[time-nuts] Can I get 1 millisecond accuracy with a USB GPS-18

George Lu lu at goodxense.com
Mon May 13 13:04:59 EDT 2013

On Mon, May 13, 2013 at 9:25 AM, Chris Albertson
<albertson.chris at gmail.com>wrote:

> On Mon, May 13, 2013 at 9:04 AM,  <lists at 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     3 u   50   64  377   56.110   13.134   2.715
-clock01.laca02.   2 u   16   64  377   25.002   17.157   1.003
+cheezum.mattnor       2 u   20   64  377   57.517   11.404   1.817
+gatekeeper.tss.      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
post under that topic).


> 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 at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.

More information about the time-nuts mailing list