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 PWM out or GPIO. In GPIO they can be used for measuring frequency. Unfortunately they they all have to be one or another 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.
The ADC ports don't have interrupts capability, meaning that they can't be used for frequency measurement.
This means that for the current application, an off-board 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.
The RPM will be measured and reported using a PX4 Application. The software is part of the PX4 Firmware fork