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Re: [Discuss-gnuradio] Question about USRP2 Tx procedure
From: |
Nowlan, Sean |
Subject: |
Re: [Discuss-gnuradio] Question about USRP2 Tx procedure |
Date: |
Fri, 13 Apr 2012 15:36:52 +0000 |
A few questions about this: can you wait on the uhd async message queue from a
python script that has an instance of top_block? Would you have to override
wait() with a custom version that waits on the queue (haven't checked if wait
is a virtual function)?
Does UHD also ACK time tags and start-of-burst tags?
Thanks,
Sean
-----Original Message-----
From: address@hidden [mailto:address@hidden On Behalf Of Josh Blum
Sent: Friday, April 13, 2012 11:26 AM
To: address@hidden
Subject: Re: [Discuss-gnuradio] Question about USRP2 Tx procedure
On 04/13/2012 06:54 AM, Pan, Luyuan wrote:
> Hi everyone,
> I now have some trouble in understanding how the usrp2 sent out
> the data. My scenario of the test is:
> We tried to control the usrp2 to transmit in a fixed time slot, such
> as
> 5 seconds. The code is:
> tb = usrp.transmit_path() # Create the path
> t1 = time.time()
> tb.start()
> while (time.time() - t1< 5):
> .............. # The code to send out the data
> continue;
> time.sleep(0.65) # to ensure all the data are sent
> truly out......... Question?????? WHY
> tb.stop()
> tb.wait()
> My question lies in line time.sleep(0.65). if we don't use
> time.sleep(), we can not receive all the packets, every time the
> receive side will lose about 50 packets(each one is 1500 bytes), and
> if time.sleep()<0.65s, it will still lose but less than 50. So, I want
> to know why it need such a time(about 650ms)? And we have some hypotheses:
> 1. The usrp did not send out data instantly at tb.start(), it need
> time to build the flow graph. But we do an experiment to get the time
> we received the first packet, and found the truth is not that. The
> time tb.start and (in the recv side)the time we received the packet is
> almost the same. So, It send out immediately.
> 2. But we encountered another dilemma, if we let it send an
> extremely short time, like while (time.time() - t1< 0.02) (generally
> less than 50 packets), and do not use time.sleep(), it will never send
> out the data. Only add time.sleep(), it will success. Why? I really
> confused!~
> 3. Considering that we only need 650ms, no matter how long we
> send(like 10s or more). We guess there is a fixed size cache in the
> usrp and it send the cached data at a precise-controlled time, but I
> can not persuade myself.
> Can anyone interpret the strange phenomenon? Any suggestion is
> appreciated, thank you!!
>
The is buffering in the USRP2, about 1 MB. Give your sample rate and 4 bytes
per sample you can approximate the sleep time.
The more proper way to do this is to send an end-of-burst tag with the last
sample and to wait on the async message queue for a burst ACK packet.
About the TX tags for the sink block:
http://gnuradio.org/cgit/gnuradio.git/tree/gr-uhd/include/gr_uhd_usrp_sink.h#n52
Also see the async message source:
http://gnuradio.org/cgit/gnuradio.git/tree/gr-uhd/include/gr_uhd_amsg_source.h
-josh
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