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

From: rmsrms1987
Subject: Re: [Discuss-gnuradio] Using PPS as a sampling trigger
Date: Wed, 22 Feb 2012 15:47:30 -0800 (PST)

Hi Josh, 

Thank you for the reply. 

I have been trying to implement what you mentioned using the
rx_timed_samples program but am having some trouble making sense of the
results.  Here are the changes I have made to the code in order to acquire
PPS timing information. 



uhd::time_spec_t pps_time = usrp->get_time_last_pps();

These are the initial variables that are created outside of the "while"
loop.  Inside of the "while" loop I added:

        if (pps_time != usrp->get_time_last_pps()){
                pps_time = usrp->get_time_last_pps();
                std::cout << boost::format("PPS Trigger: %u full secs, %f frac 
                % pps_time.get_full_secs() % pps_time.get_frac_secs() << 

Basically what this is supposed to do is print out the time value of the
last PPS trigger, once a new trigger value has been detected. 

The problem I am having right now is that the timing information is not
matching up to the signal I am inputting to the PPS port.  I started off
with inputting a signal that has an IPP of 1ms, but each PPS Trigger output
incremented in values of 5ms.  I made additional changes to the input signal
by making the IPP 5ms, then the increment of the printed PPS trigger was 10
ms.  Inside each IPP there are also some trigger variations, but these were
never detected.  Is there something I am not doing correctly in my code in
order to detect each PPS trigger? It seems like once a PPS is detected, the
program assumes each subsequent PPS will occur in equal time increments.
Please let me know what you think. 

Thank you very much, 

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