I spent a little time reading the thread you referenced and going back to
the datasheets. You said,
David Lissiuk pointed out that my assumptions about the DS18x20's behavior
were incorrect. This may explain the rare problems with conversion,
especially when powered and unpowered sensors were mixed.
Basicallly, during powered temperature conversion, the bus is still unusable
because the data line is help low until a temperature is ready. I suspect
this is an early design decision, since the DS2450 will free the bus for
powered voltage conversions if you issue a reset.
Here is what the datasheet says:
If the DS18S20 is powered by an external supply, the master can issue
"read-time slots" (see the 1-Wire Bus System section) after the Convert T
command and the DS18S20 will respond by transmitting 0 while the temperature
conversion is in progress and 1 when the conversion is done.
It's not completely unambiguous but I conclude that the problem is that,
once the simultaneous conversion has been issued, all thermometers think
they are selected and respond to reads with a conversion status, low if
still converting and high if finished. This way they are all wire-ored
together and the result tells us if any of them are still converting.
So yes, apparently you can't use the bus during this interval unless there
is some way of deselecting the active thermometers without disturbing the
conversion in progress. I don't think the datasheet has anything to say
about that.
I mentioned the documentation because I would be happy to update the
documentation if you would tell me where you think the updates should go.
On Sun, Jul 13, 2008 at 03:34, Paul Alfille <[EMAIL PROTECTED]> wrote:
> Documented? Apparently not well. This note should help:
>
> http://www.mail-archive.com/owfs-developers@lists.sourceforge.net/msg02524.html
>
>
>
>
> On Sun, Jul 13, 2008 at 3:25 AM, Doug Collinge <[EMAIL PROTECTED]>
> wrote:
>
>> Ah, writing to '/simultaneous' actually seems to wait for the conversion
>> to complete. I didn't realize that. I'd better check the code here ...
>>
>> ... Yes, it does poll for completion of the conversion. I think that
>> explains a few things. I'll go back to the drawing board with this new info.
>>
>> Is there anywhere where this behaviour of '/simultaneous/temperature' is
>> documented? I mean, clears cache, polls for conversion.
>>
>>
>
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