n3xx: Disable gpsdo reference source when enable_gps=0

When using enable_gps=0, the power to the LTE-Lite GPS chip is turned
off, and neither the reference sources (time/clock) nor the
location/time data (via gpsd) can be used.

This commit disables the gpsdo options for the set_time_source,
set_clock_source, and set_sync_source when enable_gps=0 is used, and
adds logging to inform the user about this.

This behaviour is consistent with X310, where `gpsdo` is only a valid
reference if the GPSDO module is plugged in.

The manual was also updated accordingly.

1 files changed, 8 insertions, 1 deletions

diff --git a/host/docs/usrp_n3xx.dox b/host/docs/usrp_n3xx.dox
index 477764e49..052f20ee9 100644
--- a/host/docs/usrp_n3xx.dox
+++ b/host/docs/usrp_n3xx.dox
@@ -643,7 +643,10 @@ will look like this:
 
 The N3xx series has an onboard GPSDO as well as a 25 MHz reference oscillator,
 which can both be used as time- and clock references. The GPSDO will function
-as a reference even when there is no GPS reception.
+as a reference even when there is no GPS reception. It can be powered off to
+reduce power usage and interference by supplying the `enable_gps=0` option in
+the configuration file (in this case, `gpsdo` cannot be used as a time or clock
+reference).
 
 Note that this does not enable the synchronization of multiple devices.
 Using an internal reference is the default.
@@ -662,6 +665,10 @@ auto usrp = uhd::usrp::multi_usrp::make(
     "type=n3xx,clock_source=external,time_source=external");
 ```
 
+
+Note: When disabling the GPS with `enable_gps=0` in the configuration file, or
+when manually launching MPM, the `gpsdo` reference source is not available.
+
 \subsection n3xx_synchronization_whiterabbit White Rabbit
 
 White Rabbit is an Ethernet-based synchronization procedure; it is an extension