[time-nuts] accurate 60 hz reference chips/ckts

[Vlad]

My setup is pretty simple indeed. This is 9.830400MHZ OCXO which 
clocking MCU. Then it is Zero-Cross detector which connected to capture 
timer.
The MCU counting the intervals between of each zero-cross event and 
number of events occurred.

     if (htim->Instance == TIM5 && htim->Channel == 
HAL_TIM_ACTIVE_CHANNEL_1) {
         uwIC2Value2 = (HAL_TIM_ReadCapturedValue(htim, TIM_CHANNEL_1));
         if (uwIC2Value2 >= prevtimer) {
             uwDiffCapture = (uwIC2Value2 - prevtimer);
         } else {
             uwDiffCapture =((0xFFFFFFFF - prevtimer) + uwIC2Value2);
         }

         prevtimer = uwIC2Value2;

         // MAIN Time calculation
         if(++uwCapT > 59) {                  // Every 60 cycles (60hz)
             if(++maintime.Seconds > 59) {
                 maintime.Seconds = 0;
                 if(++maintime.Minutes > 59) {
                     maintime.Minutes = 0;
                     if(++maintime.Hours > 23)
                         maintime.Hours = 0;
                 }
             }
             uwCapT = 0;
        }

Once an hour the program prints its internal clock (MCU time), RTC time 
and MAIN time

# Uptime:       476 hours
# RTC time:     13:00:00
# MCU time:     13:00:00
# MAIN time:    13:00:01

The graph shows the "delta" between of RTC and MAIN.

On 2017-12-15 12:14, Jeremy Nichols wrote:
> I'm surprised Vlad is seeing as much as six seconds differential but
> maybe I don't understand the experiment. I've done measurements of the
> line frequency here in California and never seen much variation.
> 
> Jeremy
> 
> On Fri, Dec 15, 2017 at 9:02 AM Vlad <time at patoka.org> wrote:
> 
>> I have one of my project boxes, which monitor the main freq. Here is
>> graph which reflect the time difference between of RTC (based on
>> number
>> of pulses from OCXO) and the "MAIN TIME" which is based on number of
>> zero-cross events.
>> The observation period is 486 hours.
>> 
>> On 2017-12-14 23:13, Jim Harman wrote:
>>> On Thu, Dec 14, 2017 at 10:53 PM, Bob kb8tq <kb8tq at n1k.org> wrote:
>>> 
>>>> 
>>>> 
>>>> Of course this *assumes* an electronic approach. Given that
>> it’s
>>>> moving
>>>> pretty slow and you
>>>> only are looking at fractions of a millisecond, one *could* do an
>>>> electro
>>>> mechanical design …...
>>>> 
>>>> Bob
>>>> 
>>>> There is interesting background on power grid frequency/time
>>>> adjustment
>>> procedures here
>>> 
>> 
> https://en.wikipedia.org/wiki/Utility_frequency#Time_error_correction_(TEC)
>>> 
>>> and here
>>> https://en.wikipedia.org/wiki/Electric_clock#Accuracy
>> 
>> --
>> WBW,
>> 
>> V.P._______________________________________________
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> 
> --
> 
> Sent from my iPad 4.

-- 
WBW,

V.P.