[Discuss-gnuradio] What time is it?

[Mattias Kjellsson]

I was exited to see the vrt progress. I have compiled Josh's usrp2_vrt- 
branch, installed the required firmware/fpga- image in the sd- card, and 
can find the usrp2. Everything working so far.

But now comes the question, is there a way to read the current usrp2- 
time, so that I can do something like "start_rx_streaming(0, &future_time)".
I basically try to figure out what the time in the usrp2 is, and then 
add a number of seconds/ticks/whatever to that, in order to start 
receiving at some future instant.

To do this, I have tried two ways
First I tried to use two instances of a 16sc handler, one which just 
read one sample (writing it to /dev/null), from which I extracted the 
time, and then stopped the streaming.
Then I add a number of seconds/ticks to the time I got from the usrp2, 
and start streaming again with that incremented time as the second 
parameter to "start_rx_streaming(..,..)", and try to receive samples 
using the second handler (which was actually writing to a file).

This results in two things,
1)    During reception with the first handler, a message about bad VRT- 
header is displayed, which I tracked back to usrp2_impl.cc, but not 
further than that, since I can't really figure out under which 
conditions the message is displayed.
2)   During reception with the second handler, no samples are received. 
I thought that it might have something to do with seconds being to long, 
or a tick being to short, so I tried adding different values but that 
didn't seem to change much, since the out- file was still 0 bytes.

In the second approach:
I noticed that there is a #define OP_READ_TIME  3, but after some 
grepping I only found it in usrp2_eth_packet.h (where it was defined) 
and in "usrp2_impl.cc, std::string opcode_to_string(int opcode)". What 
are the plans of this op- code, a "what time is it on my usrp2 right 
now"- function?

If that is the case, and it has not been written yet, I did some 
rewriting of usrp2.cc, usrp2_impl.cc, usrp2.h and usrp2_impl.h for that 
functionality (give me current time on the usrp2), using the already 
existing struct op_read_time_reply_t. But this raises another question, 
namely: What is "op_read_time_reply_t.time" actually describing? It is a 
uint32_t, but the vrt_header seems to use a uint32_t for integer 
seconds, and a uint64_t for fractional seconds, while the time_spec_t 
structure seems to use two uint32_t for ticks and seconds, and another 
structure for something I'm not sure about... I guess the question is, 
do I parse the 32 bit integer to a time_spec_t? And if so how?

I think I lost my self somewhere. However would I be very happy if 
someone could straighten my questions out.

Best regards
Mattias Kjellsson