57.8.3.3 Clock Generation

The SPI bit rate clock is generated by dividing a source clock which can be the peripheral clock or a programmable clock from the GCLK. The divider can be a value between 1 and 255.

If the SCBR field is programmed to 1 and the clock source is GCLK, the operating bit rate is peripheral clock (refer to the section “Electrical Characteristics” for the SPCK maximum frequency). Triggering a transfer while SCBR is at 0 can lead to unpredictable results.

At reset, SCBR is 0 and the user has to program it to a valid value before performing the first transfer.

The divisor can be defined independently for each chip select, as it has to be programmed in the FLEX_SPI_CSR.SCBR field. This allows the SPI to automatically adapt the bit rate for each interfaced peripheral without reprogramming.

If GCLK is selected as source clock (FLEX_SPI_MR.BRSRCCLK = 1), the bit rate is independent of the processor/bus clock. Thus, the processor clock can be changed while SPI is enabled. The processor clock frequency changes must be performed only by programming the PMC_MCKR.PRES field (refer to the section “Power Management Controller (PMC)"). Any other method to modify the processor/bus clock frequency (PLL multiplier, etc.) is forbidden when SPI is enabled.

The peripheral clock frequency must be at least three times higher than GCLK.