What do you have tied to pin 3.2? Are you sure the interrupt event is occuring and that pin 3.2 is actually going from high to lo? Is the duration of the interrupt long enough for the 8051 to detect it? Per the Intel user manual the high preceeding the interrupt should be at least 1 machine cycle long and the low level after the transition also needs to be at least one machine cycle long. If you are running at 12mHz each cycle will be 1 microsecond.


Hal