[En-Nut-Discussion] Nut 3: SD/MMC speed?

[Aaron Burt]

On Fri, Jan 27, 2006 at 09:21:03AM +0100, Harald Kipp wrote:
> At 17:16 26.01.2006 -0800, you wrote:
> 
> >Assuming the card is fast enough, how fast can the ethernut write to the
> >MMC port?  For some perspective, MMC supports a 20 MHz clock, which
> >means ~2.4 MB/s, and SD supports a 25 MHz clock and 4 bits, giving
> >~10MB/s.
> 
> Using the FTP client on Ethernut 3 I observed transfer rates of 90 kB/s
> for writing and 300 kB/s for reading. I assume, that this can be
> increased somewhat by optimizing the file system code and the MMC
> interface, which is not yet interrupt driven. Also note that this
> was pure data throughput, not including TCP/IP headers or FAT table
> updates. But as Jesper explained, MMC cards are slow in writing.

Thank you.  So it sounds like there's some work that can be done.  If I
can get 50 MHz out of the timer chip and use 4-bit transfer mode, I
could theoretically write to 133x SD cards at their full rated 20 MB/s.

Once I can get a board and start fiddling with it, I'll let y'all know
how fast I can push the SD/MMC port.

I know that with interrupts and such, there's no way the ARM can push
out that much data, but I'm planning on using a state machine in the
CPLD to pull data off an ADC on the expansion port and push it straight
to SD.

Thank you for your answers,
  Aaron