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florian.berthold 64f9e34fc3 Add firmware skeleton for RP2040

Complete working firmware including:
- CMakeLists.txt for Pico SDK build
- SSD1306 OLED driver (128x32, I2C)
- High-resolution latency measurement using hardware timer
- Debounced button with short/long press detection
- Three modes: Single, Continuous, Stats
- USB serial debugging output

Includes 8x8 font with numbers and letters for display.

2026-01-23 03:33:35 +01:00

2.9 KiB

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SN-L00 Firmware

RP2040-based firmware for the SN-L00 Eurorack latency tester.

Requirements

Raspberry Pi Pico SDK v1.5+
CMake 3.13+
ARM GCC toolchain (arm-none-eabi-gcc)

Building

Linux/macOS

# Set SDK path (add to .bashrc/.zshrc for persistence)
export PICO_SDK_PATH=/path/to/pico-sdk

# Build
cd firmware
mkdir build && cd build
cmake ..
make

Output files

After building:

sn_l00.uf2 - Flash via USB (drag to RPI-RP2 drive)
sn_l00.elf - For debugging with SWD

Flashing

Hold BOOTSEL button on RP2040-Zero
Connect USB cable (or press reset while holding BOOTSEL)
Drag sn_l00.uf2 to the RPI-RP2 drive
Module reboots automatically

Usage

Modes

Mode	Entry	Display	Behavior
Single	Default / Long press from Continuous	`SINGLE`	Press button to measure once
Continuous	Long press from Single	`CONT`	Auto-measures every 500ms
Stats	Short press from Continuous	Min/Max/Avg	Shows measurement statistics

Controls

Short press: Trigger measurement (Single mode) / Exit to Stats (Continuous)
Long press (>500ms): Toggle between Single and Continuous modes
Any press in Stats: Return to Single mode, reset statistics

USB Serial

The firmware exposes USB CDC serial for debugging:

Baud: 115200 (or any, it's USB CDC)
Shows measurement results and mode changes

# Linux
screen /dev/ttyACM0 115200

# macOS
screen /dev/tty.usbmodem* 115200

GPIO Pinout

GPIO	Function	Direction
GP0	I2C SDA (OLED)	Bidir
GP1	I2C SCL (OLED)	Output
GP2	Trigger Output	Output
GP3	Return Input	Input
GP4	Button	Input (pull-up)

Code Structure

firmware/
├── CMakeLists.txt      # Build configuration
├── include/
│   ├── config.h        # Pin definitions, timing constants
│   ├── display.h       # OLED display interface
│   ├── latency.h       # Measurement functions
│   └── button.h        # Button handling
└── src/
    ├── main.c          # Application entry, state machine
    ├── display.c       # SSD1306 OLED driver
    ├── latency.c       # Timer-based latency measurement
    └── button.c        # Debounced button with long press

Measurement Accuracy

Timer resolution: ~1µs (RP2040 @ 125MHz)
Display resolution: 0.01ms (10µs)
Polling loop: ~1-2µs per iteration
Practical accuracy: ±10µs

The main latency contributors are:

Input buffer propagation delay (~10ns, negligible)
GPIO read latency (~10ns, negligible)
Polling loop timing (~1-2µs)

Future Improvements

Configurable trigger pulse width
Sample rate display (in samples @ 44.1/48/96kHz)
CV output proportional to latency
Threshold adjustment via long button hold
EEPROM settings storage

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