Pulse Position Modulation (PPM) timing (ppm signal from Graupner MC-12 measured with oscilloscope)

   22ms (fixed)  |<-------------------------------------------------------------------------->|

   400us (fixed) |<>|

 ->|<---Init.--->|<-Ch.1->|<-Ch.2->|<-Ch.3->|<-Ch.4->|<-Ch.5->|<-Ch.6->|<-Ch.7->|<---Init.--->|<---

   +--+          +--+     +--+     +--+     +--+     +--+     +--+     +--+     +--+          +--+
   |  |          |  |     |  |     |  |     |  |     |  |     |  |     |  |     |  |          |  |
 __|8 |__________|1 |_____|2 |_____|3 |_____|4 |_____|5 |_____|6 |_____|7 |_____|8 |__________|1 |_

                 |<------>| channel pulse (modulated): 1500us for channel = 0%;
                                                       1100us/1900us for channel = -100%/+100%;
                                                       900us/2100us for channel = -150%/+150%

   |<----------->| init. pulse (modulated): 22ms minus the duration of 7 channels
                                            11500us for 7 channels = 0%
                                            14300us/8700us for 7 channels = -100%/+100%;
                                            15700us/7300us for 7 channels = -150%/+150%;

Since each pulse width is measured between two rising edge, doesn't matter if the radio has a positive
ppm signal (as the one above) or negative (level 1 and 0 inverted) and if we use or not an inverting
separator stage before RB0/INT input pin.