[time-nuts] Can I get 1 millisecond accuracy with a USB GPS-18
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 126.96.36.199 3 u 50 64 377 56.110 13.134 2.715
-clock01.laca02. 188.8.131.52 2 u 16 64 377 25.002 17.157 1.003
+cheezum.mattnor 184.108.40.206 2 u 20 64 377 57.517 11.404 1.817
+gatekeeper.tss. 220.127.116.11 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
> and follow the instructions there.
More information about the time-nuts