Streaming Video from the Mouse
Before that, I also reverse engineered parts of the USB controller firmware of the mouse. When looking into the ADNS-9800 datasheet, one is presented with the possibility of reading 30x30 greyscale images from the device, so I decided to just do that.
As far as I know, the pre-existing USB controller firmware does not have any built-in function to read the frames, which is not really surprising. So I will have to modify the firmware. When patching binaries, I find doing so in assembly is often advantageous to C (or PL/M-51, which is never advantageous) because it allows to modify existing functions or jump to the middle of a function. I’m also quite used to 8051 assembly, so writing it poses no problem.
This works without problem, and one gets to test both the firmware code and host code for invoking the special mode. Knowing that the hello world works, I could now go on to read frames from the sensor.
The ADNS-9800 communicates over SPI, but the USB controller doesn’t have any native SPI ports, so it has to bit-bang the frames. Communication involves registers, which can be read or written to by sending the address over SPI and then reading/writing the byte. Getting the frame involves writing some values to a register and then reading 900 bytes from another register, all over SPI.
