Re: [Discuss-gnuradio] Understanding USRP2 flow control

[Eric Blossom]

On Sun, Jul 18, 2010 at 06:50:47PM +0200, OE1RSA wrote:
> Dear All!
> 
> I am starting to learn my new USRP2 device.
> I bought a reasonable fast quad core PC and
> a recommended ethernet board.
> 
> I wanted to find out if I can get the interpolation 4
> working in my environment. I just connect a sine
> oszillator to the usrp2 sink.
> 
> I am observing the signal on a connected scope.
> 
> Everything works fine for a while, but then suddenly
> the output becomes heavily distorted.
> 
> I can see from the resource watch of the system
> monitor that my PC putting out data at a sustained
> rate of more than 800 Mbits/sec.
> 
> I started to try to understand the code in
> gnuradio/usrp2/host/lib and expected some flow control
> when the usrp2 input buffers are nearly full .
> Unfortunately I was not able to understand how this
> is implemented.
> 
> Is there any flow control at all?
> If not, what is supposed to happen, if the PC is
> sending (slightly) too fast?
> 
> Appreciating any information on this subject,
> Roland

Flow control is handled at the link level using ethernet PAUSE frames.

Be sure to enable real-time scheduling by calling

    if gr.enable_realtime_scheduling() != gr.RT_OK:
        print "Note: failed to enable realtime scheduling, continuing"
    
before starting or running the flow graph.

If you haven't already, ensure that /etc/security/limits.conf contains
this line:

@usrp            -       rtprio          50

and that you are a member of group "usrp".

  address@hidden ~]$ groups
  eb wheel dialout usrp

Eric