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61
2593O–AVR–02/12
ATmega644
cleared by writing a logical one to its Interrupt Flag bit (INTFn) in the EIFR Register before the
interrupt is re-enabled.
Note:
1. n = 3, 2, 1or 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.
11.1.2
EIMSK – External Interrupt Mask Register
Bits 2:0 – INT2:0: External Interrupt Request 2 - 0 Enable
When an INT2:0 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, 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.
11.1.3
EIFR – External Interrupt Flag Register
Bits 2:0 – INTF2:0: External Interrupt Flags 2 - 0
When an edge or logic change on the INT2:0 pin triggers an interrupt request, INTF2:0 becomes
set (one). If the I-bit in SREG and the corresponding interrupt enable bit, INT2:0 in EIMSK, are
set (one), the MCU will jump to the interrupt vector. The flag is cleared when the interrupt routine
is executed. Alternatively, the flag can be cleared by writing a logical one to it. These flags are
always cleared when INT2:0 are configured as level interrupt. Note that when entering sleep
mode with the INT2:0 interrupts disabled, the input buffers on these pins will be disabled. This
may cause a logic change in internal signals which will set the INTF2:0 flags. See
”Digital InputTable 11-1.
Interrupt Sense Control
ISCn1
ISCn0
Description
0
The low level of INTn generates an interrupt request.
0
1
Any edge of INTn generates asynchronously an interrupt request.
1
0
The falling edge of INTn generates asynchronously an interrupt request.
1
The rising edge of INTn generates asynchronously an interrupt request.
Table 11-2.
Asynchronous External Interrupt Characteristics
Symbol
Parameter
Condition
Min
Typ
Max
Units
tINT
Minimum pulse width for asynchronous
external interrupt
50
ns
Bit
76543210
–
INT2
INT1
INT0
EIMSK
Read/Write
RRRRR
R/W
Initial Value
00000000
Bit
76543210
–
INTF2
INTF1
INTF0
EIFR
Read/Write
R/W
RRRR
R/W
Initial Value
00000000