Date: Mon, 23 Jul 2018 09:24:38 -0600 From: Ian Lepore <ian@freebsd.org> To: Per olof Ljungmark <peo@nethead.se>, David Cornejo <dave@dogwood.com> Cc: freebsd-arm <freebsd-arm@freebsd.org> Subject: Re: rpi3 and Adafruit GPS Hat Message-ID: <1532359478.1344.142.camel@freebsd.org> In-Reply-To: <fa3a6cb6-d73e-1b7f-e470-e9c88e3ea6ee@nethead.se> References: <47f49a55-66b0-1c02-4530-4701a3bd0c43@nethead.se> <20180718170157.GA40221@night.db.net> <ba31ee27-1d17-8616-96b2-b982268f0dc2@nethead.se> <CAFnjQbt395y4xyHEp52GAHW9X9bHvdCAT9q-gKA2--RvQvqTtA@mail.gmail.com> <7a14173c-cc28-6dc7-3787-a5b77a396b30@nethead.se> <1532357176.1344.130.camel@freebsd.org> <fa3a6cb6-d73e-1b7f-e470-e9c88e3ea6ee@nethead.se>
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On Mon, 2018-07-23 at 17:20 +0200, Per olof Ljungmark wrote: > On 07/23/18 16:46, Ian Lepore wrote: > > > > On Mon, 2018-07-23 at 11:40 +0200, Per olof Ljungmark wrote: > > > > > > On 07/23/18 10:46, David Cornejo wrote: > > > > > > > > > > > > this might be a little blasphemous, but for grins I tried an Oncore with > > > > PPS to a GPIO and running the serial through a TTL-USB serial cable and > > > > that seems to work ok. > > > > > > > > there's probably some good reason that this is a bad idea. > > > Depends on what precision you are after, but for lowest possible jitter > > > you need to use the uart, the difference is in magnitudes. > > > > > Technically that may be correct, but it's meaningless. On a usb 1.x > > adapter there may be ~500us of jitter from one measurement to the next. > > On a usb 2.x adapter the jitter drops to typically ~60us. Those values > > are pretty much in the noise for ntpd, which uses a median filter to > > smooth any serious jitter out of the measurements. > > > > Here are some real-world measurements. The pps source for all 3 inputs > > is the same gps-disciplined rubidium oscillator, so all the jitter is > > within the uart, usb hardware, and freebsd drivers. The usb adapters > > are both FTDI chips, which have a fixed latency on reporting a change > > on the DCD pin (pin-change status messages are only delivered once a > > millisecond on ftdi chips). > > > > remote refid st t when poll reach delay offset jitter > > ====================================================================== > > xPPS(0) .uart. 0 l 6 16 377 0.000 1.097 0.001 > > xPPS(1) .usb1. 0 l 4 16 377 0.000 -0.051 0.773 > > oPPS(2) .usb2. 0 l 4 16 377 0.000 -0.001 0.035 > > *dvb.hippie.lan .GPS. 1 u 12 64 377 1.234 1.296 2.707 > > +utcnist2.colora .NIST. 1 u 1 64 377 13.605 3.940 2.729 > > > > You can see in this case ntpd actually chose the usb2 pps input as the > > system peer. It did so because at startup the clock offset was closer > > than the uart, and the difference in jitter between the two wasn't > > significant, so the ntpd code that prevents clock-hopping chose to > > stick with the peer with the smaller offset. > > > Yes, I was technically correct but of course you are right too - > however, the main problem is not the jitter but rather that I am unable > to switch off the serial console and stop the u-boot loader from > receiving NMEA data. The Adafruit GPS Hat is made to sit right on the Pi > 40-pin header, as you probably know. > > To quote one of the posters in the thread I linked to: > "... I am seriously baffled by how difficult (nearly impossible) it had > been to get rid of the serial console..." > > Unfortunately I am not fluid enough to figure out where to make the > changes, the advices I've seen so far is not applicable to 12-CURRENT in > an easy way. > > And that is also why I wrote bugreport 229976. It may be that all the > folks involved with FreeBSD/ARM are serious developers so they do not > see it as a problem but for me it is, being more on the > application/administration side of things. > > Thanks, > > //per > Unfortunately, I can't help with the rpi part of this, since it's related to the firmware and uboot, and that part of the rpi world has changed drastically since I was involved with it years ago. I know you can build a custom uboot that disables serial console support completely, but I don't know if there's a way to achieve that with the stock uboot. -- Ian
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