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• 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 35-8.
Programmable Delays
35.7.3.5
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.
• Fixed Peripheral Select: SPI exchanges data with only 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: Data can be exchanged with more than one peripheral without
having to reprogram the NPCS field in the SPI_MR register.
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 value to write in the SPI_TDR register as the following format.
[xxxxxxx(7-bit) + LASTXFER(1-bit)
+ xxxx(4-bit) + PCS (4-bit) + DATA (8 to 16-bit)] with PCS
equals to the chip select to assert as defined in
(SPI Transmit Data Register) and
LASTXFER bit at 0 or 1 depending on CSAAT bit.
Note:
1. Optional.
CSAAT, LASTXFER and CSNAAT bits are discussed in
Section 35.7.3.9 ”Peripheral Deselec-
If LASTXFER is used, the command must be issued before writing the last character. Instead of
LASTXFER, the user can use the SPIDIS command. After the end of the DMA transfer, wait for
the TXEMPTY flag, then write SPIDIS into the SPI_CR register (this will not change the configu-
ration register values); the NPCS will be deactivated after the last character transfer. Then,
another DMA transfer can be started if the SPIEN was previously written in the SPI_CR register.
DLYBCS
DLYBS
DLYBCT
DLYBCT
Chip Select 1
Chip Select 2
SPCK
Содержание SAM9X25
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