On Thu, 2011-09-29 at 14:49 -0400, V G wrote:
> Looks good. I'm going to try my SPI SD card method first. I have a good
> feeling about this. SD says max 50 MHz via SPI which is more than enough =
for
> me.

You may have to try a few cards.

While 50MHz is the physical clock rate, you won't get anywhere near that
throughput.

The reason is protocol overhead (which isn't the big) and write time
(which is HUGE). You'll have to hunt for a card with as short and as
consistent a write time as possible (IIRC SD cards have a write block of
512 bytes).

Even bit banging, I'd notice some cards would take many queries of "are
you done" before they were done writing.=20

For example, here is a web site where someone measured the performance
of their 2GB SD card:

http://forum.xda-developers.com/archive/index.php/t-242802.html

Details:
Card tested "Storage Card"
Min. read time: 0.25ms
Max read time: 18.00 ms
Avg. read time 1.87ms
Sector/block: variable 4-64
Min write time: 0.50ms
Max write time: 16.00 ms
Avg. write time: 4.02ms
Read/write ratio: 0.46
Sector size: 512 bytes
Start Sector: 0
End Sector: 3910656
Total Sectors read: 2504
Total data read: 1.00MB
Total read time: 4673 ms
Total sectors written: 2504
Total data written: 1.00MB
Total write time: 10057ms

Notice the HUGE variability of read and write times? That's the killer
for apps the need consistent write performance.

I'd assume newer cards are better, but I'd love to hear back what stats
you end up getting?

TTYL

--=20
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