[time-nuts] TAPR Thunderbolt Temperature Reporting Question

[Mark Sims]

Search the archives for discussions on the DS1620 chip issue:
Google "site:febo.com DS1620"

The key to the problem is explained in:
http://www.febo.com/pipermail/time-nuts/2009-February/036520.html


Basically the old rev chips have a high resolution mode of around 0.01C.  The new rev chips have a resolution of 0.0625C,  but the Tbolt firmware is not compatible with some changes made in the new chips,  and it winds up with the temperature quantitized to 1C steps.  

The old rev chips had two different ways of accessing the high-res temp registers.  The new chips have only one way.  Of course,  the Tbolt firmware uses the method that was eliminated from the newer chips.  BTW,  Dallas Semi apparently never told anybody about the changes and their current data sheet still talks about the old way of doing things.

The rev level of the chip is found at the end of one of the lines (usually the middle line) of text etched in the package.  It will end in something like E2 or D1, etc.  Chips E and above have the crude resolution.  D and below have the fine resolution.   All chips from Dallas Semi or their distributors will be the new rev.  The one place that I found older (rev C) chips was rentron.com

The sensor and oscillator are in that metal box.  Yes they have different time constants,  but are fairly well coupled.  You are better off isolating the Tbolt as well as possible from the environment (as long as you don't let the internal temperature get too high).  I have found rapidly diminishing returns below around 0.1C internal temp changes over an air conditioning cycle.  Below that level,  system noise and GPS effects become the limiting factors.

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