Re: [Discuss-gnuradio] ptime get_time(void) precision down to millisec microsec nanosec ?

[Nick Foster]

On Thu, 2011-05-19 at 17:07 +0000, Bastien Auneau wrote:
> Hi
> 
> I think there is a misunderstanding.
> I do not want to set the time of the USRP.
> I want to use the USRP (with GPSDO) as my time reference.
> This is why I want to query the USRP using the UHD function ptime
> get_time(void) in
> /host/lib/usrp/gps_ctrl.cpp @ line 133
> 
> This function only returns date and time down to seconds, and the
> toked[] array has a bigger length than what is used :
> "
> UHD_ASSERT_THROW(toked.size() == 12); //if it's not we got something
> weird in there
> 
> now = ptime( date(
>   greg_year(boost::lexical_cast<int>(toked[9].substr(4, 2)) + 2000),
> //just trust me on this one
>   greg_month(boost::lexical_cast<int>(toked[9].substr(2, 2))),
>   greg_day(boost::lexical_cast<int>(toked[9].substr(0, 2)))
>                        ),
>   hours(  boost::lexical_cast<int>(toked[1].substr(0, 2)))
>   + minutes(boost::lexical_cast<int>(toked[1].substr(2, 2)))
>   + seconds(boost::lexical_cast<int>(toked[1].substr(4, 2)))
> );
> "
> 
> So I would like to know the format of the string returned by the
> safe_gps_read(); function,
> "
> reply = safe_gps_read();

OK, so 4 things:

1. Querying the GPS time through the device is going to net you accuracy
in the 10s of milliseconds, maybe worse. Asking for more precision in
the GPRMC sentence is meaningless, since the query has to go through
both the Ethernet transport and the serial port on the N210 on its way
to you. 

2. The GPSDO PPS output is used in the N210 to synchronize the sample
time across multiple devices. The PPS edge is the ONLY output of the
GPSDO which is guaranteed to be synchronized, and its synchronization is
good to (typical) 50ns. That output goes straight into the FPGA to
provide a sync pulse. Typically, you call get_time() to get the UTC time
in seconds, and then set_time_next_pps() to tell the FPGA to sync its
sample time to the next PPS pulse. Then you know the absolute time, and
you can calculate the absolute time of any given sample from the N210
based on the absolute time and the sample count.

3. The GPRMC sentence provides for decimal time down to the millisecond;
however, the Jackson Labs Firefly device (like most GPS devices) issues
sentences on the rising edge of PPS, and so the fractional seconds
component is always zero.

4. The toked vector gives you the GPRMC sentence components in order,
separated by comma or period.

--n

> "
> 
> or better the description of the toked[] array after it get through
> "
> tok.assign(reply);
> toked.assign(tok.begin(), tok.end());
> "
> 
> Bastien
> 
> On 2011-05-19 14:46, Josh Blum wrote:
> > 
> >> Is it possible to extract from the reply the milli ? micro ? and nano
> >> seconds ? For example using toked[2-8] or toked[10-11] (because
> > 
> > I recommend that you use query the seconds to catch the PPS edge, and
> > then set the time for the next PPS. This will set the GPSDO time into
> > the device at the precision of a clock cycle:
> > 
> > http://www.ettus.com/uhd_docs/doxygen/html/classuhd_1_1usrp_1_1multi__usrp.html#aaa80cd6ee4b3c1bf52afb9c3ef02f64d
> > 
> > -Josh
> > 
> 
> 
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