Microchip Technology MPLAB ICE, User Manual

Page: 157 / 192

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2001 Microchip Technology Inc. DS51159B-page 157
Debugging Techniques
A.4.7 Phantom interrupt
An interrupt has occurred, but there is no interrupt flag set.
If you halt program execution when an interrupt service routine is entered, but
then notice that no flags have been set, you have a phantom interrupt.
Phantom interrupts can occur when your program is in the process of clearing
interrupt flags when an interrupt comes in. Due to the pipelined architecture of
the PICmicro MCU, an interrupt can occur when an instruction to clear
interrupt flags is being fetched. The next fetch will be to branch to the ISR, but
the instruction executed then will be a clear of the interrupt flags.