SAM4L Datasheet, PDF (666/1185 Pages) ATMEL Corporation – ATSAM ARM-based Flash MCU

ATSAM4L4/L2
26.7.3.5
26.7.3.6
26.7.3.7
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 a zero to the Peripheral Select bit in MR (MR.PS).
In this case, the current peripheral is defined by the MR.PCS field and the TDR.PCS field has no
effect.
Variable Peripheral Select is activated by writing a one to the MR.PS bit . The TDR.PCS field 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 Periph-
eral DMA Controller is an optimal means, as the size of the data transfer between the memory
and the SPI is either 4 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 repro-
gramming the MR register. Data written to TDR is 32-bits wide and defines the real data to be
transmitted and the peripheral it is destined to. Using the Peripheral DMA Controller 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 CSRn 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
without any intervention of the processor.
Peripheral Chip Select Decoding
The user can configure 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 a one to
the Chip Select Decode bit in the MR register (MR.PCSDEC).
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 MR register or the TDR 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 one)
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, the CRS0
register 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.
Peripheral Deselection
When operating normally, as soon as the transfer of the last data written in TDR is completed,
the NPCS lines all rise. This might lead to runtime error if the processor is too long in responding
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