Thanks, will take a look at this thread.
Originally Posted by michaelstoops
For 3) I might hazard a guess that joining PD0/RXD and PD2/INT0 has something to do with using INT0 to trigger an interrupt when the PPM signal edges come in. Just glancing at the Atmel documentation, I'm seeing that INT0 is the second-highest priority interrupt and will wake it from special power modes.
Well, I'm totally agree about PD2/INT0, but connection to PD0/RXD does not make any sense...
I have one more question: why do we need to control high and low sides separately? They can be controlled from the same MCU pin, aren't they?