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

[Ralph Mason]

Well Even at 16Mhz a AVR isn't going to approach anywhere near shifting 
20 megabytes a second.

Max you have 16,000,000 instructions per second to play with.  And you 
ain't going to shift a byte in less that 4 instructions, probably more 
like 50-100 instructions per byte (assuming you need to read the data 
from somewhere) - this gives you 16 KB - 32KB a second.  Big difference.

Ralph


> Howdy.
> I'm developing a very fast data-recorder.  Ultimately, I hope I can
> write 20MB /sec to a 133x CF or SD card, but for now, I'll settle
> for what I can get.
>
> 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.
>
> Can the CPLD get those kinds of clock rates?  It looks like I could
> use a state machine to handle the interface and just feed it data from
> the CPU or the expansion port.
>
> Also, if I'm reprogramming the expansion port, is the CPLD used for
> anything other than SD/MMC and some interconnects?
>
> Thank you for your time,
>   Aaron Burt
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