Date: Sat, 30 Nov 2019 07:45:10 +1100 From: Peter Jeremy <peter@rulingia.com> To: Ross Alexander <rwa@athabascau.ca> Cc: "freebsd-arm@freebsd.org" <freebsd-arm@freebsd.org> Subject: Re: rpi3 clock drift Message-ID: <20191129204510.GC37113@server.rulingia.com> In-Reply-To: <alpine.BSF.2.21.99999.352.1911282333250.90234@autopsy.pc.athabascau.ca> References: <alpine.BSF.2.21.99999.352.1911271039470.281@autopsy.pc.athabascau.ca> <MWHPR06MB3134EC22EC3148DA800B2B7DAA440@MWHPR06MB3134.namprd06.prod.outlook.com> <alpine.BSF.2.21.99999.352.1911272214050.28592@autopsy.pc.athabascau.ca> <20191129052800.GA37113@server.rulingia.com> <alpine.BSF.2.21.99999.352.1911282333250.90234@autopsy.pc.athabascau.ca>
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--RASg3xLB4tUQ4RcS Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 2019-Nov-28 23:51:53 -0700, Ross Alexander <rwa@athabascau.ca> wrote: >I recall that some of the uBlox parts are designed for timing apps >rather than navigation - they would be the ones to go with. Yes, there are. Unfortunately, they are going to be a lot more expensive (the NEO-M8T is the cheapest and the bare module is $52 in 100-off). The cheapest board I've found is about =E2=82=AC70. >I have an identical (pi-2, ult hat, external patch antenna, debian) >system running at work where I can get only a view of half the sky and >it has significantly more jitter. A broader sky view gives more >confidence in the position domain (more birds, and they are more >spread out), which translates into lower time jitter. I've got a NEO-6M sitting against an external wall and it typically sees 11-12 satellites and uses 8-9. ntpd is reporting about 2=C2=B5s jitter (PPS input connected to a PC Engines APU2 - AMD GX-412TC) - which seems good enough. (I'm working on building a NTP server around an ESP32 as a second datapoint). >BTW, another *significant* source of jitter is the brand and age of >the sd/mmc card used. As they age, the write speed decreases and >block write latency gets less uniform; this shows up as system clock >jitter in the loopstats. As with Ian, I don't understand this. The only possibilities I can think of is that the PPS interrupt priority is too low and/or you're not using kernel PPS time capture. (Is this on FreeBSD or Debian?) --=20 Peter Jeremy --RASg3xLB4tUQ4RcS Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQKTBAEBCgB9FiEE7rKYbDBnHnTmXCJ+FqWXoOSiCzQFAl3hg1BfFIAAAAAALgAo aXNzdWVyLWZwckBub3RhdGlvbnMub3BlbnBncC5maWZ0aGhvcnNlbWFuLm5ldEVF QjI5ODZDMzA2NzFFNzRFNjVDMjI3RTE2QTU5N0EwRTRBMjBCMzQACgkQFqWXoOSi CzQ+Dg//fP37CskTeoxwqPl0LjQymwaw10faYkbWaEH93XIDKwwO0piuegMYsedR ai58HHkYqwDlXyl5Nb+9mVBamCS9aatjz4AmwhK0t0TyGXBkm+X0Ba2Yp4+9WnpV PrtR/08PVKpXHjZU3Wlye8QpX5xMlQ9k3/e2EFXKZ3ATZw20aBDbJUuVb7eKp+UW yPEeGgGXUV5lTQHQbUMGcoOxnXSfCfLy6Ci1ARWCOiQUU8UJla3ODCO4qApqjVky ECmouh7oRpcKOcfWbUzIUF+pfjcU5CWdPmiuiRUbsEggYMnzvUymp/Dj4qpC+Hvg EE6KGgbnTlM6LLyIDhUskGXYY3rmKMBU3V4xIGcCEVKXyo/Pxfowmkxq4Ilh/f+j 1RKTxb0QKCbFCU+WDKPdYsLOH0ypOvETgmOUCQs5gXtFZtuJTNZWQBI5DAMAoonj 3FOKplizjPNeH4AKwAEIY4vx8vLVpESeWbyoAkpuL3dimfVOC7USSJBrNpGloSft tR+qpocqF9L55BWTTEj1Zxyy8xP01oqLCvu7fCXQRwC3rW+FSXFaPV93m+tRZ69q K9HGxIIdqDmEybmVB1szltTIsoI89YonC6R0q1Im+NXpESkClmfcedEtGeiOiAJc aynKd1udS2PhXbbdLijNeDj/9KcUhz+bGO0CA4T5wfle6p27CC4= =RS3n -----END PGP SIGNATURE----- --RASg3xLB4tUQ4RcS--
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