85
7799D–AVR–11/10
ATmega8U2/16U2/32U2
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.
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.
13.2.2
EICRB – External Interrupt Control Register B
Bits 7:0 – ISC71, ISC70 - ISC41, ISC40: External Interrupt 7:4 Sense Control Bits
The External Interrupts [7:4] are activated by the external pins INT[7:4] 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 13-2. The value on the INT[7:4] 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 inter-
rupt. Observe that CPU clock frequency can be lower than the XTAL frequency if the XTAL
divider is enabled. If low level interrupt is selected, the low level must be held until the comple-
tion 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 = 7, 6, 5 or 4.
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.
Table 13-1.
Interrupt Sense Control
(1)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.
Bit
76543210
ISC71
ISC70
ISC61
ISC60
ISC51
ISC50
ISC41
ISC40
EICRB
Read/Write
R/W
Initial Value
0
Table 13-2.
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.