PPS and accurate headless timing

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PPS and accurate headless timing

Postby WITS-vlf » Thu Sep 17, 2015 6:49 pm

Any ideas of how to set-up a stratum1 time server on the board? I am looking to deploy the board headlessly without an internet connection and need to have a very accurate timing. Currently I've got the board to take in PPS via gpio24 (using this http://vanheusden.com/time/rpi_gpio_ntp/ ), should I have rather changed the kernel ? Can the pps signal be taken in with the rest of the gps signal somehow? I'm not so sure it can be done, because usb does not have enough pins nor does the half-serial on the board , right ? I guess I have all the element and just need a shove in the right direction. To sum up, I can get a pps signal through a gpio pin and the satellite time from the usb-connected part of the gps, how would I integrate the two ?
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Re: PPS and accurate headless timing

Postby vpeter » Fri Sep 18, 2015 6:51 am

Can't you just read time directly from GPS receiver via USB? I'm sure Linux based time servers with GPS works like that (no GPIO).
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Re: PPS and accurate headless timing

Postby WITS-vlf » Fri Sep 18, 2015 12:39 pm

The problem with usb is that the pps does not transfer over usb only the timestamp. I need the timing to be super accurate and the only way to get the accuracy I need is to use the pps. I have been doing some reading online and I think I may have found a solution but please give me your thoughts. I've read that I could send the pps over the RTS line and linux should pick-up the signal, is this correct ?
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Re: PPS and accurate headless timing

Postby vpeter » Fri Sep 18, 2015 1:55 pm

Because this is interesting for me too I checked little more. Yes, you need to feed PPS signal. You can use serial port for this (DCD - Data Carrier Detect pin) or in case of embedded devices you can use GPIO pin. For this you need to enable PPS in kernel and also set in device tree.
Maybe one day I will implement this :)
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Re: PPS and accurate headless timing

Postby jury093 » Fri Sep 18, 2015 3:54 pm

vpeter wrote:Because this is interesting for me too I checked little more. Yes, you need to feed PPS signal. You can use serial port for this (DCD - Data Carrier Detect pin) or in case of embedded devices you can use GPIO pin. For this you need to enable PPS in kernel and also set in device tree.
Maybe one day I will implement this :)

2-3 years ago I make home ntp server. Based on Atmel at91sam9260 and china gps reciever (model gps02, uart+pps) + external antenna
in kernel sources (hmm.. may be 3.6.0) I add serial S1 for uart support and free pin for pps-gpio..
apt-get install ntp
I write ntpd.conf for my hardware and homemade ntp server share time to my flat (x86 computers, arm kits, NAS and internet router)

but for pps-gpio required to rebuild ntpd sources with support pps-gpio and make config..

in my case I see high jitter for pps-gpio and ntpd regularly rejected my pps-gpio for stratum data source and switch source time to poll of free internet servers..
I do not know the exact cause - may be china gps fail or slow ARM 9260. but I disable pps-gpio
And now my ntpd get nmea data from reciever from uart and everyone is happy :)

PS: Now I can get SIM5360E (3G/GPS/GLONAS + pps of course) and may be incorporated in my home project "wandbook".. may be..
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Re: PPS and accurate headless timing

Postby mikasa5678 » Tue Nov 03, 2015 2:40 am

I need the timing to be super accurate and the only way to get the accuracy I need is to use the pps.
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Re: PPS and accurate headless timing

Postby stevenfalco » Tue Dec 15, 2015 3:55 pm

I have built an NTP clock using yocto fido. It includes the PPS interface and provides microsecond accurate timing to my home network.

You can grab the following:

https://www.dropbox.com/s/1z4zr7kvhamsg ... ar.gz?dl=0

In that tar you will find meta-falco which includes my patches. You probably won't need all of them, as I also hacked ntp to display the satellites, signal strength, etc. on the touch-screen. I used a 3DR GPS module. It contains a uBlox chip, and while they don't advertise the PPS connection, I was able to white-wire it based on the uBlox datasheet.

I used TP126 for PPS input GPIO (2,13), which is pad SD4_DAT5.

I also added a chronodot to provide a battery backed real time chip. I wired that into the same I2C bus that the audio chip uses.

There are a few other files in the tar that you will need. bblayers.conf is what connects up meta-falco, and local.conf brings in pps tools that are handy for debug.

There are also a few jpgs that show what the display looks like and some of the white-wires.
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