[Dprglist] Stability of CPU oscillator

[John Swindle]

Murray,
Thank you for making me look at GPS again. I did not realize what the PPS signal was, and I did not know a receiver was that small. PPS will work well for my application. That one signal would be the most expensive part of the application, but I can deal with that later.
If the oscillators varied as much as some people worry, then a variety of industry specifications would be violated in consumer products. Those products would fail the compliance checklists and could not carry logos such as PCI Express, USB, etc., without being sued for false advertising. Most people write about accuracy, and not many write about stability.
Thank you very much for the GPS PPS idea.
John Swindle

-----Original Message-----
From: Murray Altheim via DPRGlist <dprglist at lists.dprg.org>
To: dprglist at lists.dprg.org
Sent: Fri, Jun 5, 2020 2:47 am
Subject: Re: [Dprglist] Stability of CPU oscillator

Hi John,

What I was trying to say is that the CPU's oscillator is well known to be
unstable, so if that's a requirement for your application that would most
certainly fail. This is also known to be exacerbated when running on an
unstable power source such as might be used in a robot, i.e., clock
stability is also related to the stability of the power source. This is
also affected by temperature, so as the Intel chip warms up its clock
speed will change, e.g. as a source:

    Clock Quality (Network Time Foundation's NTP Support Wiki)
    http://www.ntp.org/ntpfaq/NTP-s-sw-clocks-quality.htm#AEN1220

The values shown on the above page may help you judge whether the amount
of known drift is acceptable.

I was suggesting a GPS clock as a stable time source because it does seem
to fit your requirements. A typical GPS unit such as Adafruit's is US$29,
is 16mm x 16mm x 5mm, weighs 4 grams, and uses 20mA:

  https://www.adafruit.com/product/790

Indoors it will never track satellites but you don't need to be outdoors
and use the GPS feature itself to take advantage of the clock.

Cheers,

Murray

On 5/06/20 5:13 pm, John Swindle via DPRGlist wrote:
> Murray and Dave,
> 
> Thank you both for your ideas.
> 
> I've taught classes on Intel and AMD Family 6 Instruction Set Architecture and several microarchitectures of that family. I know how to correctly use the Time Stamp Counter and the Performance Monitoring Counters. The TSC will work 
> perfectly well for my application, but only if the CPU's oscillator is stable.
> 
> An oven-controlled crystal oscillator (OCXO) or GPS-disciplined oscillator (GPSDO) would be great, but I want something that fits on a robot, doesn't weigh much, doesn't draw much current, and doesn't cost much. FM radio 19kHz stereo pilot 
> works great, but FM does not penetrate metal buildings. AC line frequency jitters far too much and is only guaranteed to be stable on a 24-hour basis. TSC is by far the simplest solution, so long as the XTAL is stable. I wonder what the 
> typical spec is for the stability of the 100MHz reference clock in a consumer PC.

...........................................................................
Murray Altheim <murray18 at altheim dot com>                      = =  ===
http://www.altheim.com/murray/                                     ===  ===
                                                                    = =  ===
    In the evening
    The rice leaves in the garden
    Rustle in the autumn wind
    That blows through my reed hut.
            -- Minamoto no Tsunenobu

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