locked Re: Clock Sync #Timesync


Wish I had something to add to this discussion.  I don’t.  I have a couple questions though:

  1. What would be a good GPS hat for a Raspberry Pi 4?  I bought one for a purpose that didn’t work out.
  2. Where would I find a procedure for building a time source like has been mentioned in this thread?



I recognize that I don’t need the level of accuracy provided by a good local time source, but I’m looking for a small project.


Dan – K4SHQ



From: main@WSJTX.groups.io <main@WSJTX.groups.io> On Behalf Of Kevin McQuiggin (VE7ZD/KN7Q)
Sent: Sunday, July 4, 2021 12:53 PM
To: main@wsjtx.groups.io
Subject: Re: [WSJTX] Clock Sync #Timesync


On 04/07/2021 15:40, Jim Pennino via groups.io wrote:

Re fake chips: I have no idea and don't care though u-blox might. The only features used by ntp is the output of NMEA sentences so any other capability the u-blox 8 series has is irrelevant to ntp. FYI u-blox makes dozens of different chips with different capabilities all of which identify as u-blox 8.


Actually ntpd doesn’t work very well using NMEA sentences via USB or serial port as input.  This is because of variance in the timing of the serial data stream and the way that the machine's CPU responds to interrupts on the serial or USB port from the GPS where the GPS unit is connected.  Timing can be all over the map due to CPU workload and other tasks which are running.


If you have a local GPS receiver, then a FAR better approach is to use a pulsed output from the device to generate a high priority interrupt that is handled directly by the time management processes.  This involves another daemon called gpsd and use of a GPS receiver that outputs a nanosecond-accurate “pulse per second” signal.  Cheap receivers usually do not provide a PPS signal but several slightly more expensive ones do.  


There is a good writeup on how to do this at https://www.lammertbies.nl/comm/info/gps-time.  


I use a Garmin Garmin 18x LVC GPS Navigator Unit, which is available on Amazon for about USD$68.  See the link for how to connect what to where!




Kevin VE7ZD/KN7Q






Hi Jim,

one issue with "fake" u-blox boards (note they usually have real u-blox GPS/GNSS receivers but lack on the rest of the board) is that some have insufficiently sized SPI memory chips and as a result cannot be upgraded to newer firmware versions. That can be annoying if a new satellite constellation needs to be supported, for example.



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