ff-mcp23s17: Debugging SPI Signals

I've made progress, but the initial test of driving GPIO outputs has not yet been successful. That can mean I am sending the wrong data over SPI, or that the SPI communication is transmitted correctly. For the latter case, a multi-input oscilloscope is helpful. Here is one byte of oscilloscope data I need to analyze:

Testing SPI Signals

The purple signal is the SPI MOSI signal, the green signal is the SPI CLK, and the yellow signal (mostly hidden) is Chip Select.

I discovered today that the Chip Select signal was being driven incorrectly, and fixed that. Tomorrow, God willing, I'll look at the curve characteristics (to confirm my SPI settings are correct). If that looks good, I can check to make sure I am sending the correct bytes of data.