SAM7X128_14 Datasheet, PDF (252/662 Pages) ATMEL Corporation – ARM-based Flash MCU

28.6.3.3 Clock Generation
The SPI Baud rate clock is generated by dividing the Master Clock (MCK), by a value between 1 and 255.
This allows a maximum operating baud rate at up to Master Clock and a minimum operating baud rate of MCK divided by
255.
Programming the SCBR field at 0 is forbidden. Triggering a transfer while SCBR is at 0 can lead to unpredictable results.
At reset, SCBR is 0 and the user has to program it at a valid value before performing the first transfer.
The divisor can be defined independently for each chip select, as it has to be programmed in the SCBR field of the Chip
Select Registers. This allows the SPI to automatically adapt the baud rate for each interfaced peripheral without
reprogramming.
28.6.3.4 Transfer Delays
Figure 28-7 shows a chip select transfer change and consecutive transfers on the same chip select. Three delays can be
programmed to modify the transfer waveforms:
 The delay between chip selects, programmable only once for all the chip selects by writing the DLYBCS field in the
Mode Register. Allows insertion of a delay between release of one chip select and before assertion of a new one.
 The delay before SPCK, independently programmable for each chip select by writing the field DLYBS. Allows the
start of SPCK to be delayed after the chip select has been asserted.
 The delay between consecutive transfers, independently programmable for each chip select by writing the
DLYBCT field. Allows insertion of a delay between two transfers occurring on the same chip select
These delays allow the SPI to be adapted to the interfaced peripherals and their speed and bus release time.
Figure 28-7. Programmable Delays
Chip Select 1
Chip Select 2
SPCK
DLYBCS DLYBS
DLYBCT
DLYBCT
28.6.3.5 Peripheral Selection
The serial peripherals are selected through the assertion of the NPCS0 to NPCS3 signals. By default, all the NPCS
signals are high before and after each transfer.
The peripheral selection can be performed in two different ways:
 Fixed Peripheral Select: SPI exchanges data with only one peripheral
 Variable Peripheral Select: Data can be exchanged with more than one peripheral
Fixed Peripheral Select is activated by writing the PS bit to zero in SPI_MR (Mode Register). In this case, the current
peripheral is defined by the PCS field in SPI_MR and the PCS field in the SPI_TDR has no effect.
Variable Peripheral Select is activated by setting PS bit to one. The PCS field in SPI_TDR is used to select the current
peripheral. This means that the peripheral selection can be defined for each new data.
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.
SAM7X Series [DATASHEET]
6120K–ATARM–11-Feb-14
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