[time-nuts] 1PPS to 32.768 khz

Tom Van Baak tvb at LeapSecond.com
Wed Oct 19 16:51:23 EDT 2016

Hi Brooke,

> So why not bypass the 32768 and drive the clock directly from the 1 PPS you now have.

Right, but that trick only works with analog stepper motor clocks. OP has a "big digital clock" with 8-bit cpu and 32 kHz xtal. He didn't mention the make/model of digital clock but in my experience very few commodity clocks actually accept a 1PPS input. These clocks use 32 kHz:

1) to drive the MCU which computes day / date / hh:mm:ss, or manages alarms
2) to maintain timekeeping
3) to multiplex digits of the LED / LCD display (e.g., at 128 to 1024 Hz)
4) to create the short bipolar stepper motor pulse (e.g., 1/32 kHz * 512 = 1/64 s = 15.6 ms).
5) to create the sound for the alarm/buzzer (some PWM based on 32 kHz)

The problem is that all these functions are usually integrated into one chip or even raw die/epoxy as in COB (Chip On Board). When hacking these sort of clocks it is often impossible to separate 32 kHz frequency features from the 1 Hz timing feature.

So when your goal is to improve timekeeping accuracy in these self-contained digital clocks it's usually easier and less invasive to make the clock use your precise 32 kHz signal instead of its own cheap xtal. You almost always have access to the xtal, but rarely access inside the MCU.

Note that you don't even need to unsolder the xtal -- you can "jam" the existing signal with an external 32 kHz sine or square wave applied to the XI pin (xtal in) of the MCU. Your external GPSDO/32kHz signal will "pull" the cheap xtal for free. Best yet, if your external signal goes away the clock keeps running using its own xtal without skipping a beat, like getting hold-over for free.

For a "no solder" or "no wires" solution, I have also tried to acoustically discipline a tuning fork xtal with an GPS-based 32 kHz signal and ultrasonic transducer. Poor results. I think I needed better coupling between the transducer and the xtal tuning fork. But in theory it should work. Plus it would keep small mammals and insects away from your clock.


----- Original Message ----- 
From: "Brooke Clarke" <brooke at pacific.net>
To: "Lee - N2LEE" <lee at n2lee.com>; "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Wednesday, October 19, 2016 12:18 PM
Subject: Re: [time-nuts] 1PPS to 32.768 khz

Hi Lee:

32768 can easily be divided down to drive the clock.  So why not bypass the 32768 and drive the clock directly from the 
1 PPS you now have.

Have Fun,

Brooke Clarke
The lesser of evils is still evil.

-------- Original Message --------
> First let me say this is first time I have posted to the group so go easy on me. :)
> Secondly I want everyone to know that you guys make me feel so NORMAL for
> wanting to use and understand accurate timing devices.
> I thought there was something seriously wrong with me now I know there are others
> affected with the same disease. hehe
> Now my questions.
> 1. Does anyone know of a device that will take a 1PPS GPS timing signal and generate a 32.768 kHz sine wave output ?
> I have big digital clock that uses an 8 bit micro processor and an external 32.768 crystal. Obviously the external crystal is
> awful for accuracy.
> I have searched every where using as many search terms as I can think of and can’t believe there is not a device that performs
> this function.
> I have found a couple of Epson RTC chips that might come close that 1pps but I don’t think that corrects the 32khz just the clock time.
> 2. 10 Mhz Freq Standard
> I am not in the same league as the majority of the list members and just starting to dabble in GPSDO stuff.
> I have tried to find a thunderbolt as a starting device but it appears those are either dried up or people want too much money.
> I wanted to get the opinion of anyone who has tried the Leo Bodnar GPSDO ?
> For the money it appears to offer a beginner a lot of features.
> Thanks and reading the daily digest of what you guys are working on.
> I have a feeling if I am not careful you guys could cost me a lot of money. hehe
> Lee

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