Port expander (MCP23017) and setWatch

do I simply setwatch B13 and anytime button is pressed on the matrix an event will fire?

...not exactly, but closely...

Depending how you setup the interrupt, it detects a change of a pin or it detects a difference with a preset value.

I can go a bit more into detail when you describe how you plan your circuitry.

PortExpannder has two ways to generate an interrupt: a pin changes state (from low to high or high low) or a pin's state is different from the specification in the in register in which you write the resting state: all high (1)... the same as your inputs have with no button pressed. Then you activate the interrupt. As soon as a port changes (is pulled low by pressing the switch), PortExpander notices a difference and fires the interrupt. Take a look at the data sheet , pages 12 and following.