====== 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".

<code>
void signal_name_transfer(const uint8_t *data_send, 
                          const uint8_t *data_receive, 
                          const uint32_t length )</code>
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".