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We've all probably come across these kinds of electronic stopwatches at some point:

They almost always claim to count centiseconds (although this one counts in milliseconds), but do they really have such precision internally? And even if they do, does their display actually refresh to keep up with its counting speed, or does it use something slower, like 60 Hz (or less)?

ayane_m
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    Read the manufacturer's documentation and specs. Also note that every manufacturer's models are most likely different from other units. – Dwayne Reid May 13 '21 at 02:15
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    Most RTCs use 32768 Hz as the internal clock frequency, so the clock resolution will typically be about 30 microseconds. Those cheap passive LCD displays have relatively slow refresh times though, so they're incapable of anything like that update rate. You'd have to look at the spec sheet or take it apart to know what any specific model uses though. – user1850479 May 13 '21 at 02:16
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    Internally, they are probably using a 32.768kHz oscillator. A counter counts this to keep track of the time. Whether the counter literally keeps track of every pulse, or whether it runs through a divider first before being incremented/recorded into the counter (i.e. a divide ratio of 1024 means it is counting every 1ms.) is up to the designer. I have no idea what the refresh rate is on a reflective monochrome LCD. Presumably it's faster than an LCD monitor, but whether they bother to run it that fast is another matter. If I were them I would run it slooowww. – DKNguyen May 13 '21 at 02:17
  • Related question: https://electronics.stackexchange.com/questions/177844/why-do-we-use-32-768-khz-crystals-in-most-circuits – user1850479 May 13 '21 at 02:20
  • And why 32.768kHz. It's divisible by 2 and if a clock is counting time, there is a significant chance there is a 32.768kHz crystal inside. – StainlessSteelRat May 13 '21 at 02:23
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    @DKNguyen Dividing by 1024 would be only 32Hz. This is kind of an interesting question. If they have an MCU they can take the number and convert it with some math. If it's a simple ASIC it might be easier to use a 32.000kHz crystal. – Spehro Pefhany May 13 '21 at 02:45
  • what difference does the display refresh rate make? ... it has nothing to do with timekeeping – jsotola May 13 '21 at 02:55
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    @jsotola "Time reading" – DKNguyen May 13 '21 at 03:08
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    @DKNguyen nothing to do with displaying elapsed time when the stop button is pressed ... illusion of seeing accurate elapsed time when the stopwatch is running ... for most people, anything more than 20 Hz refresh rate is just a blur on the least significant digits – jsotola May 13 '21 at 04:07
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    If you had something that updated very fast, you could put it in the frame of a high-speed camera to help document what is going on. Of course you can count frames, too, but maybe it would be more convenient to have time displayed in the video. – user57037 May 13 '21 at 04:10
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    @jsotola And yet they're still displayed while the clock is ticking so you can't blame someone for trying to read them – DKNguyen May 13 '21 at 04:11
  • Unfortunately humans can't read at that refresh rate, so it may well be displaying junk at the precision part :D – Mitu Raj May 13 '21 at 05:00
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    The display update only has to be fast enough that when you've pressed the 'stop' button, you see a stable figure within maybe 200 mS that looks 'instant' to you eye. The display timing has nothing to do with the timing resolution, which is the responsibility of the MCU and the stop button. It's easy to measure its timing to its system clock, which could be MHz. Whether anything finer than 1/1000th of a second is meaningful to a mechanical button and human thumb is another matter. – Neil_UK May 13 '21 at 06:01

2 Answers2

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99% of the watches uses a 32768kHz crystal with a suitable divider (i.e. 15 bits for a 1Hz final clock).

How do they do the centisecond? Dithered counters. If you, for example, divide by 2 and on the next cycle divide by 4 the net result is one third of the frequency. With a suitable state machine for sequencing the dividers you can obtain any wanted ratio. Of course duty cycle and jitter are the most horrible things seen on this planet but who cares?

As for the display refresh rate, you have two frequency to handle: the segment clock (needed for the LCD to not burn off) and the commons duty (used to multiplex the 'digits', if there is a common for each digit, for example). The whole frame cycle is a full sequence of common drives. In short, it can be whatever is a compromise between contrast, flicker and power consumption. In ULP microcontrollers (like the MSP430) often you even low the refresh while the display is 'idle' and raise it when there's something interesting to see. I suppose that 32Hz and 64Hz would be popular choices (for obvious reasons).

Lorenzo Marcantonio
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  • Excellent answer. This video: https://www.youtube.com/watch?v=c7tb1QEdL_Y leads me to believe that 16 Hz is a more common choice than either 32 or 64 – ayane_m May 13 '21 at 06:23
  • Well, 16Hz is slightly below the flicker rate but if the crystal is slow to turn over you wouldn't notice it. I've seen FSTN panels with reaction time of 50ms so it's plausible. The MSP430x4 series guide has all the scary math for driving a glass – Lorenzo Marcantonio May 13 '21 at 06:25
  • Many false assumptions and conclusion here – Tony Stewart EE75 May 13 '21 at 10:40
  • This stopwatch does not measure in centiseconds, it measures in milliseconds – Tony Stewart EE75 May 13 '21 at 23:25
  • the refresh rate of the LCD has no contributing error on the accuracy – Tony Stewart EE75 May 13 '21 at 23:26
  • @TonyStewartEE75 I just noticed that! I was trying to find a graphic of a centisecond stopwatch, but somehow i picked a millisecond one without looking closely... They're also pretty uncommon – ayane_m May 14 '21 at 07:29
  • @ayane_m You can also have a nanosecond time interval counter with a 15 Hz refresh LCD display designed for low flicker if you wanted. – Tony Stewart EE75 May 14 '21 at 13:04
  • if you plan to use the visual section (as myself) to measure different things on a super slow motion camera, the capacity of a slow display will minimize the resolution and the accuracy. so the display will refresh every 60ms on a 16hz making it a terrible low resolution solution for video capture measurements. – javqui Apr 24 '23 at 13:12
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  • This stopwatch does not measure in centiseconds, it measures in milliseconds
  • the refresh rate of the LCD has no contributing error on the accuracy of the time interval counter. This is only limited by eye-on-target to hand coordination, without ever looking at the display until it has stopped.
  • a 1kHz PLL counter clock has a spec accuracy of 1ms in 10hr yet an untrained human error of pushing a button is far greater
  • The 1kHz PLL divided by 3-decade counters can lock onto a 1 Hz clock divided from 32kHz and retain the same accuracy.
  • the LCD display refresh rate has nothing to do with the time interval clock.
Tony Stewart EE75
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