Dear Marcus, Thanks a lot for your reply and hope you are enjoying the Holidays.
I am trying to have the Tx and Rx of a radio start at the same time. I (currently) need to have a well-defined, consistent sample offset between Tx and Rx in my flowgraph. When the Tx and Rx start is not important, just that they start at the same time. What you said makes a lot of sense. I left out what is probably a critical detail, which is that the USRP in question is synchronized to an Octoclock. As long as I have the "sync" option set to "Unknown PPS" should I not need any further function calls to ensure Tx and Rx start at the same time in a consistent manner? Thanks again On Thu, Dec 23, 2021 at 9:39 AM Marcus D. Leech <patchvonbr...@gmail.com> wrote: > On 2021-12-23 00:30, Manav Kohli wrote: > > Hello, > > Hope everyone is doing well. I am encountering an issue regarding late > packet arrivals when using Function Probe blocks to call set_start_time and > set_time_next_pps. > > I have a flowgraph with a USRP Source and Sink and I am applying the > following two function calls to each (so four function probes in total). > The function probes have their rate set to 1e-9 (so they should only ever > run once). > > set_time_next_pps(uhd.time_spec_t(0.0)) > set_start_time(uhd.time_spec_t(2.0)) > > The USRP Sink is fed by a file source and the USRP Source feeds a QT > Frequency Sink. There is nothing else going on and the Sink and Source are > on the same subdev. If I run the flowgraph, the following will invariably > occur: a cascade of LLLL... printed to stdout and the following warning: > "gr::log :WARN: usrp_source0 - USRP Source Block caught rx error code: 2" > > Disabling the Function Probes with set_start_time(uhd.time_spec_t(2.0)) or > removing the time_spec_t function parameter in the set_start_time function > call let the flowgraph run fine. > > I've tried to look up exactly what is going on behind these two function > calls but I have been unable to make any headway. I saw a previous thread > here > <https://lists.gnu.org/archive/html/discuss-gnuradio/2015-10/msg00035.html> > which > doesn't seem to have a resolution. > > GNU Radio & USRP environment: > Ubuntu 20.04 > UHD 4.0.0.0 > GNU Radio 3.8 > USRP 2974 > > I think this may have something to do with the USRP and PC clocks drifting > apart, but this happens as soon as the Tx and Rx should turn on, i.e. two > seconds after running the flowgraph, so I am not sure. Any help would be > greatly appreciated, and thank you for your time in reading through this > post. > > Take care > Manav > > My suspicion is that your approach is "all wrong". What is it you're > trying to actually do? > > The gr-uhd module already does this when you select "unknown_pps" or > "next_pps" in the "SYNC" option in the source/sink blocks. Your approach > is going to cause multiple > resets of the system clock on the radio, at unpredictable times, likely > leading to the results you're seeing. > > >
