Russell asked :What cpu are you using? How does your code operate? Do you write in the correct sequence to the PCA registers?

let me add
1) are all other interrupts low priority and the PCA interrupt high priority?
2) just in case: You, I hope, do not suffer from interruptifobia, this IS interrupt driven?
3) you are aware of the required sequence of loading 'H' and 'L' registers in the PCA?

Erik