# 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