83
7647H–AVR–03/12
Atmel ATmega16/32/64/M1/C1
10.2 External Interrupt Control Register A – EICRA
The External Interrupt Control Register A contains control bits for interrupt sense control.
• Bit 7..0 – ISC31, ISC30 - ISC01, ISC00: Interrupt Sense Control 0 Bit 1 and Bit 0
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 interrupt are defined in Table 10-1. Edges on INT3..INT0 are registered asynchro-
nously. The value on the INT3:0 pins are sampled before detecting edges. If edge or toggle
interrupt is selected, pulses that last longer than one clock period will generate an interrupt.
Shorter pulses are not guaranteed to generate an interrupt. Observe that CPU clock frequency
can be lower than XTAL frequency if the XTAL divider is enabled. 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 interrupt request as
long as the pin is held low.
Note: 1. n = 3, 2, 1 or 0.
When changing the ISCn1/ISCn0 bits, the interrupt must be disabled by clearing its Interrupt
Enable bit in the EIMSK Register. Otherwise an interrupt can occur when the bits are changed.
10.2.1 External Interrupt Mask Register – EIMSK
• Bit 7..4 – Res: Reserved Bits
These bits are unused bits in the ATmega16/32/64/M1/C1, and will always read as zero.
• Bit 3..0 – INT3 - 0: External Interrupt Request 3:0 Enable
When an INT3 – INT0 bit is written to one and the I-bit in the Status Register (SREG) is set
(one), the corresponding external pin interrupt is enabled. The Interrupt Sense Control bits in the
External Interrupt Control Register A - EICRA defines whether the external interrupt is activated
on rising or falling edge or level sensed. Activity on any of these pins will trigger an interrupt
request even if the pin is enabled as an output. This provides a way of generating a software
interrupt.
Bit 76543210
ISC31 ISC30 ISC21 ISC20 ISC11 ISC10 ISC01 ISC00 EICRA
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
Table 10-1. Interrupt Sense Control
(1)
ISCn1 ISCn0 Description
0 0 The low level of INTn generates an interrupt request.
0 1 Any logical change on INTn generates an interrupt request.
10
The falling edge between two samples of INTn generates an interrupt
request.
11
The rising edge between two samples of INTn generates an interrupt
request.
Bit 76543210
– – – – INT3 INT2 INT1 INT0 EIMSK
Read/Write RRRRRRR/WR/W
Initial Value 0 0 0 0 0 0 0 0
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