clocking SPI for SD cards

[Dan Moos]

A lot of the tutorials on using SPI to talk to SD cards seem to show code that sends a 8 or so "reads" to the device before writing data.

Almost always anyway. I have gotten the beginnings of an explanation here and there, but nothing really descriptive.

The best I can gather is that you need to do some fake reads just to get the device a look at the clock so it can be ready when the writes come in.

First, is that about right?

Second, should I precede EVERY write operation with some of these pretend reads?

Third, 8 reads seems  common. Is that a good rule of thumb?

Finally, if I'm completely wrong about the whole thing, care to set me straight?

[fchk]

Just get the official standard document and look it up for yourself:

https://www.sdcard.org/downloads/pls/

You will need the first document "Physical Layer Simplified Specification", and SPI mode is described in section 7.

fchk

[Dan Moos]

Already have, and have been reading said doc. Maybe it's in there, but I can't find it. I thought maybe it was an SPI thing rather than just an SD card specific thing, but who knows.