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ATmega64A [DATASHEET]
8160D–AVR–02/2013
13. External Interrupts
The External Interrupts are triggered by the INT7:0 pins. Observe that, if enabled, the interrupts will trigger even if
the INT7:0 pins are configured as outputs. This feature provides a way of generating a software interrupt. The
external interrupts can be triggered by a falling or rising edge or a low level. This is set up as indicated in the spec-
ification 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. Note
that recognition of falling or rising edge interrupts on INT7:4 requires the presence of an I/O clock, described in
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 mode, the changed level must be held
for some time to wake up the MCU. This makes the MCU less sensitive to noise. The changed level is sampled
twice by the Watchdog Oscillator clock. The period of the Watchdog Oscillator is 1 s (nominal) at 5.0V and 25
C.
held until the end of the start-up time. The start-up time is defined by the SUT Fuses as described in
“Clock Sys-disappears before the end of the start-up time, the MCU will still wake up, but no interrupt will be generated. The
required level must be held long enough for the MCU to complete the wake up to trigger the level interrupt.
13.1
Register Description
13.1.1
EICRA – External Interrupt Control Register A
This Register can not be reached in ATmega103 compatibility mode, but the Initial Value defines INT3:0 as low
level interrupts, as in ATmega103.
Bits 7:0 – ISC31, ISC30 - ISC00, ISC00: External Interrupt 3 - 0 Sense Control Bits
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
the minimum pulse width given in
Table 13-2 on page 63 will generate an interrupt. Shorter pulses are not guaran-
teed 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. If enabled, a level triggered interrupt will generate an inter-
rupt 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 re-enabled.
Bit
765
43210
ISC31
ISC30
ISC21
ISC20
ISC11
ISC10
ISC01
ISC00
EICRA
Read/Write
R/W
Initial Value
000
00000