Editing Interrupt (section)

====Spurious interrupts====
A ''spurious interrupt'' is a hardware interrupt for which no source can be found.  The term "phantom interrupt" or "ghost interrupt" may also be used to describe this phenomenon.  Spurious interrupts tend to be a problem with a [[wired-OR]] interrupt circuit attached to a level-sensitive processor input.  Such interrupts may be difficult to identify when a system misbehaves.

In a wired-OR circuit, [[parasitic capacitance]] charging/discharging through the interrupt line's bias resistor will cause a small delay before the processor recognizes that the interrupt source has been cleared.  If the interrupting device is cleared too late in the interrupt service routine (ISR), there will not be enough time for the interrupt circuit to return to the quiescent state before the current instance of the ISR terminates.  The result is the processor will think another interrupt is pending, since the voltage at its interrupt request input will be not high or low enough to establish an unambiguous internal logic 1 or logic 0.  The apparent interrupt will have no identifiable source, hence the "spurious" moniker.

A spurious interrupt may also be the result of electrical [[glitch|anomalies]] due to faulty circuit design, high [[electrical interference|noise]] levels, [[crosstalk]], timing issues, or more rarely, [[Erratum#CPU logic|device errata]].<ref name="RT_concepts">{{cite book |last1=Li |first1=Qing |last2=Yao |first2=Caroline |title=Real-Time Concepts for Embedded Systems |date=2003 |publisher=CRC Press |isbn=1482280825 |page=163}}</ref>

A spurious interrupt may result in system deadlock or other undefined operation if the ISR does not account for the possibility of such an interrupt occurring.  As spurious interrupts are mostly a problem with wired-OR interrupt circuits, good programming practice in such systems is for the ISR to check all interrupt sources for activity and take no action (other than possibly logging the event) if none of the sources is interrupting.