There is a better way. If you look in gr_uhd_usrp_sink.cc, it's expecting to see tags indicating start of burst ("tx_sob") and end of burst ("tx_eob") as well as a transmit time ("tx_time"). To issue a start of burst, for instance, use:
add_item_tag(0, //stream ID
nitems_written(0)+i, //absolute sample number to tag
pmt::pmt_string_to_symbol("tx_eob"), //tag name
pmt::pmt_from_bool(1), //set this true to indicate start of burst
d_me //block src id
);
Similar syntax applies for marking the end of burst (use tx_eob instead). To use the tx_time tag, use a pmt::pmt_from_tuple where the bool is above. The tuple is (uint64_t, double) where the integer value is seconds and the double value is fractional seconds.
The block source ID d_me above is really just a convenience identifier and one way to put it together is:
std::stringstream id;
id << name() << unique_id();
d_me = pmt::pmt_string_to_symbol(id.str());
It does sound a little complicated but it works well in practice. Feel free to keep asking questions. The best reference is gr_uhd_usrp_sink.cc, and you'll probably want to look at gruel/src/include/gruel/pmt.h.
--n
On Tue, Jun 19, 2012 at 6:08 AM, Andrew Senior
<address@hidden> wrote:
Hello all,
I've been trying to use the set_start_time() method of usrp_source to start my USRP N210 streaming data at a known time. The USRP has the GPSDO fitted and the time is set automatically from this, so I can specify an absolute time for when I want streaming to begin. I've done this successfully using UHD in C++ programs, but I couldn't get it to work in GNURadio. For the record, I'm using GNURadio 3.6.1 and UHD 3.4.2.
The problem seemed to be that despite calling set_start_time(), streaming didn't start when it should, yet if I specified a start time in the past, the program failed with an error as I'd expect, so the start time seemed to be interpreted correctly.
Having studied gr_uhd_usrp_source.cc, I realised that the problem is that the timeout for the first call to the recv() function in UHD is very short so that if my start time is many seconds in the future, the call times out and the block is done.
My question is: is this the intended behaviour and if so, how do I avoid my flowgraph stopping before it's even started, so to speak? I could delay starting the flowgraph until the time is sufficiently close to the wanted start of streaming, but that sounds a bit risky.
I have made a small patch to gr_uhd_usrp_source.cc to automatically set an initial receive timeout based on the device time when the streaming command is issued and the wanted streaming start time. Being new to GNURadio, I'm not sure if this is the right way to solve the problem but I'm happy to share the patch if anyone thinks it might be useful.
I would welcome any comments and suggestions on this.
Thanks,
Andrew.
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