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ATMEGA8L-8AN Datasheet(PDF) 66 Page - ATMEL Corporation |
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ATMEGA8L-8AN Datasheet(HTML) 66 Page - ATMEL Corporation |
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66 / 331 page  66 2486AA–AVR–02/2013 ATmega8(L) External Interrupts The external interrupts are triggered by the INT0, and INT1 pins. Observe that, if enabled, the interrupts will trigger even if the INT0..1 pins are configured as outputs. This feature provides a way of generating a software interrupt. The external interrupts can be triggered by a falling or ris- ing edge 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. Note that recognition of falling or rising edge interrupts on INT0 and INT1 requires the presence of an I/O clock, described in “Clock Systems and their Distribution” on page 25. Low level interrupts on INT0/INT1 are detected asynchronously. This implies that these interrupts can be used for waking the part also from sleep modes other than Idle mode. The I/O clock is halted in all sleep modes except Idle mode. Note that if a level triggered interrupt is used for wake-up from Power-down mode, the changed level must be held for some time to wake up the MCU. This makes the MCU less sensitive to noise. The changed level is sampled twice by the Watchdog Oscillator clock. The period of the Watchdog Oscillator is 1 µs (nominal) at 5.0V and 25 C. The frequency of the Watchdog Oscilla- tor is voltage dependent as shown in “Electrical Characteristics – TA = -40°C to 85°C” on page 235. The MCU will wake up if the input has the required level during this sampling or if it is held until the end of the start-up time. The start-up time is defined by the SUT Fuses as described in “System Clock and Clock Options” on page 25. If the level is sampled twice by the Watchdog Oscillator clock but disappears before the end of the start-up time, the MCU will still wake up, but no interrupt will be generated. The required level must be held long enough for the MCU to com- plete the wake up to trigger the level interrupt. MCU Control Register – MCUCR The MCU Control Register contains control bits for interrupt sense control and general MCU functions. • Bit 3, 2 – ISC11, ISC10: Interrupt Sense Control 1 Bit 1 and Bit 0 The External Interrupt 1 is activated by the external pin INT1 if the SREG I-bit and the corre- sponding interrupt mask in the GICR are set. The level and edges on the external INT1 pin that activate the interrupt are defined in Table 31. The value on the INT1 pin is 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. If low level interrupt is selected, the low level must be held until the completion of the currently executing instruction to generate an interrupt. Bit 765 4321 0 SE SM2 SM1 SM0 ISC11 ISC10 ISC01 ISC00 MCUCR Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 000 0000 0 Table 31. Interrupt 1 Sense Control ISC11 ISC10 Description 0 0 The low level of INT1 generates an interrupt request 0 1 Any logical change on INT1 generates an interrupt request 1 0 The falling edge of INT1 generates an interrupt request 1 1 The rising edge of INT1 generates an interrupt request |
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