38
ATtiny26(L)
1477B–AVR–04/02
External Interrupt The External Interrupt is triggered by the INT0 pin. Observe that, if enabled, the interrupt
will trigger even if the INT0 pin is configured as an output. This feature provides a way of
generating a software interrupt. The External Interrupt can be triggered by a falling or
rising edge, a pin change, or a low level. This is set up as indicated in the specification
for the MCU Control Register – MCUCR. When the External Interrupt is enabled and is
configured as level triggered, the interrupt will trigger as long as the pin is held low.
Pin Change Interrupt The pin change interrupt is triggered by any change on any I/O pin of Port B and pins
PA3, PA6, and PA7, if the interrupt is enabled and alternate function of the pin does not
mask out the interrupt. The bit PCIE1 in GIMSK enables interrupt from pins PB[7:4],
PA[7:6], and PA[3]. PCIE0 enables interrupt on digital pins PB[3:0].
The pin change interrupt is different from other interrupts in two ways. First, pin change
interrupt enable bits PCIE1 and PCIE0 also mask the flag if they are not set. The normal
operation on most interrupts is that the flag is always active and only the execution of
the interrupt is masked by the interrupt enable.
Secondly, please note that pin change interrupt is disabled for any pin that is configured
as an alternate function. For example, no pin change interrupt is generated from pins
that are configured as AREF, AIN0 or AIN1, OC1A, OC1A
, OC1B, OC1B, XTAL1, or
XTAL2 in a fuse selected clock option, Timer0 clocking, or RESET
function. See Table
17 for alternate functions which mask the pin change interrupt and how the function is
enabled. For example pin change interrupt on the PB0 is disabled when USI Two-wire
mode or USI Three-wire mode or Timer/Counter1 inverted output compare is enabled.
If the interrupt is enabled, the interrupt will trigger even if the changing pin is configured
as an output. This feature provides a way of generating a software interrupt. Also
observe that the pin change interrupt will trigger even if the pin activity triggers another
interrupt, for example the external interrupt. This implies that one external event might
cause several interrupts.
The value of the programmed fuse is “0” and unprogrammed is “1”. Each of the lines
enables the alternate function so “or” function of the lines enables the function.
Table 17. Alternative Functions
Pin Alternate Function
Control Register[Bit Name] which
set the Alternate Function
(1)
Bit or Fuse
Value
(2)
PA3 AREF ADMUX[REFS0] 1
PA6 Analog Comparator ACSR[ACD] 0
PA7 Analog Comparator ACSR[ACD] 0
PB0 USI Two-wire mode
USI Three-wire mode
TC1 compare/PWM
USICR[USIWM1]
USICR[USIWM1,USIWM0]
TCCR1A[COM1A1,COM1A0,PWM1A]
1
01
011
PB1 USI Three-wire mode
TC1 compare/PWM
USICR[USIWM1,USIWM0]
TCCR1A[COM1A1]
TCCR1A[COM1A0]
01
1
1
PB2 USI Two-wire mode
USI Three-wire mode
TC1 compare/PWM
USICR[USIWM1]
USICR[USIWM1,USIWM0]
TCCR1A[COM1B1,COM1B0,PWM1B]
1
01
011
PB3 TC1 compare/PWM TCCR1A[COM1B1]
TCCR1A[COM1B0]
1
1
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