A pair of physical buttons to control the brightness of a pair of external monitors over HDMI.

My USB-C port replicator does not pass on brightness commands to my monitors.

HDMI has a control channel called DDC. Hook this board up to a spare HDMI port on a monitor and use the buttons to control the brightness.

Here are some design considerations that were taken into account.

The board has a footprint for a 3.3 V regulator and also a footprint for a 0 Ohm bypass to use 5 V directly from USB. HDMI signaling is 5 V and the CH32V003 can run at 5 V, but both also work at 3.3 V. I have used the 5 V option.

On my monitors, the I2C lines are already pulled up to 5 V internally. The PCB has footprints for additional pull-up, but I found it works fine without populating them.

The board has a USB connector for power. The data lines are connected to GPIO pins for possible future use.

Only GPIOs 0-4 can be used for USB D+/D- due to limits of the c.andi instruction which can only hold a 5 bit value. You cannot use PC5-PC7 and PD5-PD7 for D+/D-!

GitHub - cnlohr/rv003usb: CH32V003 RISC-V Pure Software USB Controller

There are three firmware options:

1. The proof-of-concept written in Micropython on a Raspberry Pi RP2040.
2. The prototype firmware written in Platformio/Arduino C++ on a WCH CH32V003 devkit.
3. The final firmware written in Rust on the custom PCB.

DDC commands are hard-coded in the firmware. The payloads were generated from a slightly modified version of python-ddcci