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

[Harald Kipp]

Aaron,

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.

Often the write transfer dropped to 30-40 kB/s over long periods
using no-name SD cards at 20 MHz clock speed.


>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.

Sure.



>Also, if I'm reprogramming the expansion port, is the CPLD used for
>anything other than SD/MMC and some interconnects?

It's also used for the RS232 port.
http://www.ethernut.de/en/hardware/enut3/memory.html
http://www.ethernut.de/en/documents/npl.html

Harald