..  _avr-isr-vector:

Using Interrupts in AVR GCC
###########################

When we use interavrupqts in assembly language, we need to make sure that the
interrupt vector jump table, which needs to be set up starting at address 0x00,
is properly set up. The names we need to use for each possible interrupt source
are defined in the datasheet for the chip being used. From that document (page
65) we see this table:

..  csv-table::
    :header: VectorNo, Address, Name, Definition

    0,0x0000,RESET, Power-on Reset
    1,0x0002,INT0_vect,External Interrupt Request 0
    2,0x0004,INT1_vect,External Interrupt Request 1
    3,0x0004,PCINT0_vect,Pin Change Interrupt Request 0
    4,0x0008,PCINT1_vect,Pin Change Interrupt Request 1
    5,0x000A,PCINT2_vect,Pin Change Interrupt Request 2
    6,0x000C,WDT_vect,Watchdog Time-out Interrupt
    7,0x000E,TIMER2_COMPA_vect,Timer/Counter2 Compare Match A
    8,0x0010,TIMER2_COMPB_vect,Timer/Counter2 Compare Match A
    9,0x0012,TIMER2_OVF_vect,Timer/Counter2 Overflow
    10,0x0014,TIMER1_CAPT_vect,Timer/Counter1 Capture Event
    11,0x0016,TIMER1_COMPA_vect,Timer/Counter1 Compare Match A
    12,0x0018,TIMER1_COMPB_vect,Timer/Counter1 Compare Match B
    13,0x001A,TIMER1_OVF_vect,Timer/Counter1 Overflow
    14,0x001C,TIMER1_COMPA_vect,Timer/Counter0 Compare Match A
    15,0x001E,TIMER1_COMPB_vect,Timer/Counter0 Compare Match A
    16,0x0020,TIMER0_OVF_vect,Timer/Counter0 Overflow
    17,0x0022,SPI_STC_vect,SPI Serial Transfer Complete
    18,0x0024,USART_RX_vect,USART Rx Complete
    19,0x0026,USART_UDRE_vect,USART Data Register Empty
    20,0x0028,USART_TX_vect,USART Tx Complete
    21,0x002A,ADC_vect,ADC Conversion Complete
    22,0x002C,EE_READY_vect,EEPROM Ready
    23,0x002E,ANALOG_COMP_vect,Analog Comparator
    24,0x0030,TWI_vect, 2-wire Serial Interface
    25,0x0032,SPM_READY_vect, Store Program Memory Ready

If you dig into the AVR_GCC files, you will find a file named ``Interrupts.h``
where a handy ``ISR`` macro is defined. We can use this macro to set up the
required entry in the vector table:

..  code-block:: text

    #define ISR(vector)

The actual macro is more complex than this, but this will work for our assembly
language work. The vector names we need to use are define din the table above.

As an example, here is how we would set up the required entry in the vector
table for using the counter compare interrupt for Timer1:

..  code-block:: text

    ;
            org 0x0000
    RESET   rjmp        start

            org 0x0016
            rjmp        Timer1_ISR

    start:  ; main code goes here
            ...

    Timer1_ISR:
            ; code for interrupt service routine goes here.