From owner-freebsd-arm@freebsd.org Mon Aug 19 17:26:25 2019 Return-Path: Delivered-To: freebsd-arm@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id EE40DCC39B for ; Mon, 19 Aug 2019 17:26:25 +0000 (UTC) (envelope-from rwa@athabascau.ca) Received: from NAM05-CO1-obe.outbound.protection.outlook.com (mail-co1nam05on0624.outbound.protection.outlook.com [IPv6:2a01:111:f400:fe50::624]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-SHA384 (256/256 bits)) (Client CN "mail.protection.outlook.com", Issuer "GlobalSign Organization Validation CA - SHA256 - G3" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 46C1740ryjz44V0; Mon, 19 Aug 2019 17:26:23 +0000 (UTC) (envelope-from rwa@athabascau.ca) ARC-Seal: i=1; a=rsa-sha256; s=arcselector9901; d=microsoft.com; cv=none; b=ebWAAxerkRxgsIM++9PP7J5vEor1MY7Ol6u17RpMm2+HghKC6daq9wtGYbygxKS30dK8Y8YZ0+27yS9JJB6CVUrhQlbw8V1GofauCJj+6aLtbVCQnKjQuOOH9Vc4YVddrgJe8PfgRN0Mtk2CPRNGS8D1bZg6/Bc1njrtB17JhX0Ao3UI3L7egh+Guc9AVOCLxwD7zVzY265u/aDbnVbMWqdQCRDvA5LDgOF+nfu1UCdELrMw/rzC0+94SShiKR295MftSTGxHnD62rr6cGgz5hBBxHRNBEBARpyRutHJGdf1ptMtkeADA0jv0WthgZaJXjuRV5BiUzj7N/kA0WxW5g== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=microsoft.com; s=arcselector9901; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=3cY2FDV2zHLwpknDU7C+etPwgOD329cr3+iP/z5yI8A=; b=RVBsd551N24l6xIavzLkqVkdsHJtMcoE0hcf8QH7f26ep3Cvu9g/j5yPJkzhQY95GnaMf/ltFmm7XDTO9CIrMJbp2fIJoZ+kBEcz+Pg4ShDgxR8urajpeA7IqdYIv+l21R5eeOSdrE5M6yyX/kpHMJ9agZk3cdlgGy+8cTTkH73xZGCCR+TKfIuYyvbynb5mQvZUSSzt+kuW7P+Lsem0MKmVviy+1Z12bNNEjAtN1+wXTj4hGJdWqe1uM8564NQCqviYVGhn0K6BgdcyaWB/rwet2vKScIs9WidStaDX6vuSmuAjRlmP6xHh5qumS+Izvu4ZkMqML/pSOG075yLhDQ== ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=pass (sender ip is 131.232.32.37) smtp.rcpttodomain=freebsd.org smtp.mailfrom=athabascau.ca; dmarc=bestguesspass action=none header.from=athabascau.ca; dkim=none (message not signed); arc=none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=austaff.onmicrosoft.com; s=selector2-austaff-onmicrosoft-com; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=3cY2FDV2zHLwpknDU7C+etPwgOD329cr3+iP/z5yI8A=; b=wHrjzABXnYCMU7aNXhQUZcWsuXZZ2XsnwJoRcqxx8WnuCc5yAguo0JsuI7i6NklrrIEbTLvmXo6xo0Y1O4g1A4phYiRXqj6AufW2WuS6C9Q0t0xJeyLhpAUv4NPjLgNx2jiwSalkrBUtUlXr1GofzLS3e7KoyEKah2N8zi3rKf4= Received: from MWHPR13CA0029.namprd13.prod.outlook.com (10.173.117.143) by CY4PR1301MB2166.namprd13.prod.outlook.com (10.171.240.149) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2178.15; Mon, 19 Aug 2019 17:26:19 +0000 Received: from TO1CAN01FT007.eop-CAN01.prod.protection.outlook.com (2a01:111:f400:7e5d::209) by MWHPR13CA0029.outlook.office365.com (2603:10b6:300:95::15) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2199.6 via Frontend Transport; Mon, 19 Aug 2019 17:26:19 +0000 Received-SPF: Pass (protection.outlook.com: domain of athabascau.ca designates 131.232.32.37 as permitted sender) receiver=protection.outlook.com; client-ip=131.232.32.37; helo=smtp-relay.cs.athabascau.ca; Received: from smtp-relay.cs.athabascau.ca (131.232.32.37) by TO1CAN01FT007.mail.protection.outlook.com (10.152.122.104) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2178.16 via Frontend Transport; Mon, 19 Aug 2019 17:26:18 +0000 Received: from autopsy.pc.athabascau.ca (autopsy.pc.athabascau.ca [131.232.4.80]) by smtp-relay.cs.athabascau.ca (Postfix) with ESMTPS id 08A1C2016E; Mon, 19 Aug 2019 11:26:18 -0600 (MDT) Date: Mon, 19 Aug 2019 11:26:17 -0600 (MDT) From: Ross Alexander X-X-Sender: rwa@autopsy.pc.athabascau.ca To: Ian Lepore cc: freebsd-arm@freebsd.org Subject: Re: Is it a good idea to use a usb-serial adapter for PPS? Yes, it is. 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List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 19 Aug 2019 17:26:26 -0000 On Sun, 18 Aug 2019, Ian Lepore wrote: > On Thu, 2019-08-15 at 16:02 -0600, Ross Alexander wrote: >> In <24b0eaf25b64d6098b390df092866c69e352d859.camel@freebsd.org>, >> Ian Lepore writes: >> >>> [... ed.] I arranged to use a very stable nearly-drift-free >>> frequency source instead of a cheap crystal for counting time in the >>> kernel. >> >> You have my complete and focussed attention. Say on. > > I detailed the design of the measurement system in the original > message. Basically it involves feeding a very stable external clock > signal to a block of timer hardware within the imx6 SoC and using that > timer hardware as the kernel clock. The source of the stable external > clock has these specs: > > https://www.microsemi.com/product-directory/gps-instruments/4152-syncsyst= em-4380a > > I'm a software engineer for the division of Microsemi that makes those > devices. If it wasn't the clock source you wanted to hear more about, > then let me know in more detail what you'd like to know. No, the clock source is the ticket. I've got a TAPR clockblock lying around somewhere, and have been meaning to pick up a precision 10 MHz source from eBay (or equiv.) for some time. In the meantime I've just ovenized the whole Pi, all you need is a shoebox and an old towel :). > [...] I said: >> 125 microseconds is a lot of jitter. [...] > It's negligible jitter for everyday computer use. It's a lot if you're > doing something like timestamping financial transactions, or recording > radio-astronomy observations, but the people doing that sort of thing > usually aren't using ntpd to sync their clocks (they're using ieee- > 1588, or custom solutions such as PCI irig input cards). I'm measuring packet roundtrip times to regional campii and that sort of thing. We're linked to various tier-3 carriers via a provincial gov't MPLS network that makes QOS questions rather opaque, and this is one of the things I do to probe it. I wrote: >> [...] >> The jitter is expressed in units of 1 millisecond, unless I am badly >> mistaken; for which possibility I apologize in advance. > > Yes, it's reported in milliseconds. The jitter number is the RMS of > the differences between the offset from the measurement with the lowest > delay for that peer and the other 7 offsets for that peer in the 8-step > filter. That is, it finds the lowest delay, then it does, roughly: > > jitter =3D 0; > for offset in {the other 7 samples that aren't the lowest delay} > jitter +=3D SQUARE(offset - lowest_delay_offset) > > then the final number is > > jitter =3D SQRT(jitter / 7) That is a fine thing to have clarified. I've dug into the code, but only because I wanted to run multiple sound card radio modems (two WWV and two CHU) to deal with varying HF propagation and path dropouts. Obviously those patches were a long way from the actual PLL logic at the core of the protocol. regards, Ross =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D Ross Alexander, (780) 675-6823 desk / (780) 689-0749 cell, rwa@athabascau.c= a 54.71593 N 113.30835 W, ve6pdq Order is simply a thin, perilous condition we try to impose on the basic reality of chaos. -- William Gaddis, _J R_ -- This communication is intended for the use of the recipient to whom it is a= ddressed, and may contain confidential, personal, and or privileged informa= tion. Please contact us immediately if you are not the intended recipient o= f this communication, and do not copy, distribute, or take action relying o= n it. Any communications received in error, or subsequent reply, should be = deleted or destroyed. ---