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+========================================================================
+UHD - X3x0 GPIO API
+========================================================================
+
+.. contents:: Table of Contents
+
+------------------------------------------------------------------------
+The X3x0 Front Panel GPIO
+------------------------------------------------------------------------
+The X3x0 is the first USRP device to offer an auxiliary GPIO connection on the
+motherboard itself (independent of the daughterboards). These GPIO pins are
+controlled directly by the FPGA, where they are controlled by an ATR (Automatic
+Transmit / Receive). This allows them to be toggled simultaneously with other
+radio-level changes (e.g., enabling or disabling a TX or RX mixer).
+
+
+^^^^^^^^^^^^^^^^
+Front Panel GPIO
+^^^^^^^^^^^^^^^^
+
+Connector
+:::::::::
+
+.. image:: ./res/x3x0_gpio_conn.png
+   :scale: 75%
+   :align: left
+
+Pin Mapping
+:::::::::::
+
+* Pin 1:  +3.3V
+* Pin 2:  Data[0]
+* Pin 3:  Data[1]
+* Pin 4:  Data[2]
+* Pin 5:  Data[3]
+* Pin 6:  Data[4]
+* Pin 7:  Data[5]
+* Pin 8:  Data[6]
+* Pin 9:  Data[7]
+* Pin 10: Data[8]
+* Pin 11: Data[9]
+* Pin 12: Data[10]
+* Pin 13: Data[11]
+* Pin 14: 0V
+* Pin 15: 0V
+
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Explaining ATR
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+ATR works by defining the value of the GPIO pins for certain states of the
+radio. This is the "automatic" part of it. For example, you can tell UHD that
+when the radio is transmitting and receiving (full duplex), GPIO6 should be
+high, but when it is only transmitting, GPI06 should be low. This state machine
+is set up using a series of GPIO attributes, with paired values and a mask,
+which you will want to define for the GPIO pins you intend to use. To set up
+the ATR, you use the **multi_usrp** function *set_gpio_attr*. 
+
+* **CTRL**: Is this pin controlled by ATR (automatic), or by manual control
+  only?
+* **DDR**: "Data Direction Register" - defines whether or not a GPIO is an
+  output or an input.
+* **OUT**: Manually set the value of a pin (only to be used in non-ATR mode).
+* **ATR_0X**: The status of the pins when the radio is **idle**.
+* **ATR_RX**: The status of the pins when the radio is only **receiving**.
+* **ATR_TX**: The status of the pins when the radio is only **transmitting**.
+* **ATR_XX**: The status of the pins when the radio is in **full-duplex** mode.
+
+The counterpart to setting the ATR (the "getter"), is called *get_gpio_attr*.
+It has the exact same attributes as above, and has one more:
+
+* **READBACK**: Readback the GPIOs marked as inputs.
+
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+An Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+The front panel X3x0 GPIO bank is enumerated in the motherboard property tree
+("*<mb_path>/gpio/FP0/*"), and so is easily accessible through the standard
+**multi_usrp** UHD interface.
+
+You can discover this using the *get_gpio_banks* function in **multi_usrp**.
+This will tell you that there is a GPIO bank on your X3x0 called "FP0". This is
+the bank we want to set-up.
+
+Let's say we want to use GPIO6 for an external amp. We want it to be
+automatically controlled by ATR as an output, and we want it to be high when we
+are transmitting, and low in all other cases. We are also using GPIO4, which
+we want to control manually, as an output. We can set this up with the following
+code:
+
+::
+
+    // set up our masks, defining the pin numbers
+    #define AMP_GPIO_MASK   (1 << 6)
+    #define MAN_GPIO_MASK   (1 << 4)
+
+    #define ATR_MASKS       (AMP_GPIO_MASK | MAN_GPIO_MASK)
+
+    // set up our values for ATR control: 1 for ATR, 0 for manual
+    #define ATR_CONTROL     (AMP_GPIO_MASK & ~MAN_GPIO_MASK)
+
+    // set up the GPIO directions: 1 for output, 0 for input
+    #define GPIO_DDR        (AMP_GPIO_MASK & ~MAN_GPIO_MASK)
+
+    // assume an existing USRP device handle, called "usrp_x300"
+
+    // now, let's do the basic ATR setup
+    usrp_x300->set_gpio_attr("FP0", "CTRL", ATR_CONTROL, ATR_MASKS);
+    usrp_x300->set_gpio_attr("FP0", "DDR", GPIO_DDR, ATR_MASKS);
+
+    // let's manually set GPIO4 high
+    usrp_x300->set_gpio_attr("FP0", "OUT", 1, MAN_GPIO_MASK);
+
+    // finally, let's set up GPIO6 as we described above
+    usrp_x300->set_gpio_attr("FP0", "ATR_0X", 0, AMP_GPIO_MASK);
+    usrp_x300->set_gpio_attr("FP0", "ATR_RX", 0, AMP_GPIO_MASK);
+    usrp_x300->set_gpio_attr("FP0", "ATR_TX", 0, AMP_GPIO_MASK);
+    usrp_x300->set_gpio_attr("FP0", "ATR_XX", 0, AMP_GPIO_MASK);
+
+After the above code is run, the ATR in the FPGA will automatically control
+GPIO6, as we have described, based on the radio state, and we have direct
+manual control over GPIO4.
+
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Further Information
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+For more information, see the Doxygen API documentation:
+
+* `multi_usrp API <./../../doxygen/html/classuhd_1_1usrp_1_1multi__usrp.html>`_