SPI

Different variants exist. Usually SPI refers to the Motorola Serial Peripheral Interface. Other variants are the Texas Instruments Serial protocol (SSP) and the National Instruments Microwire.

Protocol

Most significant bit first

Modes

There are 4 Modes. Defied by the two settings “clock polarity” CPOL and “clock phase” CPHA.

“clock polarity” CPOL = 0:

Clock line is low when idle.

“clock polarity” CPOL = 1:

Clock line is high when idle.

“clock phase” CPHA = 0:

first data bit is output on /CS activating (going low).

data bits change when clock goes to idle level.

data bits are read when clock comes from idle level.

“clock phase” CPHA = 1:

first data bit starts on first edge after /CS activates.

data bits change when clock comes from idle level.

data bits are read when clock goes to idle level.

mode	CPOL	CPHA	write bit	read bit
0	0	0	clock goes low, and when /CS activates	clock goes high
1	0	1	clock goes high	clock goes low
2	1	0	clock goes high, and when /CS activates	clock goes low
3	1	1	clock goes low	clock goes high

API

The part “signal_name” in the described functions will be replaced by the name of the signal. E.g. if your signal is called “audio” then the API function “signal_name_transfer” will become “audio_transfer”.

void signal_name_transfer(const uint8_t *data_send,
const uint8_t *data_receive,
const uint32_t length )

Will send “length” bytes from the buffer “data_send” out on MOSI and will receive the same amount of bytes from MISO into the buffer “data_receive”.