Thanks for your observations, I'll try not to be so messy with my codes... What I pretend to do is an IV curve tracer, so I have to take measures every hour from 7 am to 7 pm (10 pm on the sketch for testing), first I get a direct value from my voltage source (a PV), afterwards I connect a load to the PV, and take data again.
I've uploaded my sketch (the one without the prescaler change) into another UNO board already, and it doesn't work either! I'm collapsing! Could it be the serial port? It gives me this:
Desconectando carga
Card Failure
VOC = Desconectando carga
Card Failure
VOC = Desconectando carga
Card Failure
VOC = Desconectando carga
Card Failure
VOC = Desconectando carga
Card Failure
VOC = Desconectando carga
Card Failure
VOC = Desconectando carga
Card Failure
Anf before I got voltage values (from 0 to 1023). What happened?