Interrupts on Pixhawk running ArduCopter 3.2.1

Hi all!
I’m trying to read an optical RPM sensor with the Pixhawk. The sensor outputs a high voltage when the surface is reflective and a low voltage when the surface is dark. I have reflective tape around half of the motor, and I would like to trigger an interrupt on the rising or falling edge to increment a counter.

In the ArduPilot AP_HAL library, there is a function in the GPIO header that is called “attach_interrupt”. When one looks in the AP_HAL_PX4 library at the GPIO.cpp file, “attach_interrupt” is not expanded (it just returns true). I would like to integrate this sensor on the AUX FMU pins (50-55). My questions:

  1. Is there a way to incorporate this sensor without using interrupts?
  2. How would one write the function for attaching interrupts, if they are available on the AUX pins?

Thank you for your help.