[USRP-users] UHD timestamp synchronization for multiple USRPs connected to an unlocked Octoclock-G

Urban Hakansson uhakansson at tecore.com
Fri Dec 12 14:06:56 EST 2014

Hi everybody, 

First I thought that by getting an Octoclock-G and connecting all my USRPs to the Octoclock-G (unlocked mode) would ensure UHD timestamp synchronization amongst the USRPs. 

However, after I have read http://files.ettus.com/manual/page_sync.html and thought about it carefully I still cannot figure out how to guarantee time stamp UHD sycnhronization with just the 1PPS and the 10MHz reference signals. The fact that the 1PPS signals are perfectly aligned in time amongst the USPRs don't seem to be sufficient. 

Method 1, poll USRP registers only ensures that you set the time on the next 1PPS pulse 

const uhd::time_spec_t last_pps_time = usrp->get_time_last_pps(); 
while (last_pps_time == usrp->get_time_last_pps()){ 
//sleep 100 milliseconds (give or take) 
usrp->set_time_next_pps( uhd::time_spec_t (0.0)); 
How do I know for sure that all the USRPs trigger on the same 1PPS pulse? 

Even if the USRPs boot-up just slightly slower/faster than each other I can see it easily happening that the USRPs could set t = 0.0, for two different 1PPS pulses, which is not good. 

Method 2 - query the GPSDO for seconds 
Can I query the Octoclock-G for an NMEA string? The Octoclock-G does not have a serial interface. 

//call user's function to wait for NMEA message... 
usrp->set_time_next_pps( uhd::time_spec_t (0.0)); 
Is there a sure-fire method for achieving time-stamp synchronization? It does not have to be absolute, UTC time etc.. I just want the USRPs to start counting time at the same time, preferably by calling usrp->set_time_next_pps( uhd::time_spec_t (0.0)) for the same 1PPS pulse. 

Thanks for your consideration. 

Urban Hakansson 
Senior Software Engineer 
Tecore Networks 
Phone: +1 410 872 6315 
Fax: +1 410 872 6010 
Email: uhakansson at tecore.com 

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