SAM7S256_14 Datasheet, PDF (278/775 Pages) ATMEL Corporation – ARM-based Flash MCU

The Fixed Peripheral Selection allows buffer transfers with a single peripheral. Using the PDC is an optimal means,
as the size of the data transfer between the memory and the SPI is either 8 bits or 16 bits. However, changing the
peripheral selection requires the Mode Register to be reprogrammed.
The Variable Peripheral Selection allows buffer transfers with multiple peripherals without reprogramming the
Mode Register. Data written in SPI_TDR is 32 bits wide and defines the real data to be transmitted and the periph-
eral it is destined to. Using the PDC in this mode requires 32-bit wide buffers, with the data in the LSBs and the
PCS and LASTXFER fields in the MSBs, however the SPI still controls the number of bits (8 to16) to be transferred
through MISO and MOSI lines with the chip select configuration registers. This is not the optimal means in term of
memory size for the buffers, but it provides a very effective means to exchange data with several peripherals with-
out any intervention of the processor.
28.6.3.6 Peripheral Chip Select Decoding
The user can program the SPI to operate with up to 15 peripherals by decoding the four Chip Select lines, NPCS0
to NPCS3 with an external logic. This can be enabled by writing the PCSDEC bit at 1 in the Mode Register
(SPI_MR).
When operating without decoding, the SPI makes sure that in any case only one chip select line is activated, i.e.
driven low at a time. If two bits are defined low in a PCS field, only the lowest numbered chip select is driven low.
When operating with decoding, the SPI directly outputs the value defined by the PCS field of either the Mode Reg-
ister or the Transmit Data Register (depending on PS).
As the SPI sets a default value of 0xF on the chip select lines (i.e. all chip select lines at 1) when not processing
any transfer, only 15 peripherals can be decoded.
The SPI has only four Chip Select Registers, not 15. As a result, when decoding is activated, each chip select
defines the characteristics of up to four peripherals. As an example, SPI_CRS0 defines the characteristics of the
externally decoded peripherals 0 to 3, corresponding to the PCS values 0x0 to 0x3. Thus, the user has to make
sure to connect compatible peripherals on the decoded chip select lines 0 to 3, 4 to 7, 8 to 11 and 12 to 14.
28.6.3.7 Peripheral Deselection
When operating normally, as soon as the transfer of the last data written in SPI_TDR is completed, the NPCS lines
all rise. This might lead to runtime error if the processor is too long in responding to an interrupt, and thus might
lead to difficulties for interfacing with some serial peripherals requiring the chip select line to remain active during a
full set of transfers.
To facilitate interfacing with such devices, the Chip Select Register can be programmed with the CSAAT bit (Chip
Select Active After Transfer) at 1. This allows the chip select lines to remain in their current state (low = active) until
transfer to another peripheral is required.
Figure 28-8 shows different peripheral deselection cases and the effect of the CSAAT bit.
SAM7S Series [DATASHEET]
6175M–ATARM–26-Oct-12
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