SD 3.00 Physical layer FULL specification
I was just trolling around the net trying to figure out what the Pm parameter and speed class of SD cards meant, and I came across a definitive answer in the form of a copy of Part 1 of the unredacted v3.00 specification (April 16, 2009) at
http://www.61ic.com/code/attachment.php?aid=73871&k=cfaccbbb80c338769bd7282636c0ca7e&t=1331742726
I have of course snarfed a copy, and if anyone else on the list would benefit from one, I figured it was worth sharing.
In particular, it documents the FAT update optimizations in SD cards (section 4.3.1.7, p. 112).
If anyone cares, write performance is defined in terms of updates to "allocation units" (i.e. erase blocks) 512K-4MiB in size of a card-defined size, which are made up of "recording units" (i.e. FAT clusters). Table 4-51 on p. 113 gives the limits.
An update to an allocation unit is a combination of writes of new data and and moves (copies) of old data from the pre-existing AU. Each of these has a performance standard, Pw and Pm, respectively.
For a "Class n" card, Pw and Pr must be >= n MB/s, while Pm must be
Quoted 1 line
= n/2 MB/s. (Exacpt, oddly, class 10 has NO lower bound on Pm.)
Quoted 7 lines · George Spelvin
In particular, it documents the FAT update optimizations in SD cards (section 4.3.1.7, p. 112).
If anyone cares, write performance is defined in terms of updates to "allocation units" (i.e. erase blocks) 512K-4MiB in size of a card-defined size, which are made up of "recording units" (i.e. FAT clusters). Table 4-51 on p. 113 gives the limits.
I believe these recommendations are often ignored anyway. A lot of the SD cards nowadays have erase block sizes of 1.5, 3, 6, or 8 MB. None of these can be specified with this version of the standard. Even for the rare cards that have 1MB or 2MB erase block sizes, I've never seen one that reports anything but 4MB.
Arnd