Modifying an Inexpensive Clock I like to be more accurate

[RubyRhod]

Hi.  First time poster and hope to gain some knowledge and share some as time progresses.

I have a clock I like that doesn't keep very good time gaining about a second a day.  The clock uses a HT48R30A-1 microcontroller and a 32.768 kHz crystal is used as the timing source across pins 17 and 18 of the MCU.  I am considering simply swapping out the 32.768kHz crystal for a SiTime SiT1556 TCXO with a +/- 3ppm accuracy.  Do you think I can simply inject the signal from the TCXO at the point where the crystal is soldered to as long as I can supply the appropriate VDD to the TCXO?

Also, there is noticeable differences in LED light levels across the alpha numeric characters per attached picture) that may be caused by differences in the as-measured values of the 47ohm and 330 resistors attached to the display devices.  I intend to replace those resistors with 1% tolerance jobs just to be sure.

Any thoughts would be greatly appreciated!

Thanks!

[Benta]

The oscillator should be accurate enough by itself, which leads to the question: why is C2 missing?

[ataradov]

This device has some strange and unconventional clocking options with datasheet being very brief on this. I think TCXO should work if you feed its output into the OSC1 pin.

But often simply replacing a crystal with something better is enough.

I doubt even 10% difference would be enough for appreciable difference in brightness. It is likely due to the way matrix is scanned. But replacing the resistors won't hurt.

[ataradov]

why is C2 missing?

This holtek has some weird clocking options. You can see that one capacitor is a valid configuration for "Internal RC oscillator with RTC". This is the only valid configuration for a 32 kHz clock. Full crystal oscillator needs two capacitors and a feedback resistor, but only works for higher frequencies. 

There seems to be no documented option for an external clock source.

[Gyro]

It might be worth trying replacing the missing C2 with a 2 - 22pF trimmer cap. It might give you a cheap and reasonably accurate frequency adjustment - as opposed to the manufacturer's ham fisted one!


P.S. Welcome to the forum!

[iMo]

Go with the external TCXO and feed it into the OSC1. Remove any capacitors on OSC1/2.
+/-3ppm TCXO gives you +/-8secs per month, provided my math is ok..

[RubyRhod]

Oddly the missing cap, the 10 pf, was on the other side of the board.

[RubyRhod]

Yep. The other 10 pf is on the other side of the board for some weird reason.

[RubyRhod]

Ordered from Mouser!

[MikeK]

I agree with Gyro, I'd go after the caps first.  2 x 10pF is likely too low for a typical watch crystal.

[strawberry]

43mA per LED? could be worn LEDs or something with MUX

[RubyRhod]

Thanks Strawberry.  The clock display was actually like this when brand new so I think it could probably be the muxing arrangement.  I should have sent it back but didn't and now it's too late and is a project.

[MikeK]

By the way, I made a clock not too long ago, with a microcontroller running from a cheap Chinese watch crystal.  I got it to under one second after two weeks.  I did the trimming in software, but it could have been done with the caps.  Plenty of reasonably accurate watches using those crystal.  So, it *can* be done, if you were so inclined.

[RubyRhod]

Excellent and good to know even cheap xtals can be tweaked. If I can't get the TCXO to work, I'll probably replace one of the 10 pf caps with a trimmer and start playing.

[MikeK]

Is the TXCO trimmable?  3ppm is 1.8 seconds per week.

[RubyRhod]

No but I can live with that and I'm feeling really lucky that Mouser will pick me one that is spot on    Just kidding.  I can live with that amount and, if I can't, I'll get the trimmer as mentioned above and start playing.

[golden_labels]

Also, there is noticeable differences in LED light levels across the alpha numeric characters per attached picture) that may be caused by differences in the as-measured values of the 47ohm and 330 resistors attached to the display devices.  I intend to replace those resistors with 1% tolerance jobs just to be sure.

I can see the same situation on product photos, e.g. on Amazon. So it seems it’s not that you received a unit with a particularly bad subset of resistors.

I would say it’s a design issue. Note how brightness corelates with the number of LEDs being on in a column. As if there was not enough current to supply to all rows at the same time.

Is the TXCO trimmable?

If you open the case and replace/add components

3ppm is 1.8 seconds per week.

Rubidium standard should provide better!

No but I can live with that and I'm feeling really lucky that Mouser will pick me one that is spot on    Just kidding.

Actually you have very good chances of obtaining something much better than the worst case specs. Thanks to the normal distribution.

[RubyRhod]

Also, there is noticeable differences in LED light levels across the alpha numeric characters per attached picture) that may be caused by differences in the as-measured values of the 47ohm and 330 resistors attached to the display devices.  I intend to replace those resistors with 1% tolerance jobs just to be sure.

I can see the same situation on product photos, e.g. on Amazon. So it seems it’s not that you received a unit with a particularly bad subset of resistors.

I would say it’s a design issue. Note how brightness corelates with the number of LEDs being on in a column. As if there was not enough current to supply to all rows at the same time.

No but I can live with that and I'm feeling really lucky that Mouser will pick me one that is spot on    Just kidding.

Actually you have very good chances of obtaining something much better than the worst case specs. Thanks to the normal distribution.

I agree with your points and it's probably a design issue.  As a side note, the clock has a three-setting intensity adjustment and, regardless of the intensity setting be it low, medium or high, the perceived intensity differences persist.

Yep, it's a purchase I made from Amazon.

[ledtester]

An alternate idea... replace the Holtek with an Arduino or a Wifi capable MCU module (ESP32 / RPi Pico W) and set the time using the Internet using the NTP protocol or via a WWVB module that you can get for around $8 on aliexpress/ebay.

[RubyRhod]

An alternate idea... replace the Holtek with an Arduino or a Wifi capable MCU module (ESP32 / RPi Pico W) and set the time using the Internet using the NTP protocol or via a WWVB module that you can get for around $8 on aliexpress/ebay.

That would be the ideal solution without a doubt but one that is way out of my league skill-wise.  I admire those of you who have the skill to pull off a modification like that!

I also found another solution and that is is a miniature OCXO by SiTime with a +/-3 parts-per-billion accuracy/stability that should get me to an accuracy of 1 second in 3,858.02 days (if my math is correct).

[tooki]

The matrix brightness is because of the matrix design, such that the current limiting resistor is shared across the whole column, so the lit LEDs within a column are essentially in parallel. The more LEDs lit, the less current per LED. I’ve seen the exact same thing on eBay/Aliexpress clock kits I’ve built.

[RubyRhod]

Thanks tooki.  I suspect that, in the end, I will not be able to resolve the issues with differences in dot brightness and will end up calling it a "feature" of the clock and living with it.  Or I'll toss it into the garbage bin 

[RoGeorge]

Is the clock ticking too fast, or to slow?

When we were teenagers we used to tune electronic wrist watches by adding a very small capacitor in parallel with the 32768Hz resonator. 

Since SMD parts were not yet a thing, and there was virtually no room inside a wrist-watch, the tuning capacitor was a DIY hand made:
- on a piece of 0.5-1mm thick CuEm wire of about 1-2cm long, wind 0.1-0.2mm diameter CuEm wire on the entire length of the thick piece of wire, one layer of thin wire, no interleave space
- cover all with nail lacquer and keep all tight until the lacquer solidifies
- at one side of the thick wire, remove the Em isolation for both the thick and the thin wires
- solder that in parallel with the 32768Hz resonator

Frequency adjustment is made by un-winding and cutting away a few turns from the thin wire each time, then let it run for 1-2 days to compare the time, then uncoil from the loose end a couple more turns and let it run a few more days again, and so on until you get a reasonable small time skew.  Leave uncut the empty end of the thick wire that remains after consecutive un-winding of the thin wire, always cut only the un-winded remaining of the thin wire. 

[RubyRhod]

Thanks much.  It's "ticking" too fast (about a second a day) and your suggestion makes sense.  I like the idea of building your own capacitor!  The resonator has a couple of 10pf caps in the circuit and I could replace one of them with a small variable and tweak to my heart's content.  I may do that if I can't get the TCXO already purchased ($4) to work out.

[RubyRhod]

I got the 1% resistors and the TCXOs.  The TCXOs are so darn small that I can't even "see" them let alone figure out how to solder to the pads 

I think I'll get a 2-22 pf trimmer and install it in place of one of the 10 pfs in the circuit and start tweaking.