USRP Hardware Driver and USRP Manual  Version:
UHD and USRP Manual

The Front Panel GPIO

All Generation-3 USRP 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).

For information on the X410 GPIO, see X4x0 GPIO API

X3x0 Front Panel GPIO

The GPIO port is not meant to drive big loads. You should not try to source more than 5mA per pin.

The +3.3V is for ESD clamping purposes only and not designed to deliver high currents.


X3x0 GPIO Connectors

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

E310/E312/E313 Internal GPIO


E31x GPIO Connector

Pin Mapping

  • Pin 1: +3.3V
  • Pin 2: I2C SCL (3.3 V)
  • Pin 3: Data[5]
  • Pin 4: I2C SDA (3.3 V)
  • Pin 5: Data[4]
  • Pin 6: Data[0]
  • Pin 7: Data[3]
  • Pin 8: Data[1]
  • Pin 9: 0V
  • Pin 10: Data[2]

Pin 1 is connected to the shared +3.3V power rail and can be used to draw power. The maximum current depends on the power used by the rest of the device, but 300-500 mA is generally safe. It is recommended to monitor the rail voltage when drawing power from this pin.

E320 External GPIO connector

Front Panel GPIO Connections

GPIO Mini HDMI (Type C) HDMI (Type A)
Data[0] Data 0+ - Pin 8 Pin 7
Data[1] Data 0- - Pin 9 Pin 9
Data[2] Clock 0+ - Pin 11 Pin 10
Data[3] Clock 0- - Pin 12 Pin 12
Data[4] CEC - Pin 14 Pin 13
Data[5] SCL - Pin 15 Pin 15
Data[6] SDA - Pin 16 Pin 16
Data[7] Utility - Pin 17 Pin 14

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 receiving only.
  • ATR_TX: The status of the pins when the radio is transmitting only.
  • 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(). 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/\*), the E31x internal GPIO bank as (<mb_path>/gpio/INT0/\) and so are 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, E320, or N3x0 called "FP0" (for E31x this will be called "INT0"). 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 only, 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)
// set up our values for ATR control: 1 for ATR, 0 for manual
// set up the GPIO directions: 1 for output, 0 for input
// assume an existing multi_usrp object, called "usrp"
// now, let's do the basic ATR setup
usrp->set_gpio_attr("FP0", "CTRL", ATR_CONTROL, ATR_MASKS);
usrp->set_gpio_attr("FP0", "DDR", GPIO_DDR, ATR_MASKS);
// let's manually set GPIO4 high
usrp->set_gpio_attr("FP0", "OUT", 1, MAN_GPIO_MASK);
// finally, let's set up GPIO6 as we described above
usrp->set_gpio_attr("FP0", "ATR_0X", 0, AMP_GPIO_MASK);
usrp->set_gpio_attr("FP0", "ATR_RX", 0, AMP_GPIO_MASK);
usrp->set_gpio_attr("FP0", "ATR_TX", AMP_GPIO_MASK, AMP_GPIO_MASK);
// usually, you would want to also make this pin go high when doing
// full-duplex, but not in this example
usrp->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.

To modify the example to work with the E31x's internal GPIO bank, use the bank name "INT0" instead of "FP0".