[time-nuts] 60 Hz line quirks, anybody recognize this stuff?

David davidwhess at gmail.com
Mon Sep 3 17:23:28 UTC 2012


On Mon, 03 Sep 2012 12:00:53 -0500, Graham / KE9H
<timenut at austin.rr.com> wrote:

>On 9/1/2012 1:35 AM, Hal Murray wrote:
>> The context is using the 60 Hz line for timing.
>>
>> I'm feeding 60 Hz from a wall wart transformer into a modem control signal
>> that the kernel PPS stuff watches.  Mostly, it works as expected, but
>> occasionally, it picks or drops a cycle.
>>
>> In order to understand what was going on, I fed the same signal into the
>> audio input and setup a job to capture the audio.  Here is an example of a
>> pick:
>> http://www.megapathdsl.net/~hmurray/time-nuts/line/2012-Aug-09-a-pick.png
>> http://www.megapathdsl.net/~hmurray/time-nuts/line/2012-Aug-09-a0.png
>> http://www.megapathdsl.net/~hmurray/time-nuts/line/2012-Aug-09-a1.png
>>
>> OK, that somewhat makes sense.
>>
>>
>> Something happened several days ago.  I used to get picks/drops rarely, say
>> ballpark of 1 a month.  Now I'm getting 10 or 20 per day.  So I started
>> looking closer.
>>
>> I'm now seeing stuff like this.  I've got lots and lots of examples.  I added
>> a second PC with different hardware.  It sees the same stuff.
>>
>> Does anybody recognize this?
>>
>> http://www.megapathdsl.net/~hmurray/time-nuts/line/2012-Sep-01-a0.png
>> http://www.megapathdsl.net/~hmurray/time-nuts/line/2012-Sep-01-b0.png
>> http://www.megapathdsl.net/~hmurray/time-nuts/line/2012-Sep-01-c0.png
>> http://www.megapathdsl.net/~hmurray/time-nuts/line/2012-Sep-01-d0.png
>> http://www.megapathdsl.net/~hmurray/time-nuts/line/2012-Sep-01-e0.png
>>
>>
>Hal:
>
>Two ideas:
>
>1.) You could have some process in Windows that is causing aperiodic
>blocking of the OS's ability to process real time data.  Can be many,
>many causes.
>
>It is very common for a lot of the background processes that autonomously
>run when you are not actively using the mouse and keyboard to cause these
>type of problems.  Back up utilities, virus checkers, USB WiFi accessories,
>are examples. (The worst I have seen was the little CPU inside a (Dell) 
>laptop
>battery, hanging up the USB bus, every time while it calculated the 
>charge in
>the battery.)
>
>Windows is NOT a real time operating system. A fast computer that is not
>heavily loaded can come close, but there is a lot of background stuff going
>on in Windows that can aperiodically hang up a "lightly loaded" machine.
>
>Download a DPC tester (Deferred Procedure Call tester) and watch it
>for an extended time that includes one of your "power glitches."
>
>http://www.thesycon.de/deu/latency_check.shtml

You can add to this list the SMM (system management mode) routines
stored in the BIOS.  If those are causing the problem then the OS
latency performance becomes academic.  I have had to qualify
motherboards and BIOS revisions before to avoid problems with them.

http://en.wikipedia.org/wiki/System_Management_Mode#Problems




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