Atmel SAM4S Series, Preliminary Data Sheet

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647
11100B–ATARM–31-Jul-12
SAM4S Series [Preliminary]
31.7.3.9 Peripheral Deselection with PDC
When the Peripheral DMA Controller is used, the chip select line will remain low during the
whole transfer since the TDRE flag is managed by the PDC itself. The reloading of the SPI_TDR
by the PDC is done as soon as TDRE flag is set to one. In this case the use of CSAAT bit might
not be needed. However, it may happen that when other PDC channels connected to other
peripherals are in use as well, the SPI PDC might be delayed by another (PDC with a higher pri-
ority on the bus). Having PDC buffers in slower memories like flash memory or SDRAM
compared to fast internal SRAM, may lengthen the reload time of the SPI_TDR by the PDC as
well. This means that the SPI_TDR might not be reloaded in time to keep the chip select line
low. In this case the chip select line may toggle between data transfer and according to some
SPI Slave devices, the communication might get lost. The use of the CSAAT bit might be
needed.
When the CSAAT bit is set at 0, the NPCS does not rise in all cases between two transfers on
the same peripheral. During a transfer on a Chip Select, the flag TDRE rises as soon as the con-
tent of the SPI_TDR is transferred into the internal shifter. When this flag is detected the
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
de-asserted between the two transfers. This might lead to difficulties for interfacing with some
serial peripherals requiring the chip select to be de-asserted after each transfer. To facilitate
interfacing with such devices, the Chip Select Register can be programmed with the CSNAAT bit
(Chip Select Not Active After Transfer) at 1. This allows to de-assert systematically the chip
select lines during a time DLYBCS. (The value of the CSNAAT bit is taken into account only if
the CSAAT bit is set at 0 for the same Chip Select).
Figure 31-11 shows different peripheral deselection cases and the effect of the CSAAT and
CSNAAT bits.