Since I'm too "frugal" at least at this stage, to pay for the "full" specification, which, I'm told, changes every couple of weeks, I currently use and old specification. The somewhat old but still useable and relatively complete specification v1.9 as was once published by SanDisk but has, in the meantime been withdrawn from public view, has a maximal clock rate of 25 MHz, and, therefore a data rate in 4-bit mode of 50 MBps, albeit in whatever bursts the SD card chooses to generate. That's certainly fast enough for what I need, and it's old enough to reach back to before I bought any of the SD media I presently own.

It does, of course, imply that, when devices supporting a higher clock rate become available, it could run at whatever rate they support, barring any protocol changes, though I understand that it won't necessarily impact the SPI mode. I'm presently planning to limit my device size to the 2GB variety, since that will still adhere to the old standard.

My reference to "full" speed was simply as opposed to single-bit SD mode, or the SPI mode. What's under consideration is the added hardware complexity, vs the 4x longer transfer time. Now, if one uses built-in MCU features to handle SPI communication, the speed is considerably slower. The SD card spec indicates SPI can be used at up to 25 MHz. I'm not convinced the hardware support in any 805x-class MCU can handle that high a rate. That's why external hardware has to play a role.

RE