[time-nuts] 1PPS to 32.768 khz

Lee - N2LEE lee at n2lee.com
Wed Oct 19 23:32:35 EDT 2016

Tom nailed the issue.

First problem is I was native in thinking “Oh this will be easy to interface to the NTP or GPS”.  WRONG  :) 
But the good news I am learning a lot about accurate time from you guys.

The second issue is Tom is right. This is a cheap jumbo clock that at the heart uses a Holtek HT48R30A
8 bit processor. Everything is contained in the chip except the 32khz crystal and led drivers.


This is certainly not the most sophisticated clock chip available.

My original idea was to hijack the timing signal and replace it with something more accurate. But the more info
you guys share the more I see there are a couple of ways to do this. Obviously the easiest might be to just replace the
crystal with a TCXO and hope for the best. But my guess as soon as it is off by one second from my other sources I will
be back into tearing it apart again. LOL

A lot of my other clocks are 6 digit NTP POE clocks so they are not GPS accurate but I would at like them to all agree.

Lee - N2LEE

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.


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