It seems you're right, Per ... Not that that's any surprise. Two nybbles = 1 Byte in 80 ns, which is 12.5 MHz. I guess I'm unconsciously trying to impose some truth on the various marketing claims. I guess I should apologize to Maarten for reacting as I did.

I'm clearly confused ... since I see varying claims from equipment manufacturers who use these devices, and from SD card, particularly micro-SD card makers, who claim such rates in terms of some unspecified "classic" rate, and define their parts as 20x, 40x, 80x, etc.

The spec talks about clocks up to 25 MHz, yet other documents, possibly from marketing rather than engineering, refer to aggregate transfer rates up to 400 Mbps. I haven't bought the official specification because there are many costly revisions per hour, it seems, and certainly many of them per year. The manufacturers of some equipment claim burst rates at the same rate as USB2/FireWire claims, i.e. up to 480 Mbps.

Mysteries such as these confuse more than just me, however, and the "specsmanship" in the marketing departments doesn't help at all.

If you want a look at the confusion, just GOOGLE for "SD Card" "Transfer Rate" and see what you get.

Now I knew this, having pointed it out to a colleague in email just yesterday, yet I'm sufficiently mixed up about the spec's to make this sort of a blunder ... and I'm supposed to be a mathematician ...

BTW, they are purported to be capable of 25 MHz in SPI mode, too, so I'm looking into that as well. It would be much easier to get there with SPI.

I'd really like it if we had someplace to put spec's such as the ones relevant to devices like this. The old one I'm using is ~1.15 MByte in size, so I can't post it on 8052.COM ... perhaps someone, somewhere has a more up-to-date spec ... I might even find a place to put something like that, so people can access it. I'm paying for 70GB of "personal web space" at my current ISP, and have, at last check, ~20 TB available at the one I previously used ... until they find out I still have that much ...

I'd happily make one of those seven 10GB "personal" spaces on Comcast available for manufacturers' spec's, etc. I don't know how long that would last. I'd want to be the only one to place items in that space, however, else it would be swallowed up with advertising within minutes. Maybe we should look into that.

RE