Re: [Discuss-gnuradio] What time is it?

[Doug Geiger]

Josh Blum wrote:
> There are new updates to the usrp2_vrt branch.
>
> 1) The sync at pps calls have been removed. They no longer apply and 
> have been replaced with the set time calls (see below).
>
> 2) The mimo config call has been replaced with a clock config call 
> that handles setting the pps source, pps polarity, ref source, (and 
> other options).
>
> 3) Two calls were added to set the time: set_time and 
> set_time_at_next_pps. A host pc hooked up to the serial port of a 
> GPSDO could use set_time_at_next_pps to set the usrp2 with absolute 
> time. Example mentioned in usrp2.h
>
> 4) That bug with the junk data i believe to be fixed. There was an 
> improper way to stop the streaming.
>
> 5) There is an example app usrp2/apps/rx_timed_samples
> This app will set the usrp2 time to the host system time with 
> subsecond precision. Then it will call start streaming with a time in 
> the future. The timestamps on the received packets are printed to 
> verity working.
>
> These changes require burning new fw and fpga images. I will post 
> these if there is an interest. Otherwise, I will refrain from posting 
> them as there are other kinks to work out in the codes.
>
> -Josh
Exciting! I'll take a look. Out of curiosity, do you have any sense of 
what sort of sub-second precision you're getting on setting the usrp2 
time according to the host? It looks like the sample is fairly simple, 
(i.e. just getting the current system time, then turning around and 
setting the USRP2 time ASAP to that time, taking into account the fpga 
clock rate). I wonder if the sync between host and USRP2 wouldn't be 
better improved by measuring the (rough) RTT, and taking that into 
account (e.g. measuring current system time, sending a control message 
to the USRP2 and getting a response, measuring the system time again, 
and using the difference as the RTT).
Doug

--
Douglas Geiger
Code 5545
U.S. Naval Research Laboratory
Washington, DC 20375
(202) 767-9048
address@hidden