33.6.3.5 Hardware Controlled SS
In Host mode, a single SS chip select can be controlled by hardware by writing the Host SPI Select Enable (CTRLB.MSSEN) bit to ‘1
’. In this mode, the SS pin is driven low for a minimum of one baud cycle before transmission begins, and stays low for a minimum of one baud cycle after transmission completes. If back-to-back frames are transmitted, the SS pin will always be driven high for a minimum of one baud cycle between frames.
In the following figure, the time, T, is between one and two baud cycles depending on the SPI Transfer mode.
When CTRLB.MSSEN = 0, the SS pin(s) is/are controlled by user software and normal GPIO.