Re: [Discuss-gnuradio] Transmission time control from python level

[Josh Blum]

On 07/27/2012 10:24 AM, Nazmul Islam wrote:
> Sorry for the previous email. I mistakenly hit the sent button before
> completing the mail. Here is the complete mail:
> 
> Is it possible to see the time through USRP GPSDO with
> microsecond/millisecond granularity? I want to synchronize the timing of my
> computers and run gnuradio codes based on the synchronized timing.
> 

When synchronized to a common clock reference and time, a group of USRPs
can perform actions simultaneously with accuracy down the the jitter of
GPSDO ref clock + clock synthesizer chain.

> For example, if I have ethernet based internet connection, I can use ntp
> servers to synchronize the computers and read the time (using
> "time.time()") with milisecond granularity. I am trying attain the same
> thing using GPSDO. I am using the following python code.
> 

the "magic" of the GPSDO and time synchronization is that there is a
shared "pulse" aka PPS. This pulse is used to latch a time value into
the FPGA at a very specific moment. From that point on, a group of usrps
share the same concept of time.


> ****
> 
> class TimeFind :
> 
> .........
>     self.uhd_usrp_source_0 = uhd.usrp_source(
>             device_addr="",
>             stream_args=uhd.stream_args(
>                 cpu_format="fc32",
>                 args="peak=0.5",
>                 channels=range(1),
>             ),
>         )
> 
> ...........................
> ..............................
> 
> if __name__ == '__main__':
> 
> tb = TimeFind()
> 
> print "current time :" , tb.uhd_usrp_source_0.get_time_now()
> print "last pps time :" , tb.uhd_usrp_source_0.get_time_last_pps()
> print "GPS Time: " ,
> tb.uhd_usrp_source_0.get_mboard_sensor("gps_time").to_int()
> tb.Run(True)
> 
> *************
> 
> I have the following issues:
> 
> 1. The current time and last pps time show the following output:
> 
> current time : <gnuradio.uhd.uhd_swig.time_spec_t; proxy of <Swig Object of
> type 'uhd::time_spec_t *' at 0x331fcf0> >
> last pps time : <gnuradio.uhd.uhd_swig.time_spec_t; proxy of <Swig Object
> of type 'uhd::time_spec_t *' at 0x331fcf0> >
> 
> How can I get the time value?
see member functions:
http://files.ettus.com/uhd_docs/doxygen/html/classuhd_1_1time__spec__t.html
> 
> 2. "GPS Time: " shows the time with 1 second granularity. Can I get higher
> resolution that 1 second? Is there an option that allows to see the exact
> time instead of the integer time?
> 

GPS time in seconds is actually a blocking call that waits until the
seconds increments to return you the time. That way you know very
precisely that the next pulse will be time_read + 1 seconds.

-josh