Hello Peter,

I guess it has to be key bouncing. You can check this by connecting a 74HCT counter via a comparator (to meet the input specs of the 8574!) to the switches.

The 8574 fires an interrupt, whenever the logical state on its input is changed, maybee this can help you.

At least: don't connect anything direct to an interrupt pin! If you can't poll external events by an internal timer driven interrupt routine, use a flip flop to signal this external events - with this, you can react to external events with an interrupt routine and avoid multiple interrupts occuring in a very short time wich can cause interrupt overflows. Be very careful with external interrupts!

Good luck!

Mike