ATmega644PA datasheet(68/468 Pages) ATMEL

68 / 468 page

8152G–AVR–11/09

ATmega164PA/324PA/644PA/1284P

12. External Interrupts

12.1

Overview

The External Interrupts are triggered by the INT2:0 pin or any of the PCINT31:0 pins. Observe

that, if enabled, the interrupts will trigger even if the INT2:0 or PCINT31:0 pins are configured as

outputs. This feature provides a way of generating a software interrupt.

The Pin change interrupt PCI3 will trigger if any enabled PCINT31:24 pin toggle, 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. PCMSK3, PCMSK2, PCMSK1 and PCMSK0 Registers control which pins

contribute to the pin change interrupts. Pin change interrupts on PCINT31:0 are detected asyn-

chronously. 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 (INT2:0).

When the external interrupt is enabled and is configured as level triggered, the interrupt will trig-

ger as long as the pin is held low. Low level interrupts and the edge interrupt on INT2:0 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 inter-

rupt 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 30.

12.2

12.2.1

EICRA – External Interrupt Control Register A

The External Interrupt Control Register A contains control bits for interrupt sense control.

• Bits 7:6 – Reserved

These bits are reserved in the ATmega164PA/324PA/644PA/1284P, and will always read as

zero.

• Bits 5:0 – ISC21, ISC20 – ISC00, ISC00: External Interrupt 2 - 0 Sense Control Bits

The External Interrupts 2 - 0 are activated by the external pins INT2:0 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 12-1. Edges on INT2..INT0 are registered asynchro-

nously. Pulses on INT2:0 pins wider than the minimum pulse width given in ”External Interrupts

Characteristics” on page 334 will generate an interrupt. Shorter pulses are not guaranteed to

Bit

76543210

(0x69)

–

ISC21

ISC20

ISC11

ISC10

ISC01

ISC00

EICRA

Read/Write

R/W

Initial Value

00000000

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