BLE-to-USB-HID Bridge for XIAO-nRF52840

Firmware that connects to a Corne wireless keyboard via Bluetooth and presents itself as a USB HID keyboard for use with Deskhop KVM.

Features

• BLE Central mode with HID over GATT Protocol (HOGP) client
• USB HID keyboard device (boot protocol, 8-byte reports)
• Passkey pairing support (displayed via USB serial console)
• Bond storage in flash for automatic reconnection
• Low latency: 7.5-15ms BLE interval, 1ms USB polling

Prerequisites

Install nRF Connect SDK

# macOS dependencies
brew install cmake ninja gperf python3 ccache qemu dtc wget

# Install west (use pipx if pip3 fails due to externally-managed-environment)
pipx install west
# or: pip3 install west

# Initialize SDK workspace
mkdir ~/ncs && cd ~/ncs
west init -m https://github.com/nrfconnect/sdk-nrf --mr v2.6.0
west update
west zephyr-export

# Zephyr SDK toolchain (for Apple Silicon use aarch64 variant)
cd ~
# For Intel Mac:
# wget https://github.com/zephyrproject-rtos/sdk-ng/releases/download/v0.16.5/zephyr-sdk-0.16.5_macos-x86_64.tar.xz
# For Apple Silicon:
wget https://github.com/zephyrproject-rtos/sdk-ng/releases/download/v0.16.5/zephyr-sdk-0.16.5_macos-aarch64.tar.xz
tar xf zephyr-sdk-*.tar.xz
cd zephyr-sdk-*
./setup.sh

# Add to ~/.zshrc
export ZEPHYR_BASE=~/ncs/zephyr

Build

cd ~/ncs
west build -b xiao_ble /path/to/ble-to-hid

Flash

1. Double-tap the reset button on XIAO-nRF52840 to enter bootloader mode
2. Copy build/zephyr/zephyr.uf2 to the USB drive that appears

Usage

1. Connect XIAO to your computer via USB
2. Open serial console: screen /dev/tty.usbmodem* 115200
3. Put your Corne keyboard in pairing mode
4. When prompted, enter the displayed 6-digit passkey on the Corne
5. Once paired, keystrokes from Corne will appear on the USB host

LED Status

• Blinking: Scanning for BLE devices
• Solid: Connected to keyboard
• Brief flash: Keystroke forwarded

Serial Commands

While connected to the serial console, you can use these commands:

• c - Clear all Bluetooth bonds (requires confirmation with y)

Project Structure

ble-to-hid/
├── CMakeLists.txt        # Build configuration
├── prj.conf              # Kconfig settings
├── app.overlay           # Devicetree overlay
└── src/
    ├── main.c            # Entry point
    ├── usb_hid.c/h       # USB HID keyboard
    ├── ble_central.c/h   # BLE scanning/connection
    ├── hogp_client.c/h   # HID over GATT client
    ├── pairing.c/h       # Passkey authentication
    └── hid_bridge.c/h    # BLE->USB forwarding

Troubleshooting

Device not found: Ensure Corne is in pairing mode and close enough.

Pairing fails: Clear bonds on both devices and try again. On the dongle, press c then y in the serial console to clear bonds.

Keys stuck: Disconnect and reconnect USB to reset HID state.

References

• nRF Connect SDK Central HIDS Sample
• Zephyr USB HID Keyboard Sample