Date: Thu, 28 Nov 2019 23:51:53 -0700 (MST) From: Ross Alexander <rwa@athabascau.ca> To: Peter Jeremy <peter@rulingia.com> Cc: James Shuriff <james@opentech.cc>, "freebsd-arm@freebsd.org" <freebsd-arm@freebsd.org> Subject: Re: rpi3 clock drift Message-ID: <alpine.BSF.2.21.99999.352.1911282333250.90234@autopsy.pc.athabascau.ca> In-Reply-To: <20191129052800.GA37113@server.rulingia.com> References: <alpine.BSF.2.21.99999.352.1911271039470.281@autopsy.pc.athabascau.ca> <MWHPR06MB3134EC22EC3148DA800B2B7DAA440@MWHPR06MB3134.namprd06.prod.outlook.com> <alpine.BSF.2.21.99999.352.1911272214050.28592@autopsy.pc.athabascau.ca> <20191129052800.GA37113@server.rulingia.com>
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On Fri, 29 Nov 2019, Peter Jeremy wrote:
> On 2019-Nov-27 22:20:50 -0700, Ross Alexander <rwa@athabascau.ca> wrote:
>> An Adafruit "ultimate gps hat" (typical commercial hyperbole, sigh.)
>> It's about $65 after you add the 40 pin molex connector, the
>> micro-whatever to SMA cable, and the external amplified patch antenna.
>
> You can get a uBlox 7 or 8 module with a patch antenna and PPS output
> for O($10). It should be possible to wire the serial data to the PL011
> UART and preserve the serial console UART but you'd to write a driver.
Agreed. The hat spares you the wiring and mounting challenge. The
s/w issues are why I'm running the GPS box on Debian. I do plan to go
back and try it again on that same h/w using 12-stable at some point.
I recall that some of the uBlox parts are designed for timing apps
rather than navigation - they would be the ones to go with.
> I have a metal roof and find putting the GPS module with a patch
> antenna near a window (or even near an external wall) is sufficient.
I have an identical (pi-2, ult hat, external patch antenna, debian)
system running at work where I can get only a view of half the sky and
it has significantly more jitter. A broader sky view gives more
confidence in the position domain (more birds, and they are more
spread out), which translates into lower time jitter. I'm arguing
from a very small sample size here, of course. I would love to
have a larger population to test.
BTW, another *significant* source of jitter is the brand and age of
the sd/mmc card used. As they age, the write speed decreases and
block write latency gets less uniform; this shows up as system clock
jitter in the loopstats. After a few years (3 or 4), the box becomes
a complete falseticker and you need to replace the sd/mmc card.
regards,
Ross
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=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
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Ross Alexander, (780) 675-6823 desk / (780) 689-0749 cell, rwa@athabascau.c=
a
54.71593 N 113.30835 W
Order is simply a thin, perilous condition
we try to impose on the basic reality of chaos.
-- William Gaddis, _J R_
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