I use int0 & int1 to measure the width of a pulse. My program works fine in the simulator with inputs from an include file. When I run the program on a breadboard using an ATMEL AT89C1051 I do not get a response to the external interrupt inputs. I can see these strobes with a 'scope. I have put p1.x toggles in the two isr's involved and do not see any toggling. In my init part of the program I do select edge triggering, int0 & int1 enabled, master interrupt enabled. Does anyone know what more must be done for this mode of operation? - ATMEL chip, A51 assembler, external interrupts.