# SN-L00 Design Document

## Overview

The SN-L00 is a 6 HP Eurorack module that measures round-trip latency in audio/CV signal chains.

## Block Diagram

```
+12V ──┬── LDO ──── 3.3V ──── RP2040-Zero ──── OLED Display
       │                          │    │
       │                          │    └── I2C (SDA/SCL)
       │                          │
GND ───┴──────────────────────────┤
                                  │
              ┌───────────────────┼───────────────────┐
              │                   │                   │
         TRIG OUT            RETURN IN            BUTTON
              │                   │                   │
         74LVC1G17            MCP6001            Tactile
         (buffer)            (buffer)             switch
              │                   │
              ▼                   ▼
         3.5mm Jack          3.5mm Jack
```

## Signal Flow

### Trigger Output
1. RP2040 GPIO generates 5ms pulse (configurable)
2. 74LVC1G17 Schmitt buffer cleans edge, provides drive strength
3. Output: 0-3.3V trigger compatible with Eurorack gate inputs

### Return Input
1. Eurorack signal (0-5V typical, can be ±5V audio)
2. Protection diodes clamp to 3.3V rail
3. MCP6001 op-amp buffers signal
4. Voltage divider if needed for >3.3V signals
5. RP2040 GPIO reads with internal Schmitt trigger

### Measurement
1. RP2040 starts timer on trigger output rising edge
2. Timer stops on return input rising edge
3. Latency = timer count × timer resolution
4. Display updates with result in ms

## Specifications

| Parameter | Value |
|-----------|-------|
| Width | 6 HP (30.00mm) |
| Depth | <25mm (skiff-friendly) |
| Power +12V | ~20mA |
| Power -12V | 0mA (not used) |
| Trigger output | 0-3.3V, 5ms default |
| Input threshold | ~1.5V (adjustable in firmware) |
| Resolution | <10μs (RP2040 @ 125MHz) |
| Range | 0.01ms - 999ms |
| Display | 128x32 OLED, I2C |

## Operating Modes

### Mode 1: Single Shot
- Press button to trigger measurement
- Display shows result until next measurement
- LED blinks on trigger

### Mode 2: Continuous
- Hold button 2s to enter continuous mode
- Triggers every 500ms, displays live reading
- Shows min/max/average
- Press button to exit

### Mode 3: Statistics
- After measurements, short press cycles through:
  - Last reading
  - Minimum
  - Maximum
  - Average (last 10)

## Panel Layout (6 HP)

```
┌─────────────────────┐
│      SN-L00         │
│                     │
│   ┌───────────┐     │
│   │   OLED    │     │
│   │  12.4 ms  │     │
│   └───────────┘     │
│                     │
│       [BTN]         │
│                     │
│   ○ OUT    ○ IN     │
│   TRIG    RETURN    │
│                     │
│        ● PWR        │
└─────────────────────┘
```

## Firmware Architecture

```
main.c
├── init_gpio()          # Configure pins
├── init_i2c()           # OLED communication
├── init_timer()         # High-resolution timer
│
├── trigger_pulse()      # Generate output pulse
├── wait_for_return()    # Monitor input with timeout
├── calculate_latency()  # Timer math
│
├── display_update()     # OLED rendering
├── button_handler()     # Mode switching
└── main_loop()          # State machine
```

## GPIO Assignments (RP2040-Zero)

| GPIO | Function | Notes |
|------|----------|-------|
| GP0 | I2C SDA | OLED |
| GP1 | I2C SCL | OLED |
| GP2 | Trigger Out | Via 74LVC1G17 |
| GP3 | Return In | Via MCP6001 |
| GP4 | Button | Internal pullup |
| GP5 | Power LED | Optional |

## Future Expansion Ideas

- CV output proportional to latency
- MIDI output (latency as CC)
- USB logging to computer
- Multiple trigger patterns (burst, random)
- Frequency response mode (sweep test)