/*! \page page_gpio_api X3x0 GPIO API

\tableofcontents

\section xgpio_fpanel 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).

\subsection xgpio_fpanel_gpio Front Panel GPIO

\subsubsection xgpio_fpanel_conn Connector

\image html x3x0_gpio_conn.png "X3x0 GPIO Connectors"

\subsubsection xgpio_fpanel_pins 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

\subsection xgpio_fpanel_atr 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 uhd::usrp::multi_usrp::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
uhd::usrp::multi_usrp::get_gpio_attr().
t has the exact same attributes as above, and has
one more:

-   **READBACK**: Readback the GPIOs marked as inputs.

\subsection xgpio_fpanel_xample 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 uhd::usrp::multi_usrp UHD interface.

You can discover this using the uhd::usrp::multi_usrp::get_gpio_banks() function.
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:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
    // 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.

*/
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