SPI Modes relate to the relationship between the clock and the data on the interface during a transfer. In standard practice there are four modes that include the combinations of whether the clock is rising edge or falling edge active and whether the data is clocked on the leading edge or trailing edge of the clock pulse.

Sometimes it takes a bit of experimentation at coding time with a scope and the data sheets in hand to work out the correct combination of the control bits at the master side.

I have found on occasion that some peripheral devices that claim to be fully compatible with one or more of the "standard" SPI modes are in fact not compliant and then extra futzing around is required to get it to work. For example I had a recent experience working with an MCU connected to a tilt sensor where it turned out there was no way to get the SPI peripheral to work with one sensor and so had to resort to bit-banging the interface so I could strobe the data in at a time that worked. (Unfortunately that also slows things down a bit running the MCU at 80MHz does help that problem).

Michael Karas