[time-nuts] 1PPS to 32.768 khz

Bob Camp kb8tq at n1k.org
Wed Oct 19 18:14:32 EDT 2016


As has been already mentioned, a lot depends on what you have. The drop dead cheapest way to do it:

Start with an MCU with an internal oscillator. There are *lots* to pick between. Which sort really does not matter. 
For example, I’ll use one that starts at 4 MHz. 

Divide the 4 MHz down to 32,768, or as close as you can get. The common clocks aren’t going to divide
straight to 32768 so you will need to do two divides. One will be a bit fast, the other a bit slow. You flip between the 
two in a fixed pattern to get the result to average out correctly. 

Next take the same 4 MHz and run a counter /timer off of it. Capture the pps edge with the timer. It will drift
a bit since the clock in the MCU is not perfect. Based on the drift, modify your dividers to correct the outcome. 
If you are “good” to 1 part in 32,768 in each second, that’s close enough for a wall clock. You will have no net
error long term if you do it right. 

That’s all a sub $1 solution….


> On Oct 18, 2016, at 10:03 PM, Lee - N2LEE via time-nuts <time-nuts at febo.com> wrote:
> 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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