Thank you both for replies.

For Charles. The schematic that you pointed to me it's exactly what I'm using right now. Anyway, in the mean time Erik gave me the solution ...

For Erik. I was stupid of course. I don't need an interrupt to read the keyboard.

But, as a curiosity, I'll try the same circuit using a Philips chip (right now I'm using Texas Instruments PCF8574A), to see if the problem persist.

Daniel