AVR Interrupts

Each of the pins on the AVR may have multiple functions. For example, pin D0 and D1 are able to serve as receive and transmit pins for serial port one, respectively. Similarly, a number of pins on the AVR are able to transmit interrupt requests to the CPU when they receive a particular signal. How to implement this functionality is shown below.

Register Summary

Register Name	Purpose of Register
PCICR	Pin Change Interrupt Control Register (Enable interrupts for a particular set of PCINT ports)
PCIFR	Pin Change Interrupt Flag Register (When a logic change triggers an interrupt on a particular PCINT pin, the corresponding flag will be set)
PCMSK0	Pin Change Mask Register (Select whether interrupts are enabled on a particular PCINT pin)

Bit Summary

Register Name	Bit Name	Purpose of Bit
PCICR	PCIE0 - 3	Enable interrupts for each set of 8 pins
PCIFR	PCIF0 - 3	Interrupt flags for each set of 8 pins
PCMSK0	PCINT0 - 7	Enable masks for each of 8 pins in set 0

Handling Interrupts Example

// Integer defined as volatile to prevent compiler optimizations
volatile int changeable_digit;

ISR (PCINT0_vect) {
    // Interrupt handler code goes here
}

int main(void) {
    // Enable interrupts for PCINT[7:0]
    PCICR |= (1 << PCIE0);
    
    // Clear the interrupt flag for PCINT[7:0]
    PCIFR |= (1 << PCIF0);
    
    // Enable interrupts on PCINT0 (PIN A0)
    PCMSK0 |= (1 << PCINT0);
    
    // Enable interrupts globally
    sei();
}

AVR Timer/Counter

The Atmel ATMega324A microcontroller contains three Timer/Counter devices available for use. The second Timer/Counter, labeled T/C 1 has a width of 16-bits, as opposed to 8-bits.

Output Compare Value formula:

The following formula can be used to calculate the required Output Compare value for a particular desired frequency.

In this case, O is the resulting Output Compare Value to be placed in register OCR1A (in decimal), C is the clock rate of the device (typically a value such as 8 000, 000 -- corresponding to 8MHz), P is the value of the selected pre-scaler (for example 8, or 64, or 256) and r is the desired rate at which you would like the Timer/Counter comparator to trigger some sort of event (this could be a value such as 100, for an event every 10ms (corresponding to 100Hz)).

Timer/Counter Register Summary

The following summary assumes that Timer/Counter 1 is being used. Three Timer/Counters are available on the AVR, labeled 0 - 3. To access the 8-bit Timer/Counter 0, TCCR0A would be used instead of TCCR1A.

Register Name	Purpose of Register
TCCR1A	Timer/Counter Control Register (Set compare output mode and define how counting sequence occurs)
TCCR1B	Timer/Counter Control Register (Define how counting sequence occurs and select a pre-scaler)
TCNT1H, TCNT1L	Direct access to the counter value (be careful about writing)
OCR1AH, OCR1AL	Output compare register for channel A (holds value to be compared with)
OCR1BH, OCR1BL	Output compare register for channel B
TIMSK1	Timer interrupt mask (set interrupt if compare match)
TIFR1	Timer interrupt flag register (contains input/match/overflow flags)

Timer/Counter Bit Summary

Register Name	Bit Name	Purpose of Bit
TCCR1A	COM0A0, COM0A1	Compare output mode bits for channel A (disable/toggle/clear/set OC1A pin on compare match)
TCCR1A	COM0B0, COM0B1	Compare output mode bits for channel B (disable/toggle/clear/set OC1B pin on compare match)
TCCR1A	WGM11, WGM10	Lower two bits of Waveform Generation Mode selection (how counting sequence occurs. Normal Mode, Clear Timer on Compare match (CTC), Pulse Width Modulation (PWM))
TCCR1B	WGM13, WGM12	Upper two bits of Waveform Generation Mode selection (see above)
TCCR1B	CS12, CS11, CS10	Prescaler selection bits (001 for no prescaler)
TIMSK1	OCIE1A	Enable timer compare interrupts for channel A
TIMSK1	OCIE1B	Enable timer compare intterupts for channel B
TIFR1	OCF1A	Set when a compare match occurs for channel A
TIFR1	OCF1B	Set when a compare match occurs for channel B

Timer/Counter Example

In order to setup a timer that matches a compare value and triggers an interrupt 100 times per second (every 10ms), the compare value can be set to 9999, with a pre-scaler value of 8.

// (Clock / (pre-scaler * rate)) - 1
// (8MHz / (8 * 100)) - 1
OCR1A = 9999;
 
// Don't toggle any pins
TCCR1A = 0;
 
// Clear timer once matched, pre-scaler of 8
TCCR1B = ((1 << WGM13) | (1 << WGM12) | (1 << CS11));
 
// Enable channel A interrupts
TIMSK1 = (1 << OCIE1A);
 
// Clear interrupt flag for channel A
TIFR1 = (1 << OCF1A);
 
// Enable interrupts globally
sei();

fpigerre/interrupts.md