rpi3 clock drift

[James Shuriff]

Can you help me determine where the timecounter frequency is being pulled from? I believe that 54 MHz value is being pulled from the FDT I just don't know where specifically. My board is supposed to be 19.2 MHz. I booted up FreeBSD's own RPI3 image and I got 19.2 MHz and didn't have any drift issues. Only when I use the environment I built from source do I get 54 MHz, as if an RPI 4 DTB somehow made it into the build.

I'm also interested in getting an RTC hat. I know how to build support for it in the bootloader and OS but how do you tell FreeBSD to start using it for system timing once it's connected?

- James Shuriff

-----Original Message-----
From: owner-freebsd-arm at freebsd.org <owner-freebsd-arm at freebsd.org> On Behalf Of Ross Alexander
Sent: Friday, November 29, 2019 12:56 PM
To: Ian Lepore <ian at freebsd.org>
Cc: freebsd-arm at freebsd.org
Subject: Re: rpi3 clock drift

On Fri, 29 Nov 2019, Ian Lepore wrote:

> On Thu, 2019-11-28 at 23:51 -0700, Ross Alexander wrote:
>> On Fri, 29 Nov 2019, Peter Jeremy wrote:
>>
>>> [...]
>>
>> BTW, another *significant* source of jitter is the brand and age of
>> the sd/mmc card used. [...]
>
> I'm having a real hard time with this one, conceptually.  This is
> exactly what every one of our products at $work does:  precision
> timing including ntpd and kernel time tracking UTC(GPS) to within a few nanos.
> We've had products in the field for 15 years still using the original
> sdcard and still hitting the same on-time performance numbers as the
> day they were shipped (tracking UTC(GPS) +-10ns RMS).

This is purely empirical.  $WORK machine got jitterier and jitterier.
Copied old SD card onto new SD card (straight dd, not a tar/untar), problem went away.  Beats me, but thats how it went.

regards,
Ross

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