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Re: [Discuss-gnuradio] Using PPS as a sampling trigger

From: Josh Blum
Subject: Re: [Discuss-gnuradio] Using PPS as a sampling trigger
Date: Mon, 13 Feb 2012 10:36:18 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:9.0) Gecko/20111229 Thunderbird/9.0

On 02/13/2012 08:21 AM, rmsrms1987 wrote:
> Hello Everyone, 
> I am a relatively new user to the USRP2 and currently would like to make
> some changes to how an external PPS signal is deciphered.  As of now, I
> believe that once the first rising edge of a PPS signal is detected, the
> USRP2 will continually collect data until it is programmed to stop.  What I
> am hoping to do is collect data when the PPS sends a rising edge and pause
> the data collection when a falling edge is detected.  Basically the external
> PPS signal is going to act as a TR switch.  So far I have been looking
> through the rx_timed_samples.cpp and the header file clock_config.hpp
> programs to plan out my approach.  The problem where I am getting stuck is
> telling the USRP to pause data collection on a falling edge.  I have not
> found any function in the libraries that is able to do this.  Has anybody
> been able to program the PPS input to act as a TR switch or have any ideas
> about how to about this? Any information on this would be greatly
> appreciated. 
> Thank you very much, 
> Rob 

Generally, this falls into the realm of customizing FPGA, since there is
no stock way to trigger streaming from the PPS input.

However, while continuously streaming, you can poll get_time_last_pps(),
which will tell you the time when you pulsed the PPS input. Use this
information and the time stamps on the RX packets.


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