[Discuss-gnuradio] Multiple USRP2 sync

if I have two (or more) usrp2s, and I do a set_time_next_pps(), there is (or is there?) a chance that the pps could occur after the first call, so the second call will set the second usrp a second later. I would need to know when there is a pps or seconds transition prior to setup, then have one or two seconds to call set_time_next_pps(). Ok, I could read the system time, but I would need a gps with pps/ref out, and sync'd using ntpd or gpsd, and this may not be guaranteed. Or I could just monitor the pps level via the serial port (DCD, like ntpd does with nmea stream) to guarantee when the seconds tick over. If the pps state could be read back from the usrp (approx 80ns rtt) however, this would simplify the system and hardware requied (USB dongle, TTL to RS232 convertor, extra wires, etc...). So...

while (get_pps_level() == 0); // wait for edge
while (get_pps_level() == 1);

u1->set_time_next_pps(...); // We are guaranteed at least one second to issue this to both usrp at once
u2->set_time_next_pps(...);

Thanks
David