Hi there,
as the title says, I would like to emulate/simulate the behavior of 555 not using the IC itself, but using just Arduino Due and no external capacitance, but just the resistor on which depend the time costant.
I'm a beginner as a programmer, and I have a lot of troubles with the flow chart. I think there's something already written on the web, but I could not find it.
I hope you can help me.
Any advice is welcome 
Thank you very much.
There's more to a 555 than just a resistor to set the timebase. The combination of resistor and capacitor set both the frequency and the duty cycle. The two are tightly interrelated.
You would need two resistive dividers connected to analogue inputs (could be potentiometers) - one to "set" the resistance, and one to "set" the capacitance (or don't have them at all and just work entirely in software - enter values through the serial console). Then you use the standard 555 calculation formulae to calculate a frequency and duty cycle (google them), so you can adjust the frequency and duty cycle of a PWM signal.
majenko:
There's more to a 555 than just a resistor to set the timebase. The combination of resistor and capacitor set both the frequency and the duty cycle. The two are tightly interrelated.You would need two resistive dividers connected to analogue inputs (could be potentiometers) - one to "set" the resistance, and one to "set" the capacitance (or don't have them at all and just work entirely in software - enter values through the serial console). Then you use the standard 555 calculation formulae to calculate a frequency and duty cycle (google them), so you can adjust the frequency and duty cycle of a PWM signal.
First of all, thank you for your answer.
My idea is to enter the value of the capacitance through the serial console, and to use one resistive divider connected to an analog input to "set" the resistance. I know the formulas (I need high and low semi-periods), but I have some problems with the flow chart managing the several statements (if, else, etc. ).