Date: Thu, 8 Aug 2019 22:26:44 +0200 From: Per Hedeland <per@hedeland.org> To: freebsd-arm@freebsd.org Subject: Re: Is it a good idea to use a usb-serial adapter for PPS? Yes, it is. Message-ID: <69a9bed3-4d0a-f8f6-91af-a8f7d84ee307@hedeland.org> In-Reply-To: <alpine.BSF.2.21.99999.352.1908071046410.98975@autopsy.pc.athabascau.ca> References: <alpine.BSF.2.21.99999.352.1908071046410.98975@autopsy.pc.athabascau.ca>
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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...").
--Per
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