uhd: added information in reference sources to sync docs

1 files changed, 26 insertions, 13 deletions

diff --git a/host/docs/sync.rst b/host/docs/sync.rst
index 152349990..9284d8e33 100644
--- a/host/docs/sync.rst
+++ b/host/docs/sync.rst
@@ -22,23 +22,44 @@ USRPs take two reference signals in order to synchronize clocks and time:
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 Provide reference signals
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-Most USRPs have front panel SMA connectors to source these reference signals.
+USRPs have two primary means of providing synchronization:
+
+**Method 1:**
+Connect the front panel SMA connectors to the reference sources.
 Typically, these signals are provided by an external GPSDO.
-Some USRP models can provide these signals from an optional internal GPSDO.
-For users generating their own signals,
+However, some USRP models can provide these signals from an optional internal GPSDO.
+
+**Method 2:**
+Use the MIMO Expansion cable to share reference sources (USRP2 and N-Series).
+The MIMO cable can be used synchronize one device to another device.
+Users of the MIMO cable may use method 1 to synchronize multiple pairs of devices.
+
+**Note:**
+For users generating their own signals for the external SMA connectors,
 the pulse-per-second should be clocked from the 10MHz reference.
 See the application notes for your device for specific signal requirements.
 
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 Set the clock configuration
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-In order to synchronize to an external clock, configure the USRP device
-using the "external" clock configuration:
+In order to synchronize to an external clock,
+configure the USRP device using the "external" clock configuration:
 
 ::
 
     usrp->set_clock_config(uhd::clock_config_t::external());
 
+Sometimes the delay on the PPS signal will cause it to arrive inside the timing
+margin the FPGA sampling clock, causing PPS edges to be separated by less or
+more than 100million cycles of the FPGA clock. If this is the case,
+you can change the edge reference of the PPS clock with the clock_config_t:
+
+::
+
+    uhd::clock_config_t clock_config = uhd::clock_config_t::external();
+    clock_config.pps_polarity = uhd::clock_config_t::PPS_NEG;
+    usrp->set_clock_config(clock_config);
+
 ------------------------------------------------------------------------
 Synchronizing the device time
 ------------------------------------------------------------------------
@@ -48,14 +69,6 @@ or to an absolute time such as GPS time or UTC time.
 For the purposes of synchronizing devices,
 it doesn't matter what time you initialize to when using set_time_next_pps(...).
 
-Some GPSDOs synchronize their PPS with the rising edge of the 10MHz clock,
-and some synchronize with the falling edge of the clock.
-You should find out which edge your GPSDO uses,
-and then pass that in as an argument to set_time_next_pps(...).
-
-Here are two examples of how to set the PPS time to synchronize USRPs
-to a clock source.
-
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 Method 1 - poll the USRP time registers
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^