gr-usrp2: implement start_streaming_at(usrp2::fpga_timestamp time)

This new method on usrp2.sink_* causes the first TX sample data to be
sent at the FPGA clock time specified, with all further data immediately
following.

u = usrp2.sink_32fc() # or 16sc
...configure sink here...
u.sync_to_pps()
...delay a second for PPS to have happened
u.start_streaming_at(int(100e6)) # start TX stream one second later
...start flowgraph here...

If this function is not called, all transmit data will be sent
immediately (the prior behavior).

1 files changed, 12 insertions, 4 deletions

diff --git a/gr-usrp2/src/usrp2_sink_16sc.cc b/gr-usrp2/src/usrp2_sink_16sc.cc
index 1e7c54dcd..75cc1f4a6 100644
--- a/gr-usrp2/src/usrp2_sink_16sc.cc
+++ b/gr-usrp2/src/usrp2_sink_16sc.cc
@@ -67,12 +67,20 @@ usrp2_sink_16sc::work(int noutput_items,
     return 0;
 
   usrp2::tx_metadata metadata;
-  metadata.timestamp = -1;
-  metadata.send_now = 1;
+
+  // Set TX metadata to either start time or now
+  if (d_should_wait == true) {
+    metadata.timestamp = d_tx_time;
+    metadata.send_now = 0;
+    d_should_wait = false;
+  }
+  else {
+    metadata.timestamp = -1;
+    metadata.send_now = 1;
+  }
   metadata.start_of_burst = 1;
 
-  bool ok = d_u2->tx_16sc(0,  // FIXME: someday, streams will have channel numbers
-			  in, noutput_items, &metadata);
+  bool ok = d_u2->tx_16sc(0, in, noutput_items, &metadata);
   if (!ok){
     std::cerr << "usrp2_sink_16sc: tx_16sc failed" << std::endl;
     return -1;	// say we're done