57.8.3.5 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 writing the FLEX_SPI_MR.PS bit to zero. In this case, the current peripheral is defined by the FLEX_SPI_MR.PCS field, and the FLEX_SPI_TDR. PCS field has no effect.
• Variable Peripheral Select Mode: Data can be exchanged with more than one peripheral without having to reprogram FLEX_SPI_MR.PCS.
Variable Peripheral Select Mode is enabled by setting the FLEX_SPI_MR.PS bit to one. The FLEX_SPI_TDR.PCS field 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 FLEX_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 SPI Transmit Data Register (FLEX_SPI_TDR).

  Note: 1. Optional

  The CSAAT, LASTXFER and CSNAAT bits are discussed in Peripheral Deselection with 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 (FLEX_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 the FLEX_SPI_CR.SPIEN bit has previously been written.