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Re: [Discuss-gnuradio] about USRP2_MIN_RX_SAMPLES

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] about USRP2_MIN_RX_SAMPLES
Date: Tue, 15 Sep 2009 11:22:39 -0700
User-agent: Mutt/1.5.18 (2008-05-17)

On Tue, Sep 15, 2009 at 04:41:39PM +0200, Ulrika Uppman wrote:
> Hi all!
> I ran into the constant USRP2_MIN_RX_SAMPLES = 371 defined in
> usrp2_base.h, but I don't get what it is used for (it comes with the
> comment "BIG ASS FIXME: get from lower layer MTU calculation").

I believe you're referring to USRP2_MAX_RX_SAMPLES.  It's the largest
number of samples that will fit in a 1500 byte ethernet packet
including overhead.  The constant sizes some structures.  (BTW, it was
recently adjusted downward to 370 samples to ensure alignment for dual
channel apps).

> Does anyone know anything about this constant and what it affects? 
> ... what is depending on that it has the value 371?

> (I got seg faults when trying to run less than 371 items out of a
> USRP2 source block, I believe this constant has something to do with
> that perhaps?)

How we're you trying to do this?  Hint: don't change the constant.

We don't currently expose this in the gr-usrp2 interface.

We do expose it in the usrp2::usrp2::start_rx_streaming interface.
items_per_frame = 0 gets you the default (currently 370 or 371
depending on rev).  You can set it lower if you like.
usrp2_source_base::start currently passes 0 to get the default.


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