Re: [time-nuts] Can I get 1 millisecond accuracy with a USB GPS-18
On Mon, 13 May 2013 12:17:42 -0700 Hal Murray hmur...@megapathdsl.net wrote: Anybody know if uBlox over USB units are available? I just took a quick look and didn't find anything. IIRC all LEA-5 and newer have an USB port for communication. I couldnt find anything in the protocol specs whether it supports interrupts over the USB interface for PPS. Attila Kinali -- The people on 4chan are like brilliant psychologists who also happen to be insane and gross. -- unknown ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Can I get 1 millisecond accuracy with a USB GPS-18
Why not run an experiment? Connect a serial and a USB GPS-18 to the same NTP server. and run BOTH ref clocks at the same time. Let it run for a day or so them please, post the results for NTPQ here. Windows is a poor platform for timing. You are best off with Linux or BSD UNIX and a real hardware serial port. The combination of Windows and USB makes 1 millisecond about the limit. But you can get 2 - 5 uSec. with Linux or BSD and a serial port. The world record (as far as I know) broke 1 uSec but required external hardware and motherboard mods. An experiment would answer this. But you'd need to run both GPSes on the same NTP server. If you don't have the port, it is worth the effort to do a Motherboard swap. Intel makes a few Atom boards with real serial rs-232 ports and the Atom is so low powered that it can use a passive heat sync. If you run this 24x7 the power savings can pay for the new $90 board. $90 includes the cpu chip, mother broad and cooling and the serial port. What is your cost per KWH? With a hardware serial port and Linux (or BSD Unitx) single digit uSec is to be expected. A common mid tower PC can burn 100W. THat works out to 876 KWH per year. I pay 27 cents for each KWH so the cost is $234 per year. The new board is only about 10W. You can see a payback on only 6 months and you make $100 in the next 6 months. The power savings can also pay for replacing the GPS receivers with serial units. On Mon, May 13, 2013 at 7:43 AM, Jim Lux jim...@earthlink.net wrote: rummaged through archives and couldn't find anything.. I've got some GPS-18's with the RS232 and 1pps output. BUT, I'm wondering if anyone has tried to get timing with the USB version (Linux or Windows..), and if so, is getting 1 millisecond absolute accuracy feasible. The underlying USB thing has 8kHz frame rate, but I suspect that the serial port emulation (which is probably what they use) might not hold it that well. I can see there being some sort of fixed offset (going through hubs and such), but once the configuration is known, is that stable? Any practical advice? ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. -- Chris Albertson Redondo Beach, California ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Can I get 1 millisecond accuracy with a USB GPS-18
Note that MAX232 type chips add jitter. The charge pumps make substrate noise and that leaks into the receivers/transmitters. I like that addafruit solution. Note the beagle XM has a bug in the USB that RCNelson fixed. I have managed to get it in opensuse, but can't say for sure yet if it is in the distribution. -Original Message- From: George Lu l...@goodxense.com Sender: time-nuts-boun...@febo.com Date: Mon, 13 May 2013 10:04:59 To: Discussion of precise time and frequency measurementtime-nuts@febo.com Reply-To: Discussion of precise time and frequency measurement time-nuts@febo.com Subject: Re: [time-nuts] Can I get 1 millisecond accuracy with a USB GPS-18 On Mon, May 13, 2013 at 9:25 AM, Chris Albertson albertson.ch...@gmail.comwrote: On Mon, May 13, 2013 at 9:04 AM, li...@lazygranch.com wrote: You might want to avoid the older Atom boards at this point. I think 28nm or 22nm is due soon. I have.an ARM board handy with serial and usb, but never set up NTS with a GPS. Is it just NEMA commands? TO work well with NTP the borad needs a low latency PPS interrupt handler. THis means that you need both (1) the DCD line on the rs232 port tied to a CPU interrupt pin, with not much between the DCD and the CPU pin and (2) The OS has a simple low latency handler. Linux x86 has a good PPS handler. I don't know about Linux ARM. Also I don't know how the interrupt hardware works on the ARM board. But if it is as described above it will work well enough. You do need access to a hardware interrupt pin. Hi, I am a newbie on this list. I had tried to do PPS via a USB serial dongle that supported DCD. I could not get under 1ms jitter. I found that since kernel 3.2 there is a PPS-GPIO module which you could use to register an available GPIO pin for PPS interface. I had successfully implemented PPS though GPIO pin on Linux ARM (first on a beagleboard-xm, then a beaglebone). ntpq -p shows: remote refid st t when poll reach delay offset jitter == *SHM(0) .GPS.0 l4 16 3770.000 -19.922 9.783 oPPS(0) .GPIO. 0 l2 16 3770.000 -0.281 0.002 -vhost.cohesivel 128.249.1.10 3 u 50 64 377 56.110 13.134 2.715 -clock01.laca02. 216.119.63.113 2 u 16 64 377 25.002 17.157 1.003 +cheezum.mattnor 129.7.1.66 2 u 20 64 377 57.517 11.404 1.817 +gatekeeper.tss. 173.13.85.5 2 u 31 64 377 39.604 15.033 0.929 This was done first using the Garmin 18x LVC then later changed to the GPS breakout from Adafruit http://adafruit.com/products/746 which is already on 3.3V logic and the NMEA output does not need inversion. I had recently shared a more detailed write-up at https://groups.google.com/d/topic/beagleboard/bU_xZ9tWoiA/discussion(second post under that topic). George It might be NMEA or it might be some other serial protocol like TSIP. The better timing type GPS receiver avoid NMEA because NMEA lacks soe commands and timing data sentences. NMEA was designed for boat and ship navigation, not timing. -- Chris Albertson Redondo Beach, California ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Can I get 1 millisecond accuracy with a USB GPS-18
On Mon, May 13, 2013 at 7:52 PM, li...@lazygranch.com wrote: Note that MAX232 type chips add jitter. The charge pumps make substrate noise and that leaks into the receivers/transmitters. How much noise or jitter? Has anyone measured the effect? Is it at the nano or micro second level? It would be easy enough to use a transistor if it improved the timing enough to matter. -- Chris Albertson Redondo Beach, California ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
