On 18/08/2023 12:19, jmalo...@umass.edu wrote:
Hello,
I have an application where I need my logic within the FPGA to access
the gps time. According to the spec sheet, the x410 has a gps module
built in. I would like to get gps time from it, but it is unclear
where I can get it from.
I assume the gps time is stored in radio_time inside the timekeeper,
which is set to the gps time when usrp->set_time_source("gpsdo") is
used. Is this correct?
Thanks
Joe
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The time-of-day is something that is generally "managed" by the
host-side, using resources within the FPGA.
The timekeeping register is part of each radio block as far as I recall,
but they don't "know" about the GPSDO.
Generally the host application queries a "sensor" (like gps_time or
gps_gga) and then calls set_time_next_pps()
or set_time_unknown_pps() to set the time-of-day (timestamp) register
to that value at the next 1PPS pulse.
The "set_time_source("gpsdo")" simply instructs the hardware to get its
notion of 1PPS from the GPSDO rather than
the internal 1PPS or the external 1PPS on the front panel. In USRPs
the 1PPS is used simply as a *trigger* mechanism
so that calls like "set_time_next_pps()" work properly.
The time-of-day (or timestamp) register that is part of the radio block
is incremented at whatever the master clock rate is,
and is utterly oblivious of what is going on with the on-board
GPSDO. In general, it's up to the host to set that register
to GPS time (or NTP time, or the time on your grandaddy's watch, or
whatever).
Now those "sensors" ("gps_time", "gps_gpgga") etc are actually resources
within the FPGA, so there is likely a way to get
the FPGA to do what the host side would do at appropriate times, but
I'm not really and FPGA guy, so I can offer nothing
concrete in that regard.
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