BT2I2C Bridge

BLE HID keyboard → Pico W → I2C → Host.

Connects to a Bluetooth keyboard and forwards keystrokes as an I2C slave using the standard i2c_puppet register protocol.

This firmware emulates a BBQ20-compatible I2C keyboard endpoint at address 0x1F. Use existing BBQ20 host support (for example Arduino/CircuitPython BBQ10KBD libraries and other i2c_puppet-compatible clients) on the host side.

Quick Start

cd build && cmake .. && make bt2i2c

Flash build/src/bt2i2c.uf2 to a Pico W. Connect GP4 (SDA) and GP5 (SCL) to your host's I2C bus with a common GND. The device appears at I2C address 0x1F. Use an existing BBQ10KBD library to poll REG_ID_KEY (0x04) for FIFO count and REG_ID_FIF (0x09) for key events (2 bytes: state + keycode).

Project Structure

src/bt_keyboard.c    BLE scan, pair, HID report → I2C FIFO
src/main.c           Init I2C slave + BT stack + run loop
src/fifo.c           Key event FIFO (31 entries)
src/reg.c            i2c_puppet register map
src/i2c_slave.c      I2C slave on i2c0 (GP4 SDA, GP5 SCL)
src/pins.h           Pin assignments

Protocol

The device implements a subset of the i2c_puppet register map:

Register	Addr	Size	Description
REG_ID_VER	0x01	1B	Firmware version
REG_ID_CFG	0x02	1B	Configuration bits
REG_ID_INT	0x03	1B	Interrupt status
REG_ID_KEY	0x04	1B	FIFO count (lower 5 bits)
REG_ID_FIF	0x09	2B	Key event: state + keycode
REG_ID_ADR	0x12	1B	I2C address (read/write, default 0x1F)

Key state values: 1=pressed, 2=held, 3=released. Keycodes are standard HID usage IDs. Write to a register by OR-ing 0x80 with its address.

I2C at 100kHz, address 0x1F. Internal pull-ups enabled (external 4.7kΩ recommended for longer wires).

See HARDWARE.md for wiring details.