With the monoflop the way you drawn it, this not not a very big issue. When wired like this, the monoflop operates in retriggerable mode, so any bouncing during the mono-period will just result in the pulse becoming longer, which should not be a problem in this case.

Dear Rob,

you are totally right, of course, the retrigger feature of CD4538 should make any additional signal conditioning unnecessary. That was the reason, why I omitted the recommendation of using a second monoflop also at pig sensor, in my first reply.

Unfortunately, there's a very small time window, where a bounce could nevertheless present a problem (especially for a paranoid person like me), namely at the very moment, when the pulse signal at PORT3.2 goes from low to high. If right now a bounce at input 'A' again triggers the monoflop, then the micro could have problems to terminate the ISR, eventually. Yes, I confess this is very unprobable, looks like hair-splitting and maybe I'm totally wrong. But triggering input 'A' by a second monoflop at pig sensor, showing this longer pulse width, would eliminate any eventualities...

Kai