Having just read the Sandisk tech spec in detail,
Bob is correct. One MUST complete the read command
transaction in under 100 msec, writes in under 250 msec,
or, per the spec, the card times out and returns an error response.

SanDisk Secure Digital (SD) Card Product Manual, Rev. 1.9
"
5.1.9.2. Read, Write and Erase Time-out Conditions
The times after which a time-out condition for read operations occur are (card independent) either 100 times longer
than the typical access times for these operations given below or 100ms. The times after which a time-out condition
for Write/Erase operations occur are (card independent) either 100 times longer than the typical program times for
these operations given below or 250ms. A card shall complete the command within this time period, or give up and
return an error message. If the host does not get any response with the given time out it should assume the card is
not going to respond anymore and try to recover (for example; reset the card, power cycle, reject). The typical
access and program times are defined in the following sections.
"

The question that is NOT answered is does the timeout begin from the
start of the command, or from the last clock edge supplied,
since the spec includes "0" Hz as the lower clock rate bound?

Reading elsewhere, since a 'busy' card can be deselected, so
others can be addressed,
the implication is that the card self times, regardless of clock
rate.

I guess someone is going to have to try low speed communication
with a card to see what happens. And then test several different
brands and sizes to confirm that all makers interpreted the
spec the same way.

Robert


Herbert Graf wrote:

> On Tue, 2004-12-07 at 13:47 -0700, Bob Axtell wrote:
> 
>>That's new to me. My documentation seemed clear, to me, it needed a 
>>burst. How does
>>one do that- just don't send the 8 SPI clocks/data until the data is 
>>ready to be sent? holding
>>the CS line down the who;le time? Interesting approach, actually that 
>>MIGHT work.
>>
>>Herb, is that how you mean? I gotta dig into my SanDisk SD book again.
> 
> 
> Of course. SPI is a synchronous protocol. There is no definition of
> "burst" outside of the clock you feed the client. Now, there are SPI
> devices out there that probably spec a minimum clock period, but all the
> card specs I've see allow you do go down to pretty much DC. TTYL

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