The QSPI in standard SPI mode operates on the clock generated by the internal programmable baud rate generator. It fully controls the data transfers to and from the slave connected to the SPI bus. The QSPI drives the chip select line to the slave (QCS) and the serial clock signal (QSCK).
The QSPI features two holding registers, the Transmit Data register (QSPI_TDR) and the Receive Data register (QSPI_RDR), and a single internal shift register. The holding registers maintain the data flow at a constant rate.
After enabling the QSPI, a data transfer begins when the processor writes to the QSPI_TDR. The written data is immediately transferred to the internal shift register and transfer on the SPI bus starts. While the data in the internal shift register is shifted on the MOSI line, the MISO line is sampled and shifted to the internal 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 (QSPI_SR) can be discarded.
If new data is written in QSPI_TDR during the transfer, it is retained there until the current transfer is completed. Then, the received data is transferred from the internal shift register to the QSPI_RDR, the data in QSPI_TDR is loaded in the internal shift register and a new transfer starts.
The transfer of a data written in QSPI_TDR in the internal shift register is indicated by the Transmit Data Register Empty (TDRE) bit in the QSPI_SR. When new data is written in QSPI_TDR, this bit is cleared. QSPI_SR.TDRE is used to trigger the Transmit DMA channel.
The end of transfer is indicated by the TXEMPTY flag in the QSPI_SR. If a transfer delay (DLYBCT) is greater than 0 for the last transfer, QSPI_SR.TXEMPTY is set after the completion of this delay. The peripheral clock can be switched off at this time.
The transfer of received data from the internal shift register in QSPI_RDR is indicated by the Receive Data Register Full (RDRF) bit in the QSPI_SR. When the received data is read, QSPI_SR.RDRF bit is cleared.
If the QSPI_RDR has not been read before new data is received, the Overrun Error Status (OVRES) bit in QSPI_SR is set. As long as this flag is set, data is loaded in QSPI_RDR. The user must read the QSPI_SR to clear the OVRES bit.
The following figures show, respectively, a block diagram of the SPI when operating in Master mode, and a flow chart describing how transfers are handled.