[USRP-users] Trouble understanding o_tdata

Jonathon Pendlum jonathon.pendlum at ettus.com
Wed Jul 8 13:33:54 EDT 2015

Hi Jason,

You have the right idea on the meaning of the signals. I think you might
have a syntax issue though. Using the division operator with concatenation
is probably causing an issue since the resulting bitwidth is wider than
o_tdata. Try changing the code to:

assign o_tdata = {i_tdata[WIDTH-1:WIDTH/2] >>> 1,i_tdata[WIDTH/2-1:0] >>>


On Tue, Jul 7, 2015 at 5:45 AM, Jason Matusiak via USRP-users <
usrp-users at lists.ettus.com> wrote:

> In my endless pursuit to understand the inner workings of RFNoC, I am
> still having issues creating my dummy block that halves the incoming
> signal.  I can create a new FPGA bitfile and GRC script and get
> everything to run, but I am having trouble getting it to function as I
> would expect.
> If I make a script that is:
>  RFNOC: Radio -> RFNoC: FIFO -> QT GUI Freq Sink
> I can see noise and then when I turn on my exterior tone and broadcast
> it at 101MHz, see it pop up on my freq sink.
> I then modify my GRC script to add in my new block so that the stream
> looks like:
>  RFNOC: Radio -> RFNoC: FIFO -> RFNoc: halfSignal -> QT GUI Freq Sink
> Now I would expect everything to function the same, and when I turn on
> my externally broadcast tone, it would show a 3-6dB decrease in power
> due to the halving block being in place.  This does not happen, instead
> I always see a tone at the center of the freq sink, even when I am not
> broadcasting my external tone.  This leads me to believe that I am not
> manipulating the data properly in my halving block on the FPGA.
> On the FPGA, I mimic the keep_one_in_n FPGA files by creating
> halfSignal.v and halfSignal_vec.v files.  In the halfSignal_vec.v file,
> the pertinent info is:
>    assign i_tready = o_tready;
>    assign o_tvalid = i_tvalid;
>    assign o_tdata = {i_tdata[WIDTH-1:WIDTH/2]/2,i_tdata[WIDTH/2-1:0]/2};
>    assign o_tlast = i_tlast;
> Am I misunderstanding what o_tdata/i_tdata actually is used for?  In my
> above example I am assuming that i_tdata contains a concatenation of the
> I/Q incoming stream.
> When I stop the script from running on the E310, I do see an error
> message, but I don't think it is relevant to this problem.  Here it is
> anyway just in case:
> python2:
> /home/balister/fido-test/build/tmp-glibc/work/armv7ahf-vfp-neon-oe-linux-gnueabi/python/2.7.9-r1/Python-2.7.9/Modules/gcmodule.c:366:
> visit_decref: Assertion `gc->gc.gc_refs != 0' failed.
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> USRP-users at lists.ettus.com
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