[time-nuts] Data sheet for the old version of Dallas DS18S20 used in TBol...

Tue May 25 21:43:15 UTC 2010

In a message dated 25/05/2010 21:49:30 GMT Daylight Time,  
hmurray at megapathdsl.net writes:

I have  data sheets for the DS1620 from 053105 and 072099.  The text on the 

fancy temperature conversion looks the same.

The difference might  be at the command/register level.  I took a quick 
scan 
and didn't see  any differences.

Is 072099 old enough?  Does anybody know what the  critical differences is?

--------------------

If I remember rightly, not always guaranteed these days:-), Mark Sims,  the 
programmer of Lady Heather, was the first to draw attention to the lower  
temperature monitoring resolution using the later versions of the DS1620,  
and would probably be best placed to answer your question.

I got the impression from previous discussions here, that all  versions of 
the IC, at least according to the data sheets, offer the higher  precision 
facility but something has changed, either in the way it's  implemented or in 
the command structure, such that the Thunderbolt firmware  is no longer 
able to switch it into the higher resolution mode.

I've just made a quick search of the archives and these two emails from  
Mark, under the subject TAPR Thunderbolt Temperature Reporting Question, were  
posted on 26th July 2009 and explain this  in some detail.

BTW
It's not reasonable to suggest that this only affects Lady Heather's  
display of data, rather it's more that Lady Heather is capable of drawing  
attention to what might not otherwise be obvious!!

1) ---------------------

The Tbolt uses the temperature reading to fine tune the oscillator  
disciplining. It has a major effect on holdover performance (when the GPS signal  
is not available) and some effect on normal performance. You can quite 
clearly  see the effect of the temperature reading on the DAC setting in plots.

If the sensor produced a temperature reading that was directly proportional 
 to the true temperature, any error should be minor. I don't think absolute 
 accuracy is as important as relative accuracy since the Tbolt learns how 
the  oscillator responds to temperature readings, but your readings are WAY  
off.

The fault is almost certainly in the small eight pin DS1620 chip near the  
RS-232 connector. There are two versions of this chip. The ones before Rev E 
 work much better than the later ones because they report the temperature 
in a  way that can be resolved by the Tbolt firmware quite finely (around 
0.01 C raw  values). The later chips report the temperature with a very crude 1 
degree C  granularity. Search the archives for the details.

I found that the chips from:
_http://www.rentron.com/Products/Electronic-Components.htm_ 
(http://www.rentron.com/Products/Electronic-Components.htm) 
are  Rev C chips and work well in the Tbolt. The chip itself is cheap at 
$3.50...  shipping is not... it ran me around $11. A group purchase/reshipment 
program  might be useful. I know of several Tbolts that had bad DS1620 
chips. I had  several, but wound up sending them out.

2) -------------------------------------

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_ 
(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.

------------------------

Hope this helps

regards

Nigel
GM8PZR