SD driver - Write speed

Question

We've been trying to figure out why we only achieve writing speed of ~53MBps on UHS104 cards that claim 90MBps. Due to hardware constraints, clock frequency supplied to the card is only 148.5 MHz (instead of 208MHz). Does that mean that we should achieve speed of (148.5 * 4)/8 = 74.25MBps? Or is our caclulation wrong since it assumes that if card guarantees speed of 90MBps on frequency of 208MHz, then it should guarantee speed of 74.25MBps on frequency of 148.5?

Answer 1

The simplified physical layer spec states that for maximum performance you need to write full AU blocks - usually 2 or 4 MByte, otherwise the card will have to copy data around internally when writing across block boundaries. Unfortunately, most of the Speed Class Specification is missing in the 4.13 chapter.

The first AUs may have a different wear level strategy, as they are normally used for the FATs. This could make them slower to write to.