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Precision Clock intermittently garbles date-time display
avdmart Posted: 15 Feb 2020, 09:02 PM
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I found other reports on the forum of this issue and some photos posted on the topic "My precision clock doesn't work". I decided to create a new topic as my clock does work but seems to have an intermittent problem of garbling the date-time display. The photos on the other topic look exactly like what I see so I am not going to post more photos of the same issue.

The backstory:
I received my kit a few weeks ago and built the clock the same day. The assembly went very smoothly. I am very experienced with soldering and have designed and built many electronic gizmos over the years. The web instructions were very clear and the clock worked on first power up without any obvious soldering or assembly issues.
I was not able to get a GPS fix in my 'lab' so following the advice on this forum, I took the clock outside to an open area. I powered it with a USB phone battery booster brick. I let it run outside in the open for 2+ hours and still no fix. My handheld Garmin GPS unit had a rock solid fix at the same location after just a few minutes.
During this time I noticed the weird date characters and assumed that this was a symptom of not having a good GPS fix which was mentioned on this forum.
I ordered the recommended active antenna from Digikey. Once I received and installed the new antenna the GPS unit got a fix in my lab a few minutes after power up. I had blinking colons and the LED on the GPS unit and the LED on the back of the clock were blinking once a second as well. Awesome!

Now that I have a good GPS fix I find that the garbled date issue still happens intermittently. Every time I have observed the garbling the LEDs are blinking indicating a good GPS fix. The clock will display the correct date and time (right now as I type this: "2020-02-15 12:52:21.00") but it will eventually revert to garbling the date-time display. It does recover on it's own. The garbled display will persist for several minutes to hours with the weird characters changing occasionally. So far today as I type this it has been spot on with the correct date and time for the last hour or so. I have not powered down the clock since I first booted it up with the active antenna installed on the GPS unit.
I suppose it could be temperature related. I have not done any scientific study of this idea but my anecdotal observations are that the issue seems to happen when it is cold in the room where the clock is. It is running perfectly right now with the room at 68F (20C). When I have observed the garbling, the room is typically much cooler. At night this room will get down to 50F (10C).

Finally my question(s)...
What should I do to resolve this?
My best guess as to the problem is the serial data baud rate issue described in other topics that has been attributed to the microprocessor internal oscillator accuracy.
Should I add the optional external crystal oscillator and caps for the microprocessor with the assumption that this will provide more stable communications between the GPS module and the microprocessor? I am not 100% clear why this option was provided on the board...
Maybe I need a replacement microprocessor that has an internal oscillator that is closer to the nominal frequency? Something that was suggested on the forum.
Or maybe it is something else like a defective GPS module or a bad solder connection somewhere?

Please help me to get this very cool clock to work perfectly!

Dana

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mit Posted: 15 Feb 2020, 11:02 PM
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yeah whatever

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Hi Dana, that does indeed sound like a problem with the internal oscillator. I'll post another chip to you, that's the fastest solution.

I just recently wrote a calibration script for the chips, so this problem should be fixed for kits sent out in future.

The optional oscillator footprint was a last-minute addition just because there was room on the board. In order to use it the chip needs reprogramming, so there's not much point populating it without doing that.

It sounds like the GPS module is probably fine, but if you wanted to check on it you could listen to the serial data with a USB-serial device.

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avdmart Posted: 16 Feb 2020, 06:02 PM
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Thanks for quick response.

I observed the clock early this morning when it was 52F in the room and it was garbling the display. I turned the heat back on and the room is now 63F and the clock is displaying the correct date-time: "2020-02-16 10:00:34.xx"

Replacing the micro-controller sounds like the most expeditious solution to the issue.

Do you have my mailing address to send a replacement chip to?

I ordered the clock 10-Jan-2020 and I live in Poulsbo Washington USA.

Dana


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avdmart Posted: 29 Feb 2020, 10:02 PM
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I received the replacement ATtiny micro-controllers about a week ago. I install the alt-calibrated controller and ran the clock for several days. It never garbled the day-time. It has not been as cold here as when I first reported this issue. Today I put the clock out in the garage where it was 50F and I let it sit powered off for an hour. I powered it up and let it run for several hours where it remained in the low 50 F temperature range. It never garbled the display. It seems the improved oscillator calibration routine that Tim came up with has corrected the issue with my clock.

Thanks for the follow up and the effort to get my clock working perfectly.

Dana

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