From K61/K64/K66 reference manual/ datasheet, it is mentioned that the SDHC clock can be set up to 50MHz. But from the SDHC timing spec, SDHC output timing violate SD specification if SDHC clock is 50MHz, because output delay (SD6 tOD) is max 6.5ns. SD specification require setup time at SD card 6ns, then output delay should less than 4 ns if SDHC clock is 50MHz. In other word, according to the SD card specification, input set-up time is minimum 6ns, then maximum of SD6 should be 4ns in order to support 50 MHz clock frequency.
So we recommend customer to drop the SDHC clock frequency to 40MHz, then the 6.5ns output valid time will allow them to meet the 6ns setup requirement for the card.