16 EXTINT - External Interrupts

The external interrupts are triggered by the INT pins or any of the PCINT pins. Observe that, if enabled, the interrupts will trigger even if the INT or PCINT pins are configured as outputs. This feature provides a way of generating a software interrupt.

The Pin Change Interrupt Request 3 (PCI3) will trigger if any enabled PCINT[27:24] pin toggles. The Pin Change Interrupt Request 2 (PCI2) will trigger if any enabled PCINT[23:16] pin toggles. The Pin Change Interrupt Request 1 (PCI1) will trigger if any enabled PCINT[14:8] pin toggles. The Pin Change Interrupt Request 0 (PCI0) will trigger if any enabled PCINT[7:0] pin toggles. The PCMSK3, PCMSK2, PCMSK1 and PCMSK0 registers control which pins contribute to the pin change interrupts. Pin change interrupts on PCINT are detected asynchronously. This implies that these interrupts can be used for waking the part 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 Register A (EICRA). When the external interrupts are enabled and are configured as level-triggered, the interrupts will trigger as long as the pin is held low. Note that recognition of falling or rising edge interrupts on INT requires the presence of an I/O clock. Low level interrupt on INT is detected asynchronously. This implies that this interrupt can be used for waking the part from sleep modes other than Idle mode. The I/O clock is halted in all sleep modes except Idle mode.

Note: 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.