36.3.6.3 SCK Start-Up Delay

When starting an SPI data exchange, the host device asserts the SS output by either setting the SSET bit or loading the TCNT value, which then triggers the module to send data by writing SPIxTXB. These data triggers are synchronized to the clock selected by the SPIxCLK register before the first SCK pulse appears, usually requiring one or two clock periods of the selected SPI source clock.

The SPI module includes additional synchronization delays on SCK generation specifically designed to ensure that the Client Select output timing is correct, without requiring precision software timing loops. By default, this synchronization delay is ½ baud period.

When the value of the SPIxBAUD register is a small number (indicating higher SCK frequencies), the code execution delay between asserting SS and writing SPIxTXB is relatively long compared to the added synchronization delay before the first SCK edge. With larger values of SPIxBAUD (indicating lower SCK frequencies), the code execution delay is much smaller relative to the synchronization delay. Therefore, the first SCK edge after SS is asserted will be closer to the synchronization delay.

Setting the FST bit removes the synchronization delay, allowing systems with low SPIxBAUD values (and thus, long synchronization delays) to forgo this extra delay, in which case the time between the SS assertion and the first SCK edge depends entirely on the code execution delay.