[time-nuts] Tbolt temperature sensor

GandalfG8 at aol.com GandalfG8 at aol.com
Thu Feb 5 10:14:55 UTC 2009

In a message dated 05/02/2009 06:25:52 GMT Standard Time,  holrum at hotmail.com 

Actually,  what is apparently going on with the newer Thunderbolts  is worse 
than that.  The reported temperature seems to only end in 0.25C  or 0.75C,  so 
it has an effective 0.5C resolution.   The basic  DS1620 resolution is 9 
bits,  but the Tbolt firmware reduces that to 8  bits since the first step in the 
high res temp algorithm is to mask off the  lower bit.  Whatever is going on  
they are not getting the extra  resolution that they think they are,  and in 
fact they are reducing the  basic resolution of the chip.

The firmware does seem do do some  filtering on those values since whenever 
the reading steps you can see some  smoothing  going on.  A lot of times the  
temperature value  oscillatates around the step point.  The filter apparently 
does not have  any hysteresis.

The older Thunderbolts produced a nice smooth  curve.  The high res 
temperature reading (Bruce says is 12 bit/0.0625C)  coupled with the firmware filtering 
gave temperature curves with microdegree  scale resolution.  The newer ones 
clunk around with effectively 0.5C  resolution.

But have you actually established this has anything whatsoever to do with  
the oscillator conditioning and, if not, what effects are you suggesting it has  
on the conditioned output?
Isn't it likely that a temperature sensor adjacent to the RS232 connector  is 
just going to monitor unit temperature for environmental  purposes, perhaps, 
for example, to give the option for flagging up an  overheating situation, in 
which case surely 0.5C resolution is more than  adequate and the "clunkiness" 
isn't really an issue?
I agree it's always nice to know what's happening, and why, but I  suspect 
the performance of this sensor has no relevance to the  oscillator performance 

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