[time-nuts] Portable Time Standard

[Joe Hobart]

Hello Petr and others,

Thank you for the suggestions.  I have several DS3231 modules running.  I was
able to adjust one to 0.04 PPM fast, which is more than good enough for my
requirement of one second a month (this is at a constant temperature, but these
RTC chips are supposed to be temperature compensated).

I can read and write the DS3231 registers with a Raspberry Pi.  Unfortunately,
the RPi kernel sends time to the RTC every 11 1/2 minutes.  This time is usually
moderately accurate, but I have measured errors of up to 0.264 second, which is
unacceptable.

Does anyone have suggestions for a very low power controller and display for
this RTC integrated circuit?  The DS3231 has a 1 Hz square wave output that
could signal the controller to read and display the time and date from the
DS3231.  The controller could sleep between updates, which would conserve batteries.

Thanks again for all the suggestions,
Joe

By the way, the manufacturer of the high accuracy marine chronometer I was
considering will not send specifications for other than 22 +/- 1 degree C.


These DS3231 modules are a complete clock

On 2/17/2019 3:46 PM, Petr Titěra wrote:
> Hello,
> 
> as I've said these are quite new, cheap RTC modules. There is DS3231SN
> on them. Based on markings they were manufactured somewhere in 2008 but
> I've bought them recently.
> 
> I have two those modules. Both connected using I2C.
> 
> First module is connected to Orange Pi sitting on my table. Pi is
> running NTP and is controlled by NEO-6M GPS module (not timing, I know
> butat this level accuracy of PPS signal should be enough).
> 
> I measure in 10 seconds intervals difference between system time (which
> I consider stable in long term) and time reported by RTC chip as
> reported by adjtimex in compare mode.
> 
> This is a little bit naive setup because timing from RTC is noisy but in
> long run (its running for 25 days now) it should give me reasonable results.
> 
> I did modify offset register. Again using naive approach. I've let run
> unmodified module for one day and from that I did compute offset required.
> 
> For second module I'm meassuring just now I don't care about time
> reported and meassure only interval between rising edge of its PPS
> pulses and PPS pulses from another GPS. Again for this measurement I
> consider GPS PPS signal as stable.
> 
> This second one is a little bit worse (or may be wrongly calibrated) but
> I still get about 0.3PPM from my measurements.
> 
> Petr Titera
> 
> 
> 
> 
> Dne 17. 2. 2019 v 19:00 Joe Hobart napsal(a):
>> Hello Petr,
>>
>> Your accuracy results are impressive.  I have questions:
>>
>>    What manufacture or brand DS3231 do you have?
>>
>>    What do you use to communicate with the DS3231 and display time?
>>
>>    How long has your DS3231 been running?
>>
>>    Have you changed the crystal aging offset register(s) to compensate for
>> crystal aging?
>>
>> Thank you,
>> Joe
>>
>>
>> On 2/15/2019 12:51 PM, Petr Titěra wrote:
>>> Hello,
>>>
>>> sorry to reopen this old thread but it took me some time to measure. As
>>> I've said I have one RTC module sitting right on my desk in my room
>>> where temperatures can go anywhere between 3C and 20C.
>>>
>>> Using naive method of comparison of time difference between system time
>>> and RTC module I get time difference only about 4.487869e-02 seconds in
>>> 22 days.
>>>
>>> Petr Titera
>>>
>>> Dne 14. 1. 2019 v 16:11 Joe Hobart napsal(a):
>>>> Hello Petr,
>>>>
>>>> I have not found a low power RTC chip that will keep 1 second a month with
>>>> changes in temperature.  This clock will be used to synchronize a portable
>>>> communications system.
>>>>
>>>> Thank you,
>>>> Joe
>>>>
>>>>
>>>> On 1/13/2019 2:23 PM, Petr Titěra wrote:
>>>>> I will probably sound dumb but did you consider cheap RTC chips? I have
>>>>> one based on DS3231 currently on my table and although its is specified
>>>>> to have precision +/-2PPM my seems to be in range about 0.5PPM right
>>>>> from package with possibility to do some corrections.
>>>>>
>>>>> Petr Titera
>>>>>
>>>>> Dne 11. 1. 2019 v 20:57 Joe Hobart napsal(a):
>>>>>> I need a relatively lightweight, self powered, portable clock accurate to 1
>>>>>> second per month.  Temperatures may range from 10 to 35 C; altitude from 1000 to
>>>>>> 7000 feet.  Although expensive, some of the marine quartz chronometers appear to
>>>>>> meet this requirement. ~0.3 PPM.
>>>>>>
>>>>>> Does anyone have any experience with these devices?
>>>>>>
>>>>>> If so, has anyone tried to detect the probably weak magnetic field generated by
>>>>>> the clock mechanism for time comparison purposes?
>>>>>>
>>>>>> Thanks,
>>>>>> Joe, W7LUX
>>>>>>

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