Project 7: VGA Display via PIO

Generate VGA sync and pixel data using PIO + DMA to drive a classic monitor.

Quick Reference

Attribute Value
Difficulty Level 4: Expert
Time Estimate 1-2 weeks
Main Programming Language C + PIO
Alternative Programming Languages Rust
Coolness Level Level 5: “I made video signals from scratch”
Business Potential 2. Educational / retro display projects
Prerequisites PIO timing, DMA, digital video basics
Key Topics VGA timing, PIO state machines, DMA streaming

1. Learning Objectives

  1. Generate HSYNC/VSYNC with correct timing.
  2. Stream pixel data with a stable pixel clock.
  3. Build a framebuffer and render simple graphics.
  4. Measure timing correctness with a scope.
  5. Handle DMA underruns gracefully.

2. All Theory Needed (Per-Concept Breakdown)

2.1 VGA Timing (HSYNC/VSYNC, Porch, Active)

Fundamentals

VGA is an analog video standard defined by precise timing for horizontal and vertical sync pulses. A frame is composed of visible pixels plus front/back porch intervals.

Deep Dive into the concept

For 640x480@60Hz, the pixel clock is 25.175 MHz. Each line contains 800 pixel times: 640 visible + 16 front porch + 96 sync + 48 back porch. The frame is 525 lines. Your PIO program must generate HSYNC and VSYNC with the correct durations, and align pixel output to the active region. If sync timing is off, the monitor will not lock.

Key insights

  • VGA is a deterministic timing problem; accuracy matters more than complexity.

2.2 Pixel Clock Generation with PIO and DMA

Fundamentals

PIO can generate a pixel clock and shift out pixel data. DMA feeds pixel data at a fixed rate to keep the pipeline full.

Deep Dive into the concept

A typical approach uses one PIO state machine for sync signals and another for pixel data. The pixel state machine shifts out bits at the pixel clock rate; DMA fills its FIFO with scanline data. If DMA stalls, the FIFO underflows and you get visual artifacts or loss of sync.

Key insights

  • The DMA/PIO pipeline is only as stable as its slowest stage.

2.3 Framebuffer Memory and Bandwidth

Fundamentals

A framebuffer stores pixel data in memory; bandwidth determines how many pixels you can output per frame.

Deep Dive into the concept

At 640x480, even 1 bit per pixel requires ~38 KB per frame. Higher color depth increases bandwidth and RAM requirements. The RP2040 has limited SRAM, so you may use line buffers instead of a full framebuffer. A double‑buffered line approach allows rendering while outputting the current line.

Key insights

  • Resolution and color depth are bandwidth tradeoffs.

3. Project Specification

3.1 What You Will Build

A VGA signal generator that outputs a stable 640x480 (or lower) display with simple graphics (checkerboard, text, or sprite).

3.2 Functional Requirements

  1. Correct HSYNC/VSYNC timing.
  2. Pixel output aligned to active region.
  3. DMA‑fed scanline pipeline.
  4. At least one demo pattern.

3.3 Non-Functional Requirements

  • Performance: stable display for 5 minutes.
  • Reliability: no sync loss during demo.

3.7 Real World Outcome

A VGA monitor locks to your signal and displays a stable test pattern.

4. Solution Architecture

4.1 High-Level Design

PIO SM0: HSYNC/VSYNC
PIO SM1: Pixel data
DMA -> Pixel FIFO

5. Implementation Guide

5.10 Implementation Phases

  • Phase 1: Sync generator
  • Phase 2: Pixel output with test pattern
  • Phase 3: DMA streaming with line buffers

6. Testing Strategy

  • Measure HSYNC/VSYNC pulse widths
  • Display test patterns

7. Common Pitfalls & Debugging

  • Incorrect porch values
  • DMA underrun causing tearing

8. Extensions & Challenges

  • Add simple text rendering
  • Add color output with resistor DAC

9. Submission / Completion Criteria

Minimum Viable Completion:

  • Stable sync and visible test pattern.

Full Completion:

  • 640x480 with line-buffered rendering.

Excellence:

  • Animated graphics without tearing.