Friday, 16 August 2013

INTERRUPTS IN AVR MICROCONTROLLER - ATMEGA 8/16/32



 An interrupt is an external or internal event that interrupts the microcontroller to inform it that a device needs its service.

 A single microcontroller can serve several devices by two ways a) interrupt and b) polling
 ·        Interrupts
o   Whenever any device needs its service, the device notifies the microcontroller by sending it an interrupt signal
o   Upon receiving an interrupt signal, the microcontroller interrupts whatever it is doing and serves the device
o   The program which is associated with the interrupt is called the interrupt service routine (ISR) or interrupt handler
o   For every interrupt, there must be an interrupt service routine (ISR), or interrupt handler
·        Polling
o   The polling method is not efficient, since it wastes much of the microcontroller’s time by polling devices that do not need  service
 
ISR or Interrupt Service Routine or Interrupt Handler defines the task that is needed to be performed when the interrupt occurs. It is the set of codes for the controller to execute on the occurrence of the interrupt.  The following table gives the interrupt sources in AVR Atmega 8 microcontroller, but most of this is common for the Atmega series controllers such as Atmega 16, Atmega 168, Atmega 32, etc

     AVR microcontroller has many interrupt sources but each has a different vector name which is used by the AVR studio compiler. But the vector name is specific to a interrupt source, which has a different name from the interrupt source name. The following table shows the vector name used by the compiler. The ISR is function whose input is the "INTERRUPT VECTOR NAME". These Vector Name is mapped into vector addresses.

VECTOR NAME
INTERRUPT SOURCE
INT0_vect
External INT0
INT1_vect
External INT1
TIMER2_COMP_vect
Timer 2 Compare match
TIMER2_OVF_vect
Timer 2 overflow
TIMER1_CAPT_vect
Timer1 Input Capture
TIMER1_COMPA_vect
Timer1 Compare A match
TIMER1_COMPB_vect
Timer1 Compare B match
TIMER1_OVF_vect
Timer1 Overflow
TIMER0_OVF_vect
Timer0 Overflow
SPI_STC_vect
SPI complete
USART_RXC_vect
USART RX Complete
USART_UDRE_vect
USART UDR EMPTY
USART_TXC_vect
USART TX Complete
ADC_vect
ADC Conv Complete
EE_RDY_vect
EEPROM Ready/Complete
ANA_COMP_vect
Analog Comparator i/p change
TWI_vect
TWI or I2C done/complete/receive

        To add the ISR for an interrupt source, just type the vector name in the function ISR(<vect_name>).  All other interrupts are disabled when controller is servicing (executing) an ISR and automatically enabled when the execution is completed. The AVR accomplishes this by disabling the global interrupt enable {sei();} and disable {cli();} function. Don’t forget to enable global interrupt enable flag before going into the infinite program loop, else the interrupt will not be served.

Syntax:
    ISR(<vector_name>)
      {   
          //code for the ISR
      }
 Example:
ISR(INT0_vect)
{//1 int0
}

ISR(TIMER1_COMPA_vect)
{//1 int0
}

 In order to have smooth interrupt functions follow the procedure,
1.    Choose the required interrupt source and its int type & trigger, read its registers description.
2.    Set the interrupt type & trigger method of the corresponding interrupt in its control registers and status register.
3.    Set the interrupt enable bit in control register.
4.    Set the global interrupt enable bit using “ sei(); ” command.
NOTE: If two or more interrupt sources are used in the program, then the global interrupt enable bit must be set after initializing all the interrupt sources.



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