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The External Interrupts are triggered by the INT7:0 pin or any of the PCINT23..0 pins. Observe that, if enabled, the interrupts will trigger even if the INT7:0 or PCINT23..0 pins are configured as outputs. This feature provides a way of generating a software interrupt.

The Pin change interrupt PCI2 will trigger if any enabled PCINT23:16 pin toggles, Pin change interrupt PCI1 if any enabled PCINT15:8 toggles and Pin change interrupts PCI0 will trigger if any enabled PCINT7..0 pin toggles. PCMSK2, PCMSK1 and PCMSK0 Registers control which pins contribute to the pin change interrupts if Extract microcontroller code.

Pin change interrupts on PCINT23 ..0 are detected asynchronously. This implies that these interrupts can be used for waking the part also from sleep modes other than Idle mode.

The External Interrupts can be triggered by a falling or rising edge or a low level. This is set up as indicated in the specification for the External Interrupt Control Registers – EICRA (INT3:0) and EICRB (INT7:4). When the external interrupt is enabled and is configured as level triggered, the interrupt will trigger as long as the pin is held low if Extract microcontroller at89c51ic2 program.

Note that recognition of falling or rising edge interrupts on INT7:4 requires the presence of an I/O clock, described in “Clock Systems and their Distribution” on page 39. Low level interrupts and the edge interrupt on INT3:0 are detected asynchronously.

This implies that these interrupts can be used for waking the part also from sleep modes other than Idle mode. The I/O clock is halted in all sleep modes except Idle mode.

Note that if a level triggered interrupt is used for wake-up from Power-down, the required level must be held long enough for the MCU to complete the wake-up to trigger the level interrupt.

If the level disappears before the end of the Start-up Time, the MCU will still wake up, but no interrupt will be generated. The start-up time is defined by the SUT and CKSEL Fuses as described in “System Clock and Clock Options” on page 39.

The External Interrupts 3 – 0 are activated by the external pins INT3:0 if the SREG I-flag and the corresponding interrupt mask in the EIMSK is set. The level and edges on the external pins that activate the interrupts are defined in Table 31.

Edges on INT3..INT0 are registered asynchronously. Pulses on INT3:0 pins wider than the minimum pulse width given in Table 32 will generate an interrupt. Shorter pulses are not guaranteed to generate an interrupt. If low level interrupt is selected, the low level must be held until the completion of the currently executing instruction to generate an interrupt before Extract Chip.

If enabled, a level triggered interrupt will generate an interrupt request as long as the pin is held low. When changing the ISCn bit, an interrupt can occur. Therefore, it is recommended to first disable INTn by clearing its Interrupt Enable bit in the EIMSK Register.

Then, the ISCn bit can be changed. Finally, the INTn interrupt flag should be cleared by writing a logical one to its Interrupt Flag bit (INTFn) in the EIFR Register before the interrupt is reenabled.