Date: Fri, 09 Aug 2019 14:17:17 -0600 From: Ian Lepore <ian@freebsd.org> To: Per Hedeland <per@hedeland.org> Cc: freebsd-arm@freebsd.org Subject: Re: Is it a good idea to use a usb-serial adapter for PPS? Yes, it is. Message-ID: <523b6f0a0fa5f2aeec298fa74df25d3c4af66acc.camel@freebsd.org> In-Reply-To: <7312032d-2908-9414-0445-6b442c3a02e5@hedeland.org> References: <alpine.BSF.2.21.99999.352.1908071046410.98975@autopsy.pc.athabascau.ca> <69a9bed3-4d0a-f8f6-91af-a8f7d84ee307@hedeland.org> <345bae77417c2495f55799b4c7ca2784f4ece9ed.camel@freebsd.org> <7312032d-2908-9414-0445-6b442c3a02e5@hedeland.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Fri, 2019-08-09 at 21:36 +0200, Per Hedeland wrote:
> On 2019-08-09 17:28, Ian Lepore wrote:
> > 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.
>
> Oh, I certainly didn't intend to suggest that your message was
> off-topic, only that it deserved a wider audience.
>
> > 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. :)
>
> I get your point - in particular posting a "controversial" message to
> any *-geek forum can be extremely time-consuming if you try to
> address
> all the responses it will generate.
>
> Personally, although I tinkered a lot with NTP back in the days when
> computers had serial ports, and even before they had Internet access,
> I can't say that I've ever qualified as a true timing-geek - I still
> follow the trickle of posts in the abovementioned newsgroup, but it
> was probably years since I last posted anything. Would you object to
> me posting an article with a *link* to your message
> (i.e.
> https://lists.freebsd.org/pipermail/freebsd-arm/2019-August/020263.html
> )
> in the newsgroup?
>
> --Per
>
It might be better to use the link to the copy I sent to the freebsd-
usb list, since it's more directly on-topic:
https://lists.freebsd.org/pipermail/freebsd-usb/2019-August/016078.html
I also think it would be wise to add a caveat that the results are for
FreeBSD. I would expect linux performance to be similar. But for
Windows, all bets are off; Windows drivers for usb-serial devices are
said to vary wildly in quality depending on the vendor.
-- Ian
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?523b6f0a0fa5f2aeec298fa74df25d3c4af66acc.camel>