25.3.2.5 USART in Master SPI mode

Using the USART in Master SPI mode requires the transmitter to be enabled. The receiver can optionally be enabled to serve as the serial input. The XCK pin will be used as the transfer clock.

As for the USART, a data transfer is initiated by writing to the USARTn.DATA register. This is the case for both sending and receiving data since the transmitter controls the transfer clock. The data written to USARTn.DATA are moved from the transmit buffer to the Shift register when the Shift register is ready to send a new frame.

The transmitter and receiver interrupt flags and corresponding USART interrupts used in Master SPI mode are identical in function to their use in normal USART operation. The receiver error status flags are not in use and are always read as zero.

Disabling of the USART transmitter or receiver in Master SPI mode is identical to their disabling in normal USART operation.