From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-gy0-f171.google.com (mail-gy0-f171.google.com [209.85.160.171]) (using TLSv1 with cipher RC4-SHA (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority" (verified OK)) by huchra.bufferbloat.net (Postfix) with ESMTPS id 85E0620024C for ; Mon, 12 Mar 2012 14:03:10 -0700 (PDT) Received: by ghbz17 with SMTP id z17so4720064ghb.16 for ; Mon, 12 Mar 2012 14:03:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:sender:in-reply-to:references:from:date :x-google-sender-auth:message-id:subject:to:cc:content-type :content-transfer-encoding; bh=l+4isO0diyl2sP0fgRDWdm3fo5ARcz/A0KddaogBnLs=; b=GYFoPiGtTqTIYP1h1oXwyCboprg58RMgQvRvVXsczG2/9S/MVmbUjwXMApC5KyZ/Dn OYXhDSWOGJnzbpAosjZmax3x6k+XC99hrZj6yoIBVtLv6u3AsBJw3th5wvA/Hhmv3jRR wTUe48BKXqhyhv74afpV7oSZTsYXI2Dv2VQf0QECvojm5US8WkYZFMNgUsneDI4BiSqb yga7Sajv9nqdmD1Njblko1KZMfVsQxL6pWg69rZyvFyyBAZS8C5stecpc/xuwFBrWjhr KJySU3QbNn73xsow662S95vfD9E/tSLvBwtkrXY38YDrILVWz7N4mpU2VNaW1Pj+973X bUkg== Received: by 10.182.119.98 with SMTP id kt2mr9474882obb.9.1331586189405; Mon, 12 Mar 2012 14:03:09 -0700 (PDT) MIME-Version: 1.0 Sender: tz2026@gmail.com Received: by 10.182.68.164 with HTTP; Mon, 12 Mar 2012 14:02:49 -0700 (PDT) In-Reply-To: References: From: tz Date: Mon, 12 Mar 2012 17:02:49 -0400 X-Google-Sender-Auth: EVYzQd69-3DuEM8VM2QeTqFleKU Message-ID: To: Patrick Maupin Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Cc: thumbgps-devel@lists.bufferbloat.net Subject: Re: [Thumbgps-devel] Jitter and latency in a linux system might be a problem. Minimal BoM crosspost. X-BeenThere: thumbgps-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 12 Mar 2012 21:03:10 -0000 On Mon, Mar 12, 2012 at 4:44 PM, Patrick Maupin wrote: > On Mon, Mar 12, 2012 at 2:09 PM, tz wrote: > I thought sub-ms wasn't really necessary. =A0Sub-us would be useful in a > testbench for characterizing devices to make sure they meet ms... >> But to state the problem clearly, you want the clock to have 100nS >> accuracy, > > Where was that listed as a requirement? =A0I think there's some > misunderstanding here. I would think so to, but I think ESRs original post or one of the requirements comments said 100nS. It would help if we knew how hard the number was. >> Any SkyTraq GPS (I have the Venus 624 and 628, and their GLONASS/GPS >> modules), the "$" character in the first sentence can be synced to the >> UTC second and comes out at a fixed delay from the PPS edge, just over >> 1mS to stop bit with no visible jitter so technically it would be as >> accurate as the PPS itself, and the latency could be compensated for. > > That's useful. =A0With a testbench we can test this, and with a small > CPLD in conjunction with such a module and a serial to USB converter, > we might even be able to make this work over USB by wiggling CTS or > DSR. =A0In other words, if we find a module without PPS, but *with* > accurate first-character message timing, we can wiggle a modem control > line when the first character starts to force an immediate USB > interrupt response, rather than waiting on the entire message plus > some extra latency, which is what the USB chip would normally do. I've been using uS gettimeofday on Linux and with an oscilloscope connected to the PPS and data lines. > I think there are a lot of really good possibilities, but the first > thing to do is a hardware-assisted testbench to take accurate > measurements, so we know the source and size of all the latencies in > the system. =A0I haven't heard back from esr about what he thinks of > this approach, though, so I am reluctant to start work on anything > until we have some consensus on the idea that we should grab several > devices and USB modules and USB chips and see how well they really > operate. Garmin has the 18x-LVM (oem) which has a PPS and from what I recall (I haven't hooked it up in a while and I don't know where it is) the NMEA sentences didn't have a lot of jitter, but I'd need to put it back on my scope. The Sirf3 apparently doesn't work. I've seen MTK and Ublox, but I don't know about them. > This is an interesting idea. =A0I had wondered about ethernet, but the > idea of setting up anything NTPish on a tiny micro seemed daunting. =A0I > like the idea of such a UDP request/response mechanism. You don't need ntpd. You simply need precise PPS since you can then measure the actual crystal frequency, e.g. if it "10 MHz" ends up being 10.123456 MHz, you can divide the counts into this to find the time with high precision. You can play some PLL games, but I was close to syncing an SMPTE stream to PPS for my ION-GNSS entry last october where the stream edges would be synced even though the micro had a ceramic resonator. >> The SkyTraq PPS is accurate to 60nS mainly due to the 16Mhz internal clo= ck rate. > > Have you checked that over days? =A0I'm intrigued by the graph that I > just saw of things going haywire. The only problem I have seen was at 20Hz with AGPS on my motorcycle under nonrepeatable conditions - sometimes it would not get a lock though it had 10 satellites in view with strong signals, I had to reset it. If it loses lock, the PPS disappears, but I should probably try my 628 and glonass and leave both on over a weekend. >> Even if you get a $30 GPS unit you would have to break it open and do >> some soldering under a stereo microscope, assuming you can attach the >> PPS to DCD or CTS or another pin easily. > > Absolutely. =A0But esr's stated end product (which I think should be > possibly part of our range of solutions) is a tiny USB key. ... I've seen this one with the same description from a few searches: http://www.chinazrh.com/wholesale-gps-receiver-usb-dongle-for-computers-net= book-laptop-umpc-p-728.html