Date: Fri, 9 Aug 2019 21:36:11 +0200 From: Per Hedeland <per@hedeland.org> To: Ian Lepore <ian@freebsd.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: <7312032d-2908-9414-0445-6b442c3a02e5@hedeland.org> In-Reply-To: <345bae77417c2495f55799b4c7ca2784f4ece9ed.camel@freebsd.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>
next in thread | previous in thread | raw e-mail | index | archive | help
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
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?7312032d-2908-9414-0445-6b442c3a02e5>