Background: I have many years of experience with AVR and ARM Cortex PLCs and I feel very comfortable using them in projects but I am not an EE or SE. So, if I make a mistake or misunderstand something, keep that in mind. Thanks.
I cracked open the case on my Maker Select v2.1 and I noticed that I have a 2x3 header that is labeled ISP. This is fairly standard for the AVR PLCs and the Melzi board that I have uses the ATMEGA1284P.
I have asked at numerous forums and nobody seems to really know the answer (the downside to RepRap--people use but don't fully understand).
I'm wondering if I can tweak the Repetier firmware (I'm using the stock version 0.91) to use the ISP headers as a standard SPI bus. It has the SCK, MOSI, and MISO pins but it doesn't have the SS pin.
What I want to do is put a port expander on the SPI bus and break out additional usable GPIOs.
So, is there a hardware limitation or any other reason why I wouldn't be able to repurpose the ISP headers into an SPI bus?