Gatekeeper

Gatekeeper

A digital gate/trigger processor for Eurorack modular synthesizers. Gatekeeper has a two-state (0 or 5V) output driven by push button and CV input. The input response is configurable via menu settings.

This project is designed as a reference for hackers of all types. We assume hardware and coding knowledge, but those with a strong desire to learn and build will find this an approachable beginner hardware project.

Features

• Five Operating Modes:

  • Gate: Output stays high while input is held
  • Trigger: Rising edge triggers a fixed-duration pulse (1-50ms)
  • Toggle: Each press flips the output state
  • Divide: Clock divider - outputs every N rising edges (2-24)
  • Cycle: Internal clock generator

• Persistent Settings: Mode selection is saved to EEPROM and restored on power-up. Factory reset is available by holding both buttons for 3 seconds during startup.

• Debounced Input: Robust button state detection with rising and falling edge detection.

• Configuration Gestures:

  • Menu Enter: Hold Button A, then hold Button B (500ms each) to enter menu
  • Menu Exit: Hold Button A (500ms) to exit menu immediately
  • Mode Change: Hold Button A (500ms), then release to cycle modes
  • Menu Navigation: Tap Button A to change page, tap Button B to cycle values

• Hardware Abstraction Layer (HAL): Clean interface for hardware access, enabling comprehensive unit testing on the host machine.

• Unit Testing: 152 tests using the Unity framework verify functionality without hardware.

• Native Simulator: Interactive terminal UI or headless JSON output for development without hardware.

Hardware

Target: ATtiny85 @ 8 MHz (internal RC oscillator)

Pin Assignment:

LED X and Y are Neopixels connected to PB0.

Code Architecture

The firmware is organized into reusable library code (lib/) and application-specific code (src/), with headers alongside sources:

lib/                    src/
├── events/             ├── config/
├── fsm/                ├── core/
├── hardware/           ├── hardware/
├── input/              ├── modes/
├── output/             ├── output/
└── utility/            ├── app_init.c/.h
                        └── main.c

Key modules:

See docs/ARCHITECTURE.md for detailed documentation including data flow diagrams, EEPROM layout, and extension guides.

Build Instructions

Prerequisites

Firmware build:

• AVR-GCC toolchain (avr-gcc, avr-objcopy, avr-size, avr-strip)
• CMake 3.21+
• Optional: avrdude (flashing), bc + avr-nm (size analysis)

Test/Simulator build (no AVR toolchain needed):

• GCC (host compiler)
• CMake 3.21+

Install on Debian/Ubuntu:

# Tests only
sudo apt install build-essential cmake

# Firmware (minimal)
sudo apt install gcc-avr avr-libc cmake

# Firmware (full: build + flash + analysis)
sudo apt install gcc-avr avr-libc binutils-avr avrdude cmake bc

Quick Start

# Build firmware
cmake --preset firmware && cmake --build --preset firmware

# Build and run tests
cmake --preset tests && cmake --build --preset tests
ctest --preset tests

# Build and run simulator
cmake --preset sim && cmake --build --preset sim
./build/sim/sim/gatekeeper-sim

Native Simulator

The simulator is under construction.

Flashing

make flash        # Program firmware
make fuses        # Set fuse configuration
make read_fuses   # Verify fuses

Default programmer: stk500v2 on /dev/ttyACM0. Override with:

cmake -DPROGRAMMER=usbasp -DPROGRAMMER_PORT=/dev/ttyUSB0 ..

License

MIT