From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-vx0-f171.google.com (mail-vx0-f171.google.com [209.85.220.171]) (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 08FD42011CD for ; Mon, 12 Mar 2012 18:26:36 -0700 (PDT) Received: by vcbfl15 with SMTP id fl15so51436vcb.16 for ; Mon, 12 Mar 2012 18:26:36 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type; bh=Hlww1A8DAK1/sAr+3TGugqIDGTnc1xJdBRabnch+f7w=; b=U0i+goMKsV7PlgZ0LZ7j3k2QH2sUdE9NznHzX5P1znzf1d2qeBtYqe5klYgSTuUz/t wX0s9eaBWHy8He5sGt4kjcSpvzUiiKyzv+r60WmMey1CYgY/JQfYl58OtYIVO5BPjFyp IgNO6ZTWJDgWYhY2bHzCdBxMSnCJZrylzgWzoj4yJVBzXqA4yKQ4WEYfiVq48Bc/pjCh 3AhDqZogBgrrZ+nUQjSAblYhvXz2qIjbbNfc4wjNfyC0cuX9jJsAYKwkK9wFuoOgeVVg I9XxCYuvekWIQr3v5kdS88QUWtaFW5VaudYdntOkSzwJUCArPrQ0Mbumqir2wvSlVJ/6 j9YQ== MIME-Version: 1.0 Received: by 10.52.93.179 with SMTP id cv19mr15939548vdb.103.1331601995838; Mon, 12 Mar 2012 18:26:35 -0700 (PDT) Received: by 10.220.140.132 with HTTP; Mon, 12 Mar 2012 18:26:35 -0700 (PDT) In-Reply-To: References: <20120312220623.GB18256@thyrsus.com> Date: Mon, 12 Mar 2012 20:26:35 -0500 Message-ID: From: Patrick Maupin To: Dave Taht Content-Type: text/plain; charset=ISO-8859-1 Cc: thumbgps-devel@lists.bufferbloat.net Subject: Re: [Thumbgps-devel] the serial alternative/radio noise 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: Tue, 13 Mar 2012 01:26:37 -0000 On Mon, Mar 12, 2012 at 6:22 PM, Dave Taht wrote: > I direct you at the 'smile plug', which is also an ideal device for > deployment in places without reliable power. Is checking for good network connectivity where there isn't power that useful? > Earlier versions of this hardware (see the open-rd) had some > interesting connectors available in addition to usb. I don't know if > the dreamplug does - the guruplug had such lousy thermals as to rule > it out for any usage.... Agreed. I thought about that and then discarded it. > I'm not wedded to the the wndr3800 as the 'reference time ticker > beyond the internet edge' for this project. Well, I kind of like the idea of a braindead ethernet thing, too. That would require a router ethernet port, but a lot of routers don't have USB, so that could be OK. > we certainly seem to have identified a market and price gap between > shipped devices with bad time, and devices with good time, but we > haven't established that usb with PPS on 'dcd emulation' can be made > 'more right', as yet. Right, which is why the first thing we need to build is precise measurement capability for what we want. (You can buy precise measurement capability for tens of thousands of dollars, but you still have to wire up your own stuff over to it, so I don't really see the point.) > So I'm kind of looking for ways to get the right parts and evaluate > the following without having to do much soldiering, to try and isolate > where each of the latency problems are. > > PPS serial > PPS usb > kernel pps > gpsd w/wo r/t privs under load We're definitely on the same page. I'd like to be able to do this in hardware rather than software. Or rather, do the lowest level stuff in hardware, and collect data for the software to analyze. The two major pieces of this are an FPGA and a really stable frequency reference. For example, you can get a 5ppb OCXO that will drift less than 4 ms in a day, and a lot less than that for short term disturbances. Without such a thing, you can't say for sure that what Hal saw wasn't times when his computer got so loaded down that tick interrupts didn't happen. Even though the Linux drivers and firmware aren't open source, my favorite FPGA board at the moment is the 1.2M gate version of the Digilent Nexys2. I've probably bought around 40 of the Nexys and then the Nexys2 for the lab at work. You can easily build a big board that plugs right into it, and a few little boards that plug into it too. If there are no objections, this is what I plan on using for the base of the timing tool: http://digilentinc.com/Products/Detail.cfm?NavPath=2,400,789&Prod=NEXYS2 The big FPGA version is $189. I can easily build a board to attach that has the following: 1) Provision for modules for a few different FTDI chips (FT232H, FT232R, FT232x) 2) Provision for modules for a few different GPS devices from different vendors 3) Provision for connectors for a few different GPS devices (e.g. Garmin 18x) 4) Provision for an OCXO 5) Provision for input from a timing reference such as a rubidium clock I can fab 4 identical boards for ca. $200. Probably another $200-300/board for modules depending on how crazy we get. The question comes with the OCXO. Do we want to get used ones off EBAY or buy really expensive new ones? I'm contemplating that perhaps we should make it where we can use multiple ones and then check them against each other. > The venus really does look like a nice device. Agreed. Nice but expensive. > A better way to put my question is: what evidence do we have that a PPS signal over usb can provide better time? That's evidence we need to collect.