The SSI includes a programmable bit-rate clock divider and prescaler to generate the serial output clock. Bit rates are supported to 2 MHz and higher, although maximum bit rate is determined by peripheral devices. The serial bit rate is derived by dividing-down the input clock (SysClk). The clock is first divided by an even prescale value CPSDVSR from 2 to 254, which is programmed in the SSI Clock Prescale (SSICPSR) register. The clock is further divided by a value from 1 to 256, which is 1 + SCR, where SCR is the value programmed in the SSI Control 0 (SSICR0) register. The frequency of the output clock SSIClk is defined by:
For master mode, the system clock must be at least four times faster than SSIClk, with the restriction that SSIClk cannot be faster than 25 MHz. For slave mode, the system clock must be at least 12 times faster than SSIClk.