now I am beginning the hardware part which is composed by an 16*4 LCD screen and interrupts.
Interrupts are not hardware. They are software.
Sorry I mean switches, my mix of french and english makes it hard 
Ah ok, I thought the Serial.begin would have be defined in the setup function,
The think is my program is very long so I would like to separate them since I use the menu to set various parameter, and once these parameters are set the program is always running. Yes ok I will include the menu into the main program and find a way to jump between these 2.
Thank for your help.
Vince