Re: [Discuss-gnuradio] Start_rx_streaming_at patch problems

[Doug Geiger]

Eric Blossom wrote:
> You're probably trying to start "In the past".  That is, your start_at
> time has already passed (the clock starts ticking when the USRP2
> powers up).  I think we'd need to add a "get_current_time" command to
> give you a clue as to when to start.  You'd need to add a suitable
> offset to the returned value to account for the time required to
> process the previous reponse and start_at command.
>
> Eric
>
>
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To expand on that a little bit, I've found that it works best if I call 
sync_to_pps, wait a short interval (I've been calling nanosleep with at 
least 1e6 nanoseconds - but usually just over one second), then call 
start_rxstreaming_at(0, <desired # of samples, I just use 
USRP2_MAX_RX_SAMPLES here - setting it to zero should have the same 
effect>, <desired fpga start clock time>), where the fpga start time is 
at least something greater than the nanosleep interval. However, the 
real problem here is that you don't have any idea how far into the 
future the next 1PPS signal is going to arrive (i.e. it could be just 
under one second into the future, or it could be essentially zero time 
in the future), unless you have some way of getting the signal into the 
host PC as well.  So I typically sleep for just over a second, and set 
my fpga start time to be something fairly large (i.e. greater than one 
second), but not too large.  I have found that if it's too far into the 
future, the command will 'disappear' from the command queue in the 
USRP2. Perhaps a better solution will come along WRT synchronizing the 
host PC and the USRP2, but in the meantime I'm able to get it to work 
with this fairly hack-ish method.
As far as using it in gr-usrp2, I ended up writing my own custom source 
block, which started out life as a copy of the 
gr-usrp2/usrp2_source_32fc block.
 Doug

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