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Arduino Pendulum Clock Design - Comments Welcome

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John Haine19/11/2020 22:14:17
3429 forum posts
184 photos

Yes, I think that would be preferred, would save hacking about with an SMD PCB.

Michael Gilligan20/11/2020 12:00:14
16654 forum posts
725 photos

Just been having a quick look at crystal oscillators

The oven controlled ones are still pretty pricey, but I was impressed by these two: **LINK**,4294742312

[At first comparison, the cheaper one appears to be the same thing in a smaller package]

12.8MHz seems a useful starting point if you want 6.4MHz at pin_5

... and the claimed performance looks adequate for checking the behaviour of Dave’s pendulum.



Edited By Michael Gilligan on 20/11/2020 12:28:24

Michael Gilligan22/11/2020 14:53:39
16654 forum posts
725 photos

Please permit another digression ...

I’ve just been sorting some old reference documents, and found a copy of this [on the construction of an digital Sidereal Clock] from 1987 : **LINK**

Obviously there are alternative approaches to the construction, but the underlying principle may be of use.


SillyOldDuffer22/11/2020 16:06:11
6469 forum posts
1422 photos
Posted by Michael Gilligan on 22/11/2020 14:53:39:

Please permit another digression ...



Thanks Michael, this note at the end did much for my morale!


Written in 1987 so some things never change. Glad I'm not the only one having trouble with hardware!

Today's report.

I've got a Nucleo64 measuring period using the same Counter/Timer technique as the picPet. Disappointing because the results are very unstable, ±20µS at least:


If picPet was a Spitfire the Nucleo would be a Eurofighter. Nonetheless, the superchip is inferior! Reasons include:

  • Although the Nucleo clocks at 180MHz giving pulse resolution down to 0.06nS, it's derived from an ordinary 8MHz crystal. Any timing errors are multiplied by 22.5 (This may explain why the RaspberryPi is also poor in the Nanosecond region. Fine resolution comes from a slightly wobbly crystal oscillator that's been multiplied many times to get speed rather than accurate time.)
  • Nucleo counter-timers favour versatility and functionality over straightforward counting. Due to the bells and whistles I'm not convinced input events can be accurately synchronised with the system clock by this chip.
  • Rather than manage the complicated control registers directly I used a library, which might add jitter. It could be innocent - I haven't attempted to understand how the library works!
  • Nucleo boards are programmed with an adjacent snap-off ST-Link bootloader . It's aother STM-microcontroller set up to appear to the PC as a USB connected folder. Copying an executable image to the folder causes the bootloader to install it on the target STM-microcontroller. Whilst the bootloader CPU is driven by the 8MHz crystal the main CPU is clocked by an ordinary output pin on the bootloader chip. As the bootloader provides an indirect clock the target CPU's timekeeping must be worse than the original crystal. Should be fixable because the main CPU can be fitted with it's own crystal, or a precision oscillator.
  • Another possibility is Nucleo counter-timers will accept an external clock running up to 45MHz, which could be a precision oscillator. But if I read the datasheet correctly, the CPU synchronises external clocks with its own clock, which is unstable.

Copy of the program here if anyone's interested. Using it needs the STM32 environment to be installed on the Arduino IDE, and then plugged into a Nucleo64 board: I used an F446RE.

Still got some testing to do, but it appears Nucleo64 isn't good for this particular application. I could be wrong!


Michael Gilligan22/11/2020 16:21:25
16654 forum posts
725 photos
Posted by SillyOldDuffer on 22/11/2020 16:06:11:

Nucleo counter-timers will accept an external clock running up to 45MHz, which could be a precision oscillator. But if I read the datasheet correctly, the CPU synchronises external clocks with its own clock […]


Anything is possible, I suppose ... But one would hope that its own clock is synchronised by reference to the external clock.


Michael Gilligan22/11/2020 21:52:21
16654 forum posts
725 photos

Still rootling around for historic performance data ... I’ve just found this from 1902

Reifler No.56


I think the method of fine-adjustment for rate tells us a lot.


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