For MPC8541, what is the maximum bit rate clock for SSI? Is it really 12.285MHz or can it be run up to platform clock / 8?
Maximum bit rate clock for SSI is as per hardware spec i.e. 12.285MHz. This is the maximum speed at which the SSI IP is guaranteed to work.
From a system perspective it is possible to clock it at a higher speed, but for MPC8541 that is not supported.
If platform clock is 400MHz, please use appropriate values of DIV2, PSR and PM to ensure that the bit rate clock for SSI does not exceed 12.285MHz.