example
Handles invalid inputs safely
Provides clear errors and exit codes
Repeatable results across runs

References

Main guide: ARM_ASSEMBLY_LEARNING_PROJECTS.md
“RP2040 Assembly Language Programming” by Stephen Smith