49.7.3.5 Peripheral Selection

The serial peripherals are selected through the assertion of the NPCS0 to NPCS3 signals. By default, all NPCS signals are high before and after each transfer.

• Fixed Peripheral Select Mode: SPI exchanges data with only one peripheral.
Fixed Peripheral Select mode is enabled by clearing SPI_MR.PS. In this case, the current peripheral is defined by SPI_MR.PCS. SPI_TDR.PCS has no effect.
• Variable Peripheral Select Mode: Data can be exchanged with more than one peripheral without having to reprogram SPI_MR.PCS.
Variable Peripheral Select mode is enabled by setting SPI_MR.PS. SPI_TDR.PCS is used to select the current peripheral. This means that the peripheral selection can be defined for each new data. The value must be written in a single access to SPI_TDR in the following format:

  [xxxxxxx(7-bit) + LASTXFER(1-bit)⁽¹⁾+ xxxx(4-bit) + PCS (4-bit) + TD (8- to 16-bit data)]

  with LASTXFER at 0 or 1 depending on the CSAAT bit, and PCS equal to the chip select to assert, as defined in section SPI Transmit Data Register.

  Note:

  1. Optional

  
For details on CSAAT, LASTXFER and CSNAAT, see section Peripheral Deselection with another DMA.

  If LASTXFER is used, the command must be issued after writing the last character. Instead of LASTXFER, the user can use the SPIDIS command. After the end of the DMA transfer, it is necessary to wait for the TXEMPTY flag and then write SPIDIS into the SPI Control Register (SPI_CR). This does not change the configuration register values). The NPCS is disabled after the last character transfer. Then, another DMA transfer can be started if SPI_CR.SPIEN has previously been written.