24. Synchronous Peripheral Interface (SPI)

The Panel-Card features two externally available SPI ports, each with three chip selects. Be aware that on the Panel-Card 35 when connected to the Evaluation Board and on the Panel-Card 57/70 all chip selects of the second port are already used. To compensate for this, you can also use GPIO pins as chip selects with the Linux SPI driver. Additionally the first SPI port is multiplexed with the MMC controller.

The Serial Peripheral Interface (SPI) circuit is a synchronous serial data link that provides communication with external devices in Master or Slave Mode. It also enables communication between processors if an external processor is connected to the system.

The Serial Peripheral Interface is essentially a shift register that serially transmits data bits to other SPIs. During a data transfer, one SPI system acts as the "master" which controls the data flow, while the other devices act as "slaves" which have data shifted into and out by the master.

A slave device is selected when the master asserts its NSS signal. If multiple slave devices exist, the master generates a separate slave select signal for each slave (NPCS). The SPI system consists of two data lines and two control lines:

  • Master Out Slave In (MOSI): This data line supplies the output data from the master shifted into the input(s) of the slave(s).

  • Master In Slave Out (MISO): This data line supplies the output data from a slave to the input of the master. There may be no more than one slave transmitting data during any particular transfer.

  • Serial Clock (SPCK): This control line is driven by the master and regulates the flow of the data bits. The master may transmit data at a variety of baud rates; the SPCK line cycles once for each bit that is transmitted. The SPI baudrate is Master Clock (MCK) divided by a value between 1 and 255

  • Slave Select (NSS): This control line allows slaves to be turned on and off by hardware.

Each SPI Controller has a dedicated receive and transmit DMA channel.