Project 2: UART Driver from Scratch (Raspberry Pi Pico)

A serial driver in pure assembly that lets you send “Hello, World!” to your computer over USB-serial, with both polling and interrupt-driven modes.

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 / Drivers
Tooling Raspberry Pi Pico
Prerequisites Project 1 completed, basic serial terminal usage

What You Will Build

A serial driver in pure assembly that lets you send “Hello, World!” to your computer over USB-serial, with both polling and interrupt-driven modes.

Why It Matters

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

Core Challenges

  • Configuring UART peripheral registers (baud rate divisors, word length, parity)
  • Implementing polling-based TX/RX by checking status flags
  • Setting up the NVIC (Nested Vectored Interrupt Controller) for UART interrupts
  • Writing an interrupt service routine (ISR) that preserves registers correctly
  • Handling the RP2040’s dual-core memory considerations

Key Concepts

  • UART Protocol & Registers: RP2040 Datasheet Chapter 4 (UART) - Raspberry Pi Foundation
  • ARM Procedure Call Standard: “Modern Arm Assembly Language Programming” Ch. 2 - Daniel Kusswurm
  • NVIC & Interrupt Handling: “Making Embedded Systems, 2nd Edition” Ch. 4 - Elecia White
  • Status Register Polling: STM32 Cortex M0 Bare Metal Tutorial - Martin Hubacek

Real-World Outcome

$ screen /dev/ttyACM0 115200
Hello from bare-metal assembly!
Character count: 35

Implementation Guide

  1. Reproduce the simplest happy-path scenario.
  2. Build the smallest working version of the core feature.
  3. Add input validation and error handling.
  4. Add instrumentation/logging to confirm behavior.
  5. 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
  • “Making Embedded Systems” by Elecia White