SN-L00/firmware

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

1. Hold BOOTSEL button on RP2040-Zero
2. Connect USB cable (or press reset while holding BOOTSEL)
3. Drag sn_l00.uf2 to the RPI-RP2 drive
4. 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:

1. Input buffer propagation delay (~10ns, negligible)
2. GPIO read latency (~10ns, negligible)
3. 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