[time-nuts] MM5314 remote setting
tshoppa at gmail.com
Thu Oct 20 08:15:33 EDT 2016
This long thread reminded me of a technology that my employer used from the 1970's till just a few years ago.
Our system had hundreds of HH:MM LED clocks for the public and we opened in the 70's so of course they were digital clocks.
I had always imagined that there was some fancy electronics in the LED clocks because they always seemed to be well synchronized.
I was surprised during their decommissioning a few years ago, to find that the clock modules were a very simple 60Hz driven LED clock chip with the remote synchronization done by jam loading hour pulses and day pulses via audio tones decoded with twin-T audio filters. Our in-house telco system distributed the audio tones by what was effectively a systemwide PA channel. The audio tones went out in the second preceding the hour. Simple steering diodes were used to double-count at 120 Hz if the clock was running slow at the hour pulse, and to hold off 60Hz clock pulses if the clock was running fast or on time at the hour pulse. The diodes decoded the multiplexed digits not with latches, but by simply charging RC circuits. And the whole clock was just reset to 12am every night by briefly dropping power.
The clocks were replaced by LCD screen displays that for a while showed the time but due to synchronization difficulties the clock display on the LCD display has been turned off.
Ironically for both the old clocks and new signs, they were only ever displayed for the public. Train operators can never see a system clock, and station managers usually brought their own clock in because they cannot see the clock on the front of their kiosk.
Sent from my iPhone
> On Oct 19, 2016, at 11:32 PM, Lee - N2LEE via time-nuts <time-nuts at febo.com> wrote:
> 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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