The Briggs and Stratton engine used on #Goliath - A Gas Powered Quadcopter utilizes a magnet on the flywheel to trigger the ignition coils. This magnet will be utilized for measuring the engine RPM with a hall effect sensor under the engine cover.
The Pixhawk board has six auxiliary pins that can be configured as pulse width modulation (PWM) or General Purpose Input/Output (GPIO). In GPIO mode the AUX pins can be used for measuring frequency. Unfortunately the AUX pins have to be all PWM or all GPIO and can't be a mix. Since some of the AUX pins are being used for servos, this means they can't be used for frequency detection. Additionally the analog to digital (ADC) ports don't have interrupt capability, meaning that they can't be used for frequency measurement either.
This means that for the current application, an off-board frequency measurement option is needed. An Arduino will be connected to the hall effect sensor to measure the frequency. The frequency data will then be based to the Pixhawk via I2C. A 5V Trinket Pro was selected to it's small size. A sketch of the circuit is shown below.
Arduinio Sketch for Trinket Pro