BrickInterface

BrickInterface is a modern USB-C board that bridges vintage LEGO electronics where no official solution exists. As of now, it supports the LEGO Interface A and LEGO Power Functions.

The host talks to the board with a framed binary protocol over USB-CDC, and the board appears as a standard serial port.

Repository layout

firmware/BrickInterface/   CH552T firmware (ch55xduino / SDCC)
  *.ino, *.c, *.h          dispatcher, Interface A, IR engine, packet parser
  tests/                   host-side unit tests (g++)
kicad/                     KiCad hardware design (schematic, PCB, gerbers, BOM)
upload.sh                  flash helper (wchisp)

Hardware

• MCU: CH552T (TSSOP-20), internal 24 MHz RC oscillator (no crystal)
• USB: USB-C, native full-speed device. Enumerates as USB CDC ACM
• Interface A: 20-pin IDC to the 9750
• IR: Two IR LEDs in parallel with different beam angles: a narrow TSAL6100 (±10°) for range and a wider TSAL6200 (±17°) for off-axis coverage

Firmware

Built with ch55xduino (SDCC toolchain, C). Open firmware/BrickInterface/BrickInterface.ino in the Arduino IDE with the CH552 board package installed, select USB Settings > Default CDC, and compile.

Flashing

The board exposes the WCH ROM bootloader (USB VID:PID 4348:55E0) for flashing.

To enter the bootloader, send the ENTER_BOOTLOADER command over the serial port, or power-cycle while holding the BOOT button (SW1) to pull D+ high.

(Note: On board revisions v1.0.4 or lower, SW1 was incorrectly wired to the 5V rail instead of 3.3V. Resistor R9 needs to be removed to pull D+ down, however this breaks SW1. To enter BOOT, bridge R9's location (ie. with tweezers)).

Tests

Pure-logic and protocol-encoder code is unit-tested on the host (no CH552 required):

cd firmware/BrickInterface/tests
make run

Protocol

The host drives the board with a framed binary protocol on the USB-CDC serial port.

0xAA | LEN | SEQ | CMD | PAYLOAD… | CHK

Two commands, shown as hex on the wire:

AA 05 10 10 05 FF FF 00      # Interface A: outputs 0 & 2 full on
AA 06 20 20 00 01 17 00 10   # Power Functions: channel 1, output B, forward full

The board enumerates with the ch55xduino CDC identity (VID 0x1209, PID 0xC550). Use the GET_CAPABILITIES handshake to verify which hardware is supported (BrickInterface v1.0.5 and below reports 0x0037).

See PROTOCOL.md for the full wire protocol: frame format, commands, replies, error codes, and worked examples.

Status

• Interface A (6 PWM outputs, 2 counting inputs) — Working
• Power Functions IR — Working
• Legacy IR / RCX IR — Early implementation, however it's proven unreliable. Not recommended for use yet.

License

This project is dual-licensed by component:

• Software (the firmware in firmware/) is licensed under the Apache License, Version 2.0. See LICENSE.
• Hardware (the KiCad design, gerbers, and BOM in kicad/) is licensed under the CERN Open Hardware Licence Version 2 – Permissive (CERN-OHL-P v2). See kicad/LICENSE.

Both are permissive licenses with a patent grant. Copyright 2024–2026 Nathan Kellenicki.