57.8.4 SPI Client Mode

When operating in Client mode, the SPI processes data bits on the clock provided on the SPI clock pin (SPCK).

The SPI waits until NSS goes active before receiving the serial clock from an external host. When NSS falls, the clock is validated and the data are loaded in FLEX_SPI_RDR according to the configuration value of the FLEX_SPI_CSR0.BITS field. These bits are processed following a phase and a polarity defined respectively by the FLEX_SPI_CSR0.NCPHA and FLEX_SPI_CSR0.CPOL bits. Note that the BITS field, CPOL bit and NCPHA bit of the other Chip Select registers have no effect when the SPI is programmed in Client mode.

The bits are shifted out on the MISO line and sampled on the MOSI line.

When all bits are processed, the received data are transferred in FLEX_SPI_RDR and the RDRF bit rises. If FLEX_SPI_RDR has not been read before new data are received, the Overrun Error bit (OVRES) in FLEX_SPI_SR is set. As long as this flag is set, data are loaded in FLEX_SPI_RDR. The user must read FLEX_SPI_SR 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 FLEX_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 to 0.

When a first data is written in FLEX_SPI_TDR, it is transferred immediately in the shift register and the TDRE flag rises. If new data is written, it remains in FLEX_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 FLEX_SPI_TDR is transferred in the shift register and the TDRE flag rises. This enables frequent updates of critical variables with single transfers.

Then, a new data is loaded in the shift register from FLEX_SPI_TDR. If no character is ready to be transmitted, i.e., no character has been written in FLEX_SPI_TDR since the last load from FLEX_SPI_TDR to the shift register, FLEX_SPI_TDR is retransmitted. In this case the Underrun Error Status flag (UNDES) is set in FLEX_SPI_SR.

If NSS rises between two characters, it must be kept high for two MCK clock periods or more and the next SPCK capture edge must not occur less than four MCK periods after NSS rise.

In Client mode, if the NSS line rises and the received character length does not match the configuration defined in FLEX_SPI_CSR0.BITS, the SFERR flag is set in FLEX_SPI_SR.

The following figure shows a block diagram of the SPI when operating in Client mode.