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'Temperature Measurement with DS1820'
1998\10\01@200728 by Donald L Burdette

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I have extensive experience with Dallas chips, and just got done
implementing a board that includes a DS1821 (same as DS1820 but with no
ROM serial #), and which can read DS1820 and DS1920 devices.

The timing is not at all critical, and the current specs allow +-50%
tolerances on the timing.  Dallas is considering tightening the specs,
but haven't done so yet.  Actually, tightening the specs on their end
eases them on your end.

There are several timing parameters you need to get right, but I suspect
you already know that.  If you don't have a copy of Dallas'   "Book of
DS19xx Touch Memory Standards", I highly recommend you get a copy.  It
details the specs quite well.  There's also a lot of software and samples
available from Dallas, much of which is on their web site,

One bit of the timing spec that's commonly misunderstood is in the bit
read time slot.  Once the master starts a bit time slot, the slave will
hold the data for 15 to 60 microseconds.  This means that the data may go
away as soon as 15 uS after the start edge, thus the master must sample
the data BEFORE 15 uS.  The spec is often interpreted to mean that you
must sample before the 30 uS nominal or 60 uS max bit time.

Good luck.  If you have specific questions, e-mail me direct at

                               Don Burdette
                               Senior Engineer
                               TEK Industries

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