57.8.3.8 Peripheral Deselection without DMA

During a transfer of more than one data on a Chip Select without the DMA, FLEX_SPI_TDR is loaded by the processor, the TDRE flag rises as soon as the content of FLEX_SPI_TDR is transferred into the internal shift register. When this flag is detected high, FLEX_SPI_TDR can be reloaded. If this reload by the processor occurs before the end of the current transfer, and if the next transfer is performed on the same chip select as the current transfer, the Chip Select is not deasserted between the two transfers. But depending on the application software handling the SPI status register flags (by interrupt or polling method) or servicing other interrupts or other tasks, the processor may not reload FLEX_SPI_TDR in time to keep the chip select active (low). A null DLYBCT value (delay between consecutive transfers) in FLEX_SPI_CSR, gives even less time for the processor to reload FLEX_SPI_TDR. With some SPI client peripherals, if the chip select line must remain active (low) during a full set of transfers, communication errors can occur.

To facilitate interfacing with such devices, the Chip Select registers [CSR0...CSR3] can be programmed with the Chip Select Active After Transfer (CSAAT) bit to 1. This allows the chip select lines to remain in their current state (low = active) until a transfer to another chip select is required. Even if FLEX_SPI_TDR is not reloaded, the chip select remains active. To de-assert the chip select line at the end of the transfer, the Last Transfer (LASTXFER) bit in FLEX_SPI_CR must be set after writing the last data to transmit into FLEX_SPI_TDR.