[USRP-users] UHD command delay is different in the first recv stream

Michael West michael.west at ettus.com
Tue Oct 6 19:26:04 EDT 2015

Hi Dario,

Very interesting find.  I have found that changes to either the master
clock rate or sample rate affect the delay, but the delays are constant for
a given master clock rate and sample rate.  It looks like an FPGA issue of
some sort and it will take a deeper dive into the FPGA code to resolve.  We
will look into it.  Let me know if you notice any other correlations that
may help identify the root cause.


On Mon, Oct 5, 2015 at 12:57 PM, Dario Fertonani via USRP-users <
usrp-users at lists.ettus.com> wrote:

> I brought this up when UHD 3.9.0 was released, in a thread with a
> different main topic (CODED loopback issue on B210). I thought I'd open a
> specific thread, now that the issue is present in UHD 3.9.1.
> Let's call "UHD command delay" the difference between the time stamp of
> the first sample received and the time stamp of that sample as requested by
> the recv stream command (see attached code). Until UHD 3.8.5, this delay
> was constant within an error well below a microsecond. Starting with UHD
> 3.9.0, this delay is much grater for the first run (off by several us
> w.r.t. the next runs). This is not an issue for me, as long as I can assume
> that what I measure for the second stream is good (within a fraction of a
> us) for all following streams, which seems to be the case. We have been
> making this assumption and discarding the first stream from the LTE
> processing (we open/close streams every few tens of seconds).
> The absolute values depend on the sampling rate. See below the output of a
> run at 4 MHz sampling rate.
> Thanks,
> Dario
> $ g++ rxTest.cpp -o rxTest -std=c++11 -O3 -luhd
> $ ./rxTest 739 4 1 1000 128 3 > /dev/null
> Run #0 completed without errors!
> UHD command delay measured at 9625 ns.
> Run #1 completed without errors!
> UHD command delay measured at 3750 ns.
> Run #2 completed without errors!
> UHD command delay measured at 3750 ns.
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