SAM9M10_14 Datasheet, PDF (376/1385 Pages) ATMEL Corporation – Atmel | SMART ARM-based Embedded MPU

By default, the Mode Fault detection circuitry is enabled. The user can disable Mode Fault detection by setting the
MODFDIS bit in the SPI Mode Register (SPI_MR).
26.7.4 SPI Slave Mode
When operating in Slave Mode, the SPI processes data bits on the clock provided on the SPI clock pin (SPCK).
The SPI waits for NSS to go active before receiving the serial clock from an external master. When NSS falls, the
clock is validated on the serializer, which processes the number of bits defined by the BITS field of the Chip Select
Register 0 (SPI_CSR0). These bits are processed following a phase and a polarity defined respectively by the
NCPHA and CPOL bits of the SPI_CSR0. Note that BITS, CPOL and NCPHA of the other Chip Select Registers
have no effect when the SPI is programmed in Slave Mode.
The bits are shifted out on the MISO line and sampled on the MOSI line.
(For more information on BITS field, see also, the ^(Note:) below the register table; Section 26.8.9 “SPI Chip Select
Register” on page 388.)
When all the bits are processed, the received data is transferred in the Receive Data Register and the RDRF bit
rises. If the SPI_RDR (Receive Data Register) has not been read before new data is received, the Overrun Error
bit (OVRES) in SPI_SR is set. As long as this flag is set, data is loaded in SPI_RDR. The user has to read the
status register to clear the OVRES bit.
When a transfer starts, the data shifted out is the data present in the Shift Register. If no data has been written in
the Transmit Data Register (SPI_TDR), the last data received is transferred. If no data has been received since the
last reset, all bits are transmitted low, as the Shift Register resets at 0.
When a first data is written in SPI_TDR, it is transferred immediately in the Shift Register and the TDRE bit rises. If
new data is written, it remains in SPI_TDR until a transfer occurs, i.e. NSS falls and there is a valid clock on the
SPCK pin. When the transfer occurs, the last data written in SPI_TDR is transferred in the Shift Register and the
TDRE bit rises. This enables frequent updates of critical variables with single transfers.
Then, a new data is loaded in the Shift Register from the Transmit Data Register. In case no character is ready to
be transmitted, i.e. no character has been written in SPI_TDR since the last load from SPI_TDR to the Shift
Register, the Shift Register is not modified and the last received character is retransmitted.
Figure 26-13 shows a block diagram of the SPI when operating in Slave Mode.
Figure 26-13. Slave Mode Functional Bloc Diagram
SPCK
NSS
MOSI
SPIEN
SPIENS
SPIDIS
SPI_CSR0
BITS
NCPHA
CPOL
SPI
Clock
SPI_RDR
RD
RDRF
OVRES
LSB
Shift Register
MSB
MISO
SPI_TDR
TD
TDRE
376
SAM9M10 [DATASHEET]
Atmel-6355G-ATARM-SAM9M10-Datasheet_02-Sept-14