I have a HDMI switch with a PIP feature that would be very interesting for my project. Unfortunately it seems a bit more complicated to get it working with a single button. I could use an IR blaster to control the switch (it will probably end this way), but as there is a big nice unused 4 pin connector on the PCB, I would like to find out what protocol it is and see if I can control the switch this way.

My guess is either SPI or I²C. I will investigate with my BusPirate as soon as I have some time