Re: [Discuss-gnuradio] time tags for transition time using USRPs

[Marcus Müller]

Hi Henning,

to only answer the first part of your questions:

On 04/27/2016 03:22 PM, Henning Bredenberg wrote:

Hi Martin,

the time tag 'tx_time' tells the USRP when to transmit - so far so good. So I don't need any delay blocks because the USRP sink realizes the delay, right?

Nearly! In fact, the USRP itself realizes that in hardware -- which is much, much better, because that hardware "knows" and runs at a rate directly linked to the sampling rate and LO frequencies; if your PC waited the 240ms of "PC clock time", that might be anything between 238 and 300ms "USRP time", if we add non-deterministic delay of the USB transport for commands. So: there's a feature in the FPGA image that allows you to specify "hey, FPGA, I want you to execute that "start streaming" command *exactly* at the clock tick nr 12147, and it will do that.

Assume i want to add a propagation delay of 240ms: Setting 'tx_time' at the begin of the burst to 240ms will realize that?

No, you should first set the device time to something sensible (or just roll with it being set to zero on device initialization) and use relative times to that.
Minor caveat: if you're using a device with adjustable master clock rate (mainly: B2xx and E31x), and you change the MCR after setting the time, the host won't know when that change of "tick rate" happened, and the conversion between timestamps and tick numbers will be pretty random.

Best regards,
Marcus

Do I tag the burst in the code of the PDU-generator using add_item_tag() or is there another block who does it after the generator (I thought the pdu_to_tagged_stream - block tags the packet)?

The crucial code in the random_pdu_impl.cc is:
[...]
      // pick a random vector length
      int len = d_rvar();
      len = std::max(d_length_modulo, len - len%d_length_modulo);

      // fill it with random bytes
      std::vector<unsigned char> vec(len);
      for (int i=0; i<len; i++)
        vec[i] = ((unsigned char) d_bvar()) & d_mask;

      // send the vector
      pmt::pmt_t vecpmt(pmt::make_blob(&vec[0], len));
      pmt::pmt_t pdu(pmt::cons(pmt::PMT_NIL, vecpmt));

      message_port_pub(PDU_PORT_ID, pdu);
[...]

My idea then is to use add_item_tag here with 'tx_time' as key for the output vector "PDU_PORT_ID", i.d. "pdus" in GRC.

An additional question: How do I set a data rate that determines how long a packet needs to be transmitted? Is that done by USRP as well?

By now the time sink just shows a blue, continuous signal as seen in the attached picture.

Best regards,
Henning

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