35.6.7.1 SPI Mode Operations

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 client connected to the SPI bus. The QSPI drives the chip select line to the client (CS) and the serial clock signal (SCK).

The QSPI features a single internal shift register and two holding registers: the Transmit Data Register (TXDATA) and the Receive Data Register (RXDATA). 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 TXDATA. The written data is immediately transferred into 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 into the internal shift register. Receiving data cannot occur without transmitting data.

If new data is written in TXDATA during the transfer, it stays in TXDATA until the current transfer is completed. Then, the received data is transferred from the internal shift register to the RXDATA, the data in TXDATA is loaded into the internal shift register, and a new transfer starts.

The transfer of data written in TXDATA in the internal shift register is indicated by the Transmit Data Register Empty (DRE) bit in the Interrupt Flag Status and Clear register (INTFLAG.DRE). When new data is written in TXDATA, this bit is cleared. The DRE bit is used to trigger the Transmit DMA channel.

The end of transfer is indicated by the Transmission Complete flag (INTFLAG.TXC). If the transfer delay for the last transfer was configured to be greater than 0 (CTRLB.DLYBCT), TXC is set after the completion of the delay. The module clock (CLK_QSPI_AHB) can be switched off at this time.

Ongoing transfer of received data from the internal shift register into RXDATA is indicated by the Receive Data Register Full flag (INTFLAG.RXC). When the received data is read, the RXC bit is cleared.

If the RXDATA has not been read before new data is received, the Overrun Error flag in INTFLAG register (INTFLAG.ERROR) is set. As long as this flag is set, data is loaded in RXDATA.

The SPI Mode Block Diagram shows a flow chart describing how transfers are handled.