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 measurement<time-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.com>wrote: > 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 l 4 16 377 0.000 -19.922 9.783 oPPS(0) .GPIO. 0 l 2 16 377 0.000 -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.