At 04:45 PM 1/21/04 -0000, Alan wrote:
>
>Does anyone have any experience with doing this sort of access to the SD
>cards?

Yes. It is mostly straight forward.  Reading is simpler than writing as far
as the FAT is concerned. You work your way through the FAT deciphering card
size, block size, file location, etc.

Mainly, for reading, you need to figure out where the file starts and then
begin reading it.  I found in my work here that formatting with Windows and
writing a file to the card did not always put the file at the same location
even though the card was formatted each time and only one file was written
to the card.  This prevented me from using a simple scheme of looking at
one address to find the file.

To keep it simple, I decoded enough info from the FAT to determine the
start of data on the card. Then I could jump to that location and begin
searching for "my" file.  There was only one file to deal with to, again,
keep things simple.

If you are comfortable with the FAT, you will have little problem getting
data off the card.  The level shifting is easy enough as well.  I used the
'F252 to take advantage of the large amount of ram.  Writing to the cards
must be done 512 bytes at a time so the 'F252 ram was set up as two 512
byte buffers.  The SPI mode was used to talk to the card and is straight
forward as well.

Good luck!
Tom

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