External & internal Interrupts

Interrupt Sources• There are 21 different interrupts and each one has its

own vector located in a predefined location at the start of the memory space.

• 3 external interrupts (INT0, INT1 and INT2) are available at pins PD2(pin 16) and PD3 (pin 17) and PB2 (pin 3). They are Edge or Level triggered interrupts (except Int2 which is only edge triggered).

• Reset input (pin 9) which causes also an interrupt and can be both externally and internally initiated.

• If enabled, the interrupts will trigger even if the INT0 and INT1 pins are configured as outputs. This feature provides a way of generating a software interrupt (how?).

• INT0, INT1 (level) and INT2 are detected asynchronously (no Clock needed and thus they can be used to get the µC from sleep modes.

Reset and Interrupt VectorsVector No.

Program Address

Source Interrupt

Definition

1 $000 RESET External Pin, Power-on Reset, Brown-out Reset, Watchdog Reset, and JTAG AVR

Reset

2 $002 INT0 External Interrupt Request 03 $004 INT1 External Interrupt Request 14 $006 TIMER2 COMP Timer-Counter2 Compare Match5 $008 TIMER2 OVF Timer-Counter2 Overflow6 $00A TIMER1 CAPT Timer-Counter1 Capture Event7 $00C TIMER1 COMPA Timer-Counter1 Compare Match A8 $00E TIMER1 COMPB Timer-Counter1 Compare Match B9 $010 TIMER1 OVF Timer-Counter1 Overflow

Vector No.

Program Address

Source Interrupt Definition

10 $012 TIMER0 OVF Timer-Counter0 Overflow

11 $014 SPI, STC Serial Transfer Complete

12 $016 USART, RXC USART, Rx Complete

13 $018 USART, UDRE USART Data Register Empty

14 $01A USART, TXC USART, Tx Complete

15 $01C ADC ADC Conversion Complete

16 $01E EE_RDY EEPROM Ready

17 $020 ANA_COMP Analog Comparator

18 $022 TWI Two-wire Serial Interface

19 $024 INT2 External Interrupt Request 2

20 $026 TIMER0 COMP Timer-Counter0 Compare Match

21 $028 SPM_RDY Store Program Memory Ready

Notes to interrupt vector table

• If the program never enables an interrupt source, the Interrupt Vectors are not used, and regular program code can be placed at these locations.

External Interrupts:• To enable INT0/INT1/INT2:

The I-bit in the Status Register must be set. But also the corresponding External Interrupt Request Enable bit in the General Interrupt Control Register (GICR)

• When an event on the INT0/INT1/INT2 pin triggers an interrupt request, INT0-Flag/INT1-Flag/INT2-Flag (bit 6/7/5 of the General Interrupt Flag Register – GIFR) becomes set. The flag is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing a logical one to it.

General Interrupt Flag

Interrupt Triggering

The external interrupts INT0/INT1 can be triggered by a falling or rising edge or a low level.

INT2

The external interrupts INT2 can only be triggered by a falling or rising edge.

ISC2 (interrupt sense control of INT2): ISC2 = 0 -> Falling Edge ISC2 = 1 -> Rising Edge

MCU Control and Status Register

Using Interrupts in C

The access to the AVR interrupt system is implemented with the interrupt keyword.

Examples:interrupt [2] void external_int0(void) { /* Place your code here */ RETI}

interrupt [10] void timer0_overflow(void) { /* Place your code here */ RETI}

Using Interrupts in C (Contd.)

• Interrupt vector numbers start with 1.

• The compiler will automatically save the affected registers when calling the interrupt functions and restore them back on exit.

• a RETI assembly instruction is placed at the end of the interrupt function.

• Interrupt functions can't return a value nor have parameters.

• You must also set the corresponding bits in the peripheral control registers to configure the interrupt system and enable the interrupts.

Using the CodeVisionAVR Wizard by interrupts

Including Assembly Lines in the Program

Inline assembly may also be used.Example: #asm("sei") /* enable interrupts */

You can include assembly language anywhere in your program using the #asm and #endasm directives.Example:

void delay(unsigned char i) { while (i--) { /* Assembly language code sequence */ #asm nop nop #endasm };}