> Did you read all the bytes the card expected you to read, or
> did you run into a boundary condition error in your loop and
> only read 511 of 512 bytes expected for a block?

Hi Robert

Yes, I can read any number of complete and verfied blocks from
a file with a stream of CMD17. Each block is 512 data + 2 CRC.

The breakdown in comms comes when the Clk/DI/DO lines have
been idle for "some time". This can happen when a new file needs
to be accessed after time spent processing the previous one. I'm
busy cleaning up a few little s/w bugs that may be affecting my
perception of what might or might not be going wrong and causing
delays. Now that the project(s) are almost as I want them to be I
can start doing some definitive tests, with a stable working verion
for reference

> Thanks for asking the question. I learned something important
> about SD card I/O on slow processors (e.g. bit banging).

My processor isn't particularly slow. It's a dsPIC @ 33.88MHz,
with an SPI module speed of 1-bit 11MHz. It is slow compared
with the card's capability of 4-bit @ > 25MHz

Block retrieval time (actual data) is around 800us, although about
twice that time is spent waiting for the #FE token after the CMD17,
so time to get the last data bytes is > 2ms after issuing the CMD17.

It is an incentive to figure out how to get the native 4-bit transfer
working, if only for reads

One application is 16-bit signed stereo audio @ 44.1kHz, and it
manages that with time to spare using the current SPI set-up, so
it's not too shabby. But it could be better. There is a desire for
mixing two or more audio signals and very high speed would help.
If I wanted to do mixing now (eg fade-out / fade-in) I'd have to
drop the quality to 22050 for two, then 11025 for four etc so as
to maintain the same data rate as for 44100

Joe


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