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.
