Atmel ARM-Based Evaluation Kit AT91SAM9N12-EK AT91SAM9N12-EK Data Sheet

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SAM9N12/SAM9CN11/SAM9CN12 [DATASHEET]

11063K–ATARM–05-Nov-13

When configured in Master Mode, the SPI operates on the clock generated by the internal programmable baud rate
generator. It fully controls the data transfers to and from the slave(s) connected to the SPI bus. The SPI drives the chip
select line to the slave and the serial clock signal (SPCK). 

The SPI features two holding registers, the Transmit Data Register and the Receive Data Register, and a single Shift
Register. The holding registers maintain the data flow at a constant rate. 

After enabling the SPI, a data transfer begins when the processor writes to the SPI_TDR (Transmit Data Register). The
written data is immediately transferred in the Shift Register and transfer on the SPI bus starts. While the data in the Shift
Register is shifted on the MOSI line, the MISO line is sampled and shifted in the Shift Register. Receiving data cannot
occur without transmitting data. If receiving mode is not needed, for example when communicating with a slave receiver
only (such as an LCD), the receive status flags in the status register can be discarded.

Before writing the TDR, the PCS field in the SPI_MR register must be set in order to select a slave.

After enabling the SPI, a data transfer begins when the processor writes to the SPI_TDR (Transmit Data Register). The
written data is immediately transferred in the Shift Register and transfer on the SPI bus starts. While the data in the Shift
Register is shifted on the MOSI line, the MISO line is sampled and shifted in the Shift Register. Transmission cannot
occur without reception.

Before writing the TDR, the PCS field must be set in order to select a slave.

If new data is written in SPI_TDR during the transfer, it stays in it until the current transfer is completed. Then, the
received data is transferred from the Shift Register to SPI_RDR, the data in SPI_TDR is loaded in the Shift Register and
a new transfer starts. 

The transfer of a data written in SPI_TDR in the Shift Register is indicated by the TDRE bit (Transmit Data Register
Empty) in the Status Register (SPI_SR). When new data is written in SPI_TDR, this bit is cleared. The TDRE bit is used
to trigger the Transmit DMAchannel.

The end of transfer is indicated by the TXEMPTY flag in the SPI_SR register. If a transfer delay (DLYBCT) is greater than
0 for the last transfer, TXEMPTY is set after the completion of said delay. The master clock (MCK) can be switched off at
this time.

The transfer of received data from the Shift Register in SPI_RDR is indicated by the RDRF bit (Receive Data Register
Full) in the Status Register (SPI_SR). When the received data is read, the RDRF bit is cleared. 

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.

, shows a block diagram of the SPI when operating in Master Mode. 

chart describing how transfers are handled.