On 12/09/2011 1.24, IVP wrote:
>> I was not able to write at more than 64KB/s, continuous
>
> My preliminary timing tests show around that figure, if using a
> simple sequential UART fetch from source, SPI write to card
>
> Attached shows typical card behaviour, which results in an
> average storage of of ~ 61kbps
>
> This is seen as 7 fast sector writes followed by 3 slow ones (a
> looooong 170ms Busy), which really drags the average down. If
> the writes were all as fast as each of the fast 7, the average would
> go up to about 240kbps. The lower traces shows a transfer in
> more detail
It is related to the SDcard internal buffer.

>
> The simplest step to improve speed would be concurrent fetch
> and write, and I've ordered dsPICs with more I/O to try 4-bit
> parallel
I think not! You will saturate the internal buffer faster but after that yo=
u=20
will wait 170ms...
Of course the average speed will be little better BUT if you need to store =
data=20
at costant rate will be in trouble.


>
> I love to know why the card does that 7-3 thing and if / how it
> can be got around. Timing patterns are consistent and repeatable.
> Maybe it's just how the card works with SPI
I tried only SPI but different cards and the behaviour is the same only=20
different buffer size and "wait" time (better card has smaller wait time).

>
> Joe
>
>

--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.