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
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+# SN-L00 Firmware
+
+RP2040-based firmware for the SN-L00 Eurorack latency tester.
+
+## Requirements
+
+- [Raspberry Pi Pico SDK](https://github.com/raspberrypi/pico-sdk) v1.5+
+- CMake 3.13+
+- ARM GCC toolchain (`arm-none-eabi-gcc`)
+
+## Building
+
+### Linux/macOS
+
+```bash
+# 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
+
+```bash
+# 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