I have an Arduino M0 communicating to both:
- MAX31855 Thermocouple interface (SPI)
- SD Card
My issue is with the MISO line. I found my SD Card breakout board with buffer was an older one where the tri state control for the MISO was disabled (grounded).
So I had some small SD Card boards with no buffer and no 5V supply. I wired them to my setup using 3.3V drive and all the appropriate lines. There was still a conflict, when I removed the MISO wire from the SD card, the MAC31855 started to work.
Question: Is the MISO output of a "bare" SD Card tri-stated with the CS line? I downloaded some parts of the SDCARD.org spec but could not find any reference to the hardware details.
I have since, modified the buffered SD Card board and the system looks like it is working.