Date: Mon, 5 Aug 2019 15:28:53 +0930 From: "O'Connor, Daniel" <darius@dons.net.au> To: Ian Lepore <ian@freebsd.org> Cc: "freebsd-arm@FreeBSD.org" <freebsd-arm@FreeBSD.org>, "usb@freebsd.org" <usb@FreeBSD.org> Subject: Re: Is it a good idea to use a usb-serial adapter for PPS input? Yes, it is. Message-ID: <B9EFA4D4-C1AD-4181-B421-F6BD53434FA5@dons.net.au> In-Reply-To: <f1d765814f722fb2c99c9870b3cc2607b4eca2d7.camel@freebsd.org> References: <f1d765814f722fb2c99c9870b3cc2607b4eca2d7.camel@freebsd.org>
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Hi Ian, Firstly, this is a very cool test - thank you for running it :) > On 3 Aug 2019, at 06:46, Ian Lepore <ian@freebsd.org> wrote: > PPS(2) is an FTDI 232R, a USB 1.1 serial adapter, connected to a port > on a USB 2.0 hub that's connected to a USB 2.0 host port on the > Wandboard. =20 >=20 > PPS(3) is an FTDI 4232H, a USB 2.0 serial adapter, connected to a port > on the same USB hub as PPS(2). =20 <snip> > 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. =20 Does the USB clock derive from the 10MHz Rb clock? If so that would mean = you would see a lot less jitter than a 'normal' user where the USB clock = is not locked too GPS. Do you have a more detailed write up of things like the NTP = configuration file? I think I could replicate your test here although I = have a Beaglebone Black, not a Wanboard so I will need to check if it = can take an external clock. (We have GPS modules & Rb oscillators at = work to create reference clock for bi-static meteor applications). Thanks again. -- Daniel O'Connor "The nice thing about standards is that there are so many of them to choose from." -- Andrew Tanenbaum
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