48.7.3 Host Mode Operations

When configured in Host 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 client(s) connected to the SPI bus. The SPI drives the chip select line to the client and the serial clock signal (SPCK).

The SPI features two holding registers, the Transmit Data Register (SPI_TDR) and the Receive Data Register (SPI_RDR), and a single shift register. The holding registers maintain the data flow at a constant rate.

After enabling the SPI, a data transfer starts when the processor writes to SPI_TDR. The written data is immediately transferred into the internal shift register and the 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 into the shift register. Data cannot be loaded in SPI_RDR without transmitting data. If there is no data to transmit, dummy data can be used (SPI_TDR filled with ones). If SPI_MR.WDRBT is set, transmission can occur only if SPI_RDR has been read. If Receiving mode is not required, for example when communicating with a client receiver only (such as an LCD), the receive status flags in the SPI Status register (SPI_SR) can be discarded.

Before writing SPI_TDR, SPI_MR.PCS must be set in order to select a client.

If new data is written in SPI_TDR during the transfer, it is kept in SPI_TDR 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.

As soon as SPI_TDR is written, the Transmit Data Register Empty (TDRE) flag in SPI_SR is cleared. When the data written in SPI_TDR is loaded into the shift register, TDRE in SPI_SR is set. The TDRE flag is used to trigger the Transmit DMA channel.

See the figure below.

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

Note: When the SPI is enabled, the TDRE and TXEMPTY flags are set.
Figure 48-5. TDRE and TXEMPTY Flag Behavior

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

If SPI_RDR has not been read before new data is received, the Overrun Error (OVRES) flag in SPI_SR is set. As long as this flag is set, data is loaded in SPI_RDR. The user has to read SPI_SR to clear OVRES.

The following figures show, respectively, a block diagram of the SPI when operating in Host mode and a flow chart describing how transfers are handled.