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Pico-CEC

A Raspberry Pi Pico based project to bridge HDMI CEC (Consumer Electronics Control) and USB HID keyboard control (especially for use with Kodi).

Fully assembled Pico-CEC.

Motivation

Micro/mini desktops are plentiful as second hand, budget friendly media players, especially when installed with Kodi (eg. LibreELEC). However, many of these devices do not support HDMI-CEC and require the user to use additional peripherals (eg. wireless keyboard, game controller).

In this project we use a Pico to both:

  • handle the CEC protocol on the HDMI port
  • adapt the user control messages into USB keyboard inputs

What Works

  • HDMI CEC frame send and receive
  • EDID parsing to determine HDMI physical address
  • LibreELEC recognises Pico-CEC as an USB HID keyboard
  • HDMI CEC basic user control messages are properly mapped to Kodi shortcuts, including:
    • navigations arrows
    • select
    • back
    • play
    • pause
    • numbers 0-9

Cloning

To avoid cloning unneeded code, clone like this:

git -c submodule.active="lib/tinyusb" -c submodule.active=":(exclude,glob){lib,hw}/*" clone --recurse-submodules

Alternatively, clone everything in pico-sdk and tinyUSB:

git clone --recurse-submodules

Building

This project uses the 'normal' CMake based build. To build the default image:

$ git clone <blah blah as above>
$ cd pico-cec
$ mkdir build && cd build
$ cmake ..
$ make

Customising the Build

The CMake project supports three options:

  • PICO_BOARD: specify variant of Pico board, defaults to Seeed XIAO RP2040
  • CEC_PIN: specify GPIO pin for HDMI CEC, defaults to GPIO3

Example invocation to specify:

  • use Raspberry Pi Pico development board
  • use GPIO pin 11
$ cmake -DPICO_BOARD=pico -DCEC_PIN=11 ..
$ make

Installing

Assuming a successful build, the build directory will contain pico-cec.uf2, this can be written to the Pico as per normal:

  • connect the Pico to computer via USB cable
  • reset the Pico by holding 'Boot' and pressing 'Reset'
    • Pico now presents as a USB mass storage device
  • copy pico-cec.uf2 to the Pico
  • disconnect

Real World Usage

This is currently working with:

  • a Sharp 60" TV (physical address 0x1000)
    • directly connected to TV HDMI input 1
  • through a Denon AVR connected to the Sharp TV (physical address 0x1100)
    • Pico-CEC connected to Denon AVR HDMI input 1
    • Denon AVR connected to TV HDMI input 1
  • a Sony 60" TV (physical address 0x1000)
    • directly connected to TV HDMI input 1

Design

Hardware

The hardware connections are extremely simple. Both HDMI CEC and the Pico are 3.3V obviating the need for level shifters. The DDC bus (for EDID) is I2C and 5V, however, the Pico appears to be 5V tolerant.

Additionally, we rely on the GPIO input/output impedance states to read or drive the CEC bus. DDC is I2C requiring data and clock lines. Thus, we need to directly connect four wires:

  • HDMI CEC pin 13 direct to a GPIO
  • HDMI CEC ground pin 17 direct to GND
  • HDMI DDC clock pin 15 direct to SCL
  • HDMI DDC data pin 16 direct to SDA

For the Seeed Studio XIAO RP2040:

  • HDMI pin 13 --> D10
  • HDMI pin 17 --> GND
  • HDMI pin 15 --> D5
  • HDMI pin 16 --> D4

Schematic

Basic schematic.

After this we:

  • connect Pico-CEC to the HDMI output of the PC
  • connect the HDMI cable from the TV to Pico-CEC
  • connect a USB cable from Pico-CEC to the PC

Prototype

Initial prototype.

Enclosure

The enclosure is a reasonably simple three piece sandwich 3d print modelled with OpenSCAD. It is designed to be printed as three separate pieces which are bolted together with M3 nuts and bolts. An exploded preview of the result can be found in this STL.

Assembly

XIAO RP2040 with HDMI pass through and DDC.

Partially assembled Pico-CEC.

Software

The software is extremely simple and built on FreeRTOS tasks:

  • cec_task
    • interact with HDMI CEC sending user control message inputs to a queue
  • hid_task
    • read the user control messages from the queue and send to the USB task
  • usbd_task
    • generate an HID keyboard input for the USB host
  • blink_task
    • heart beat, no blink == no work

cec_task

The CEC task comprises three major components:

  • recv_frame
    • receives and validates CEC packets from the CEC GPIO pin
    • edge interrupt driven state machine
      • rewritten from busy wait loop to reduce CPU load
  • send_frame
    • formats and sends CEC packets on the CEC GPIO pin
    • alarm interrupt driven state machine
      • rewritten from busy wait loop to reduce CPU load
  • main control loop
    • manages CEC send and receive

All the HDMI frame handling was rewritten to be hardware/timer interrupt driven to meet real-time constraints. Attempts to increase the FreeRTOS tick timer along with busy wait loops were simply unable to consistently meet the CEC timing windows.

hid_task and usbd_task

These are simple FreeRTOS tasks effectively taken straight from the TinyUSB examples.

Dependencies

This project uses:

  • FreeRTOS
  • pico-sdk
    • tinyusb

Hardware

Bill of Materials

Component Quantity Price (June 2024) (AUD)
Seeed Studio XIAO RP2040 1 11.50
HDMI male/female adapter 1 4.30
M3x10mm bolt & nut 2 0.16
M3x20mm bolt & nut 2 0.17
Random short wires 2 basically free
Scunge 3D print from friend 1 mostly free
Umpteen hours of engineering 1 priceless
Total 16.13

Future

References

Inspiration and/or ground work was obtained from the following: