Date: Mon, 23 Jul 2018 11:37:56 -0400 From: Ralph Smith <ralph@ralphsmith.org> To: Ian Lepore <ian@freebsd.org> Cc: Per olof Ljungmark <peo@nethead.se>, David Cornejo <dave@dogwood.com>, freebsd-arm <freebsd-arm@freebsd.org> Subject: Re: rpi3 and Adafruit GPS Hat Message-ID: <002D469F-26E0-492A-BEB7-8A61A56ECABF@ralphsmith.org> In-Reply-To: <1532359478.1344.142.camel@freebsd.org> 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> <1532359478.1344.142.camel@freebsd.org>
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Sent from my iPhone > On Jul 23, 2018, at 11:24 AM, Ian Lepore <ian@freebsd.org> wrote: > >> 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. Not using the stock uboot, you will need to modify it. I have this working on 12-CURRENT for the Pi 2, I haven’t tried it on the Pi 3. I documented how I modified u-boot for 11-STABLE in https://docs.freebsd.org/cgi/getmsg.cgi?fetch=188164+0+archive/2018/freebsd-arm/20180204.freebsd-arm, this should apply to the current u-boot port as well. You will also meet to modify the DTB so the system doesn’t send console output to the uart as well. Ralph
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