In a message dated 25/05/2010 21:49:30 GMT Daylight Time, [email protected] 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
