docs: b200: Improve docs (auto MCR, GPIOs)

- Mention J400 and the ability to do GPIO on there
- Provide some examples on auto-MCR and its effects
- Add comments that changing MCR has ill effects on time synchronization

host/docs/usrp_b200.dox

@@ -80,6 +80,19 @@ device arguments, which many applications take:
     uhd_usrp_probe --args="master_clock_rate=52e6"
 
 The property to control the master clock rate is a double value, called `tick_rate`.
+Example:
+
+~~~{.py}
+import uhd
+my_usrp = uhd.usrp.MultiUSRP("type=b200")
+# The following two lines are synonymous:
+my_usrp.set_master_clock_rate(16e6)
+my_usrp.get_tree().access_double('/mboards/0/tick_rate').set(16e6)
+# Note that if MCR changed, then all time-synchronization is lost!
+~~~
+
+Note that changing the master clock rate will reconfigure the entire clocking
+chain, thereby losing previous synchronization configurations.
 
 \subsection b200_auto_mcr Automatic Clock Rate Setting
 
@@ -98,10 +111,28 @@ the largest possible clock rate as to enable as many half-band filters
 as possible. Expert users might have cases where a more fine-grained
 control over the resampling stages is required, in which case manually
 selecting a master clock rate might be more suitable than the automatic
-rate.
+rate. Manual selection of master clock rate may also be required to synchronize
+multiple B200 units in time.
 
 The property to dis- or enable the auto tick rate is a boolean value,
-`auto_tick_rate`.
+`auto_tick_rate`. Example:
+
+~~~{.py}
+>>> import uhd
+>>> my_usrp = uhd.usrp.MultiUSRP("type=b200")
+>>> my_usrp.get_tree().access_bool('/mboards/0/auto_tick_rate').get()
+True
+>>> my_usrp.set_master_clock_rate(16e6) # This will lock the MCR
+>>> my_usrp.get_tree().access_bool('/mboards/0/auto_tick_rate').get()
+False
+>>> U.set_rx_rate(7e6) # Not possible! We locked the MCR.
+>>> U.get_rx_rate() # The actual rate is coerced:
+8000000.0
+>>> my_usrp.get_tree().access_bool('/mboards/0/auto_tick_rate').set(True)
+>>> my_usrp.set_rx_rate(7e6) # This requires a new MCR...
+>>> my_usrp.get_master_clock_rate() # ...which is chosen automatically.
+56000000.0
+~~~
 
 \section b200_fe RF Frontend Notes
 
@@ -297,13 +328,16 @@ Component ID            |  Description               | Details
 ------------------------|----------------------------|---------------------------------------------------
  J502<sup>1</sup>       |  Mictor Connector          | Interface to FPGA for I/O and inspection.
  J503<sup>1</sup>       |  JTAG Header               | Interface to FPGA for programming and debugging.
- J504<sup>2</sup>       |  GPIO Header               | Header connected to the FPGA for GPIO purposes.
+ J504<sup>1,2</sup>     |  GPIO Header               | Header connected to the FPGA for GPIO purposes.
+ J400<sup>3</sup>       |  GPIO/UART Header          | Header connected to the FPGA for GPIO purposes, or as a serial interface to the FX3 (requires custom FPGA bitfile)
  S700                   |  FX3 Hard Reset Switch     | Resets the USB controller / System reset
  U100                   |  GPSDO socket              | Interface to GPS disciplined reference oscillator
 
 <sup>1</sup> Only on the B210
 
-<sup>2</sup> Only since rev. 6 (green board).  GPIO pinout is 1=GPIO_0, 2=GPIO_1, 3=GPIO_2, 4=GPIO_3, 5=GPIO_4, 6=GPIO_5, 7=GPIO_6, 8=GPIO_7, 9=GND, 10=GND
+<sup>2</sup> Only since rev. 6 (green board). Default voltage is 3.3V. GPIO pinout is 1=GPIO_0, 2=GPIO_1, 3=GPIO_2, 4=GPIO_3, 5=GPIO_4, 6=GPIO_5, 7=GPIO_6, 8=GPIO_7, 9=GND, 10=GND
+
+<sup>3</sup> Only since rev. 6 (green board). Default voltage is 1.8V. GPIO pinout is 1=GPIO_8, 2=GND, 3=GPIO_9. When compiling a bitfile to support a UART on this header, pinout is 1=TX, 2=GND, 3=RX.
 
 Below is a table showing the B200mini on-board connectors and switches:
 
@@ -317,6 +351,9 @@ Component ID            |  Description               | Details
 
 \section b200_known_issues Known issues
 
+- When synchronizing multiple USRPs in time, it is strongly recommended to
+  specify a master clock rate instead of relying on \ref b200_auto_mcr.
+
 - The B200 and B210 cannot support an external 10 MHz reference if a GPSDO is
   already present on the motherboard. If an external 10 MHz reference is to be
   used, the GPSDO needs to be physically removed from the device beforehand.