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71
Atmel ATmega16/32/64/M1/C1 [DATASHEET]
7647K–AVR–12/13
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 asynchronously. 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
765
4321
0
ISC31
ISC30
ISC21
ISC20
ISC11
ISC10
ISC01
ISC00
EICRA
Read/Write
R/W
Initial Value
000
0000
0
Table 10-1. Interrupt Sense Control(1)
ISCn1
ISCn0
Description
0
The low level of INTn generates an interrupt request.
0
1
Any logical change on INTn generates an interrupt request.
1
0
The falling edge between two samples of INTn generates an interrupt request.
1
The rising edge between two samples of INTn generates an interrupt request.
Bit
7
65
43
21
0
–
INT3
INT2
INT1
INT0
EIMSK
Read/Write
R
R/W
Initial Value
0
00
0