Impulse CEC

A USB CEC dongle for the Seeed XIAO ESP32-C6 and XIAO RP2350. It bit-bangs the HDMI CEC bus and exposes it over USB serial using the open P8 adapter protocol, making it work out of the box with libCEC (cec-client) and the Linux kernel pulse8-cec driver (inputattach).

Note: This project is not affiliated with or endorsed by Pulse-Eight Ltd. "Pulse-Eight" is a trademark of Pulse-Eight Ltd. The P8 serial protocol is an open, documented wire format independently implemented here.

Supports two boards:

	Seeed XIAO ESP32-C6	Seeed XIAO RP2350
CEC GPIO	GPIO21 (pad D3)	GPIO5 (pad D3)
USB	Serial/JTAG — fixed VID:PID 0x303A:0x1001	TinyUSB — 0x2E8A:0x1000
libCEC auto-detect	No (specify port with -t p)	No (PID not yet registered)
Settings storage	NVS	Flash (last sector)
USB remote wakeup	No	Yes
Build system	ESP-IDF v6.x	Pico SDK

Hardware

Wiring (both boards — same physical pin):

XIAO pad	ESP32-C6 GPIO	RP2350 GPIO	Connect to
D3	GPIO21	GPIO5	HDMI CEC pin
GND	—	—	HDMI CEC GND

The firmware enables an internal pull-up on the CEC line (~47kΩ on ESP32-C6, configurable on RP2350). No external pull-up is required for direct connections or short cables. For long cable runs, an optional 27kΩ resistor from CEC to 3.3V (paralleling to ~18kΩ) improves signal integrity.

Building

# ESP32-C6 (source ESP-IDF first)
. /path/to/esp-idf/export.sh
make esp32

# RP2350
make rp2350 PICO_SDK_PATH=~/pico-sdk

# Both
make all PICO_SDK_PATH=~/pico-sdk

Run make help for all available targets (flash, monitor, clean, …).

Toolchain setup

ESP32-C6: requires ESP-IDF v6.x. Source export.sh in each new shell before building.

RP2350: requires the Pico SDK and CMake ≥ 3.13 + an ARM toolchain. Set PICO_SDK_PATH in the environment or on the make command line.

Flashing

ESP32-C6:

make flash-esp32 PORT=/dev/ttyACM0

RP2350:

make flash-rp2350       # uses picotool if available

Or manually: hold BOOTSEL, plug in the board, then copy rp2350/build/impulse-cec.uf2 to the RPI-RP2 drive.

Usage

libCEC / cec-client

cec-client -t p /dev/ttyACM0

The -t p flag selects the P8 adapter protocol. Specify the port explicitly — neither board's VID:PID triggers libCEC auto-detection yet. On macOS use /dev/cu.usbmodem*; on Windows use COMx.

Linux kernel driver (inputattach)

The kernel pulse8-cec driver speaks the same P8 protocol and exposes the dongle as a standard Linux CEC device. TV remote button presses generate input events; CEC commands can be sent with cec-ctl.

One-time system setup

echo "options pulse8-cec persistent_config=1" | sudo tee /etc/modprobe.d/pulse8-cec.conf
sudo modprobe -r pulse8-cec && sudo modprobe pulse8-cec

persistent_config=1 is required for the driver to automatically claim a logical address on startup.

Attach the dongle

sudo inputattach -p8 /dev/ttyACM0

The driver sets PA=1.0.0.0, claims logical address 4 (Playback Device), and enables the ACK mask.

Alternative: udev rule (no modprobe change needed)

# /etc/udev/rules.d/99-cec-pa.rules
SUBSYSTEM=="cec", ACTION=="add", RUN+="/usr/bin/cec-ctl -d /dev/%k -p 1.0.0.0"

Wake-on-CEC (RP2350 only)

The RP2350 firmware can wake the PC from suspend when it receives a CEC power-on command or a Set Stream Path addressed to the dongle's physical address. For this to work, Linux must enable USB remote wakeup for the device.

Install the included udev rule to enable it automatically:

sudo cp udev/99-impulse-cec.rules /etc/udev/rules.d/
sudo udevadm control --reload-rules && sudo udevadm trigger

Verify it took effect:

cat /sys/bus/usb/devices/$(ls /sys/bus/usb/drivers/usb/ | \
  xargs -I{} sh -c 'cat /sys/bus/usb/drivers/usb/{}/idVendor 2>/dev/null | grep -q 2e8a && echo {}')/power/wakeup
# should print: enabled

Architecture

Both targets share the same protocol implementation (shared/p8_protocol.c) through a thin platform abstraction layer (shared/platform.h). The CEC bus driver is platform-specific.

USB (CDC ACM)
      │
      ▼
p8_protocol.c      shared — P8 serial protocol parser/encoder
      │
      ▼
cec_bus.c          platform-specific — open-drain bit-bang CEC driver
      │
      ▼
HDMI CEC line

ESP32-C6: GPIO ANYEDGE ISR → edge-event queue → FreeRTOS decoder task. TX uses esp_timer ISR callbacks to chain waveform phases without busy-waiting.

RP2350: GPIO IRQ stores edges in a ring buffer. Main loop drains it via cec_rx_process(). TX uses hardware alarms for phase transitions. TX results are deferred to the main loop so TinyUSB writes never happen from IRQ context.

Project Structure

impulse-cec/
├── Makefile
├── udev/
│   └── 99-impulse-cec.rules  — enables USB remote wakeup on Linux
├── shared/                 — portable sources (both targets)
│   ├── cec_bus.h           — CEC driver interface + portability typedefs
│   ├── platform.h          — platform abstraction (USB write, KV store, mutex)
│   ├── p8_protocol.h
│   └── p8_protocol.c
├── esp32/                  — ESP-IDF project (cd here for idf.py)
│   ├── CMakeLists.txt
│   ├── sdkconfig.defaults
│   └── main/
│       ├── CMakeLists.txt
│       ├── cec_bus.c       — ESP32-C6 CEC driver (GPIO ISR + FreeRTOS tasks)
│       └── main.c          — app_main: NVS init, USB Serial/JTAG, tasks
└── rp2350/                 — Pico SDK project (cd here for cmake)
    ├── CMakeLists.txt
    ├── tusb_config.h
    ├── cec_bus.c           — RP2350 CEC driver (GPIO IRQ + hardware alarms)
    ├── main.c              — main loop: tud_task + cec_rx_process + cec_bus_tick
    ├── flash_kv.h / .c     — settings in last flash sector
    └── usb_descriptors.c   — TinyUSB CDC descriptors (VID=0x2E8A PID=0x1000)

CEC Timing

Symbol	Value
Start LOW	3700 µs
Start HIGH	800 µs
Bit '0' LOW	1500 µs
Bit '1' LOW	600 µs
Bit period	2400 µs
Sample point	1050 µs from falling edge