[time-nuts] Looking for RTC or some combination of MPU/crystal for0.1ppm accuracy.

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Sep 13 00:33:56 UTC 2009

The RTC mentioned definitely has a 1 sec time stamp granularity and I
can see no easy way of improving this as their is no 1Hz output signal
synchronised to the internal clock.
Whilst the 32.768kHz output is synchronous with the input to the clock
divider, this is of little help as there is no easy way to determine the
phase of the divide by 32768 prescaler used in front of the RTC.

The 1 sec time stamp granularity probably makes it difficult to
impossible to correct this system for frequency and phase changes using
a reference time signal once every 10seconds.
It would be better to implement a very long (no more than 1 counter
rollover between successive time reference events, either using hardware
or a combination of hardware and software) continuously running  counter
that is sampled by events of interest (including your time reference
You can then process the time reference time stamps and the other time
stamps to deduce the actual event times.
One can, in principle, discipline a timer using the reference time
stamps to produce periodic interrupts synchronised to the reference time
You will also need to take into account the various propagation delays
from the reference time source to each sensor and from each sensor to
the datalogger system to ensure the various sensors are accurately
synchronised and, potentially, to avoid data collisions at the data logger.


WB6BNQ wrote:
> Rick,
> I agree with Hal in another post that you are not giving enough information for a
> properly reasoned response.  So here are some questions:
> 1. What is the environment these sensors will be experiencing [temperature -
> humidity - NOISE etc.] ?
> 2. What are the sensors measuring ?
> 3. What is the purpose for the timed measurements [that is what is expected to be
> achieved with the time stamp] ?
> 4. How did you determine your need for a time stamp resolution ?
> 5. What is the duration of the measurement process [minutes/days etc.] ?
> 6. What is the expected time frame between being able to "check/verify" each
> sensor's clock accuracy [that is how often would the sensor be brought back to a
> shop/lab to re-adjust/re-align/calibrate (whatever) the sensor's various
> adjustments if any] ?
> 7. What do you mean by "time beacon ?"
> 8. What is the purpose of the "time beacon ?"
> 9. What is the form of this "time beacon ?"
> 10. How is the "time beacon" transmitted [i.e., by wire / wireless etc.] ?
> 11. How is the sensor expected to align with this transmitted "time beacon ?"
> 12. How is the sensor sending its information "out" ?
> 13. With multiple sensors, how are they to avoid colliding with each other when
> sending their data [is the response to be asynchronous or synchronous] ?
> As you can see, a proper answer is not so simple.  The Dallas DS32B/C35 stability
> is for a temperature range from 0c to 40c and may perform much better if the
> temperature of the device were held {more or less} constant.  However, that would
> have to be measured by you as Dallas does not directly make such offerings.
> They do have a rather curious spec called "Frequency Sensitivity per LSB" that
> shows the rate of error over temperature.  One could infer that for a temperature
> between +25c and +70c that the worse case error would be +/- 0.4 ppm.  However,
> you would need to include other error terms like voltage stability, etc.
> Another point to consider is the fact that the DS32B/C35 only records time at
> "one" second intervals.  The way I see it this determines the true granularity of
> your time stamps with respect to other asynchronous sensors.  That is to say, you
> can only determine the true time difference between sensors to the "one" second
> level.  I could be all wet on this point, perhaps Bruce will comment on this ?
> Bill....WB6BNQ
> Rick Harold wrote:
>> Hi,
>> I need to find a solution for a RTC which provides 0.1 ppm accuracy in the
>> form of a RTC/MPU or some other combination, oscillator etc..
>> I'm creating some sensors which all need to do timed measurements and need
>> to be at the same time.  They can get a time beacon
>> every 10 seconds but then need to accurately perform independently 20x /
>> sec.  They cannot communicate with each other and other than the time beacon
>> are unconnected.  After a period of time they will send their time-stamped
>> data out.
>> I haven't seen anything other than RTC DS32B35/DS32C35 which perform at
>> ~2ppm.  I need an order of magnitude better.  An atomic clock
>> would be great, but the cost/size has to be low as these are sensors and it
>> would be overkill.
>> Any ideas?
>> Rick
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