Transmitter: Stopp PPM comm when transceiver signal lost, allow MultiWii to handle lost signal

[derickson2402]

MultiWii has a built-in way to deal with lost signal from a transmitter (you can modify it in config.h, but basically it just gently lowers drone to the ground). However this only works when PPM signal to MultiWii is lost, meaning that currently it can never be lost because the receiver constantly broadcasts even when not receiving data.

The only current fix in place is that the receiver will send 'safe' packets (0 throttle, no turn) but of course this means the drone can drop out of the air if someone sneezes while holding the transmitter

Ideally, receiver needs to recognize that it looses signal and stop PPM signal to MultiWii temporarily, so MultiWii is aware of loss of comm. Possible implementation is temporarily disabling interrupts whenever the radio has not received data for more than ~10ms (or some threshold to deal with intermittent noise/hardware bugs with the radio). Then the MultiWii can deal with the signal loss in a much safer way (ideally without destroying itself 🤞 )