Hi Gill,

nobody can look into your head.
So apologize us, that we can only interpret, what you have written.

Its really less informations.
So I can only add some other speculations:

Maybe, the interrupts are triggered by bouncing keys and so additional triggers occur.
On additional reset of the interrupt pending flag immediately after entering the interrupt, it occurs prior bouncing and after execution of something it occurs after bouncing. So this can cause the different behavior.

Be sure, that no mystery can be hold secret over 22 years on the 8051.


Peter