External interrupt debugging with PIC16F877A
In MPLAB, open from the menu: Windows-&gt;Simulator-&gt;IOPin. Then select to see RB0 in the I/O pins tab. On RESET RB0 will be zero (0). On the stimulus click RB0 (set High) at least once, and verify that in the IOPin tab that RB0 is high. Then in the stimulus tab click the RB0 (pulse low), and you should get an interrupt. (Remember to set a breakpoint in the interrupt code to break your code in the interrupt). Because you have selected falling edge trigger, the signal has to go from a high to a low to be able to trigger the interrupt.