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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.
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 | falling clock, and when /CS activates | rising clock |
| 1 | 0 | 1 | rising clock | falling clock |
| 2 | 1 | 0 | rising clock, and when /CS activates | falling clock |
| 3 | 1 | 1 | falling clock | rising clock |
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”.