Date: Fri, 09 Aug 2019 09:28:06 -0600 From: Ian Lepore <ian@freebsd.org> To: Per Hedeland <per@hedeland.org>, freebsd-arm@freebsd.org Subject: Re: Is it a good idea to use a usb-serial adapter for PPS? Yes, it is. Message-ID: <345bae77417c2495f55799b4c7ca2784f4ece9ed.camel@freebsd.org> In-Reply-To: <69a9bed3-4d0a-f8f6-91af-a8f7d84ee307@hedeland.org> References: <alpine.BSF.2.21.99999.352.1908071046410.98975@autopsy.pc.athabascau.ca> <69a9bed3-4d0a-f8f6-91af-a8f7d84ee307@hedeland.org>
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On Thu, 2019-08-08 at 22:26 +0200, Per Hedeland wrote:
> On 2019-08-07 18:53, Ross Alexander wrote:
> > In Message-ID: <B9EFA4D4-C1AD-4181-B421-F6BD53434FA5@dons.net.au>,
> > someone wrote [sorry, attrib trail is a little blurry ed.]:
> >
> > > > Most people are not worried about their kernel clock being 200
> > > > microseconds off from UTC, even if they're using the PPS signal
> > > > from a
> > > > GPS receiver. So I think most people should feel completely at
> > > > ease
> > > > using a USB serial adapter as the input device for a PPS
> > > > signal.
> >
> > Some people do worry, although getting PPS to work over USB is a
> > fine
> > first step and I'm grateful for the breadcrumb trail.
>
> For those that do worry, you can of course tell ntpd to correct for a
> semi-fixed offset (via the 'time1' option to the 'fudge' command) -
> once you know how large the offset is... More important is a low
> jitter, and 20-30 microseconds seems quite good.
>
> @Ian, maybe it would make sense to post your test report to some NTP
> forum, e.g. the comp.protocols.time.ntp newsgroup? I guess most
> readers of freebsd-arm@ have little interest in using a usb-serial
> adapter if they can simply use a gpio pin, with better results - but
> for PCs and the like, that no longer have serial ports, it could be
> very useful.
>
> Just last week there was a post or two to the newsgroup re-asserting
> the oft-repeated claim that using a usb-serial adapter for PPS is
> completely useless ("USB-to-RS-232 converters generally completely
> loose the precision timing abilities of traditional serial port
> circuits...").
>
I posted to arm@ because a discussion here recently brought up the usb-
serial performance questions, and also because I used arm hardware to
craft a somewhat unique measurement apparatus for it.
I have resisted joining any timing-geek newsgroups or forums for years,
because it's what I do for a living as well as being a hobby, and I've
always felt it would just suck another hour a day of my time if I
belonged to more groups/forums that might interest me that much. :)
-- Ian
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