57.8.3.9 Peripheral Deselection with DMA

DMA provides faster reloads of FLEX_SPI_TDR compared to software. However, depending on the system activity, it is never sure that FLEX_SPI_TDR is written with the next data before the end of the current transfer. Consequently, a data can be lost by the deassertion of the NPCS line for SPI client peripherals requiring the chip select line to remain active between two transfers. The only way to ensure a safe transfer in this case is the use of the CSAAT and LASTXFER bits.

When the CSAAT bit is cleared, the NPCS does not rise in all cases between two transfers on the same peripheral. During a transfer on a Chip Select, 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, FLEX_SPI_TDR can be reloaded. If this reload 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. This can lead to difficulties to interface with some serial peripherals requiring the chip select to be deasserted after each transfer. To facilitate interfacing with such devices, FLEX_SPI_CSR can be programmed with the Chip Select Not Active After Transfer (CSNAAT) bit to 1. This allows the chip select lines to be deasserted systematically during a time “DLYBCS” (the value of the CSNAAT bit is processed only if the CSAAT bit is cleared for the same chip select).

The following figure shows different peripheral deselection cases and the effect of the CSAAT and CSNAAT bits.