ATtiny Pulse Receiver

/*
 * ATtiny85_PulseReceiver.c
 *
 * Created: 10/16/2014 12:48:10 PM
 *  Author: grist.carrigafoyl

 A receiver for signals from a fibre optic connector for use with a Micro-brute
 Tesla Coil. Will respond to pin changes on PCINT0 (PB0, pin 5), but will not
 allow a pulse greater than a preset width to occur. Using an external crystal
 oscillator for the clock.

 Also incorporating a guard timer to prevent pulses ocurring too often.


 Input is on PB0, pin5
 Output is on PB1, pin 6.

 */

#define F_CPU 8000000UL // 8Mhz
#define MAX_PULSE_WIDTH 190 // microseconds
#define MIN_GUARD_TIMER 60 // min microseconds between pulses
#define OUTPUT_PIN PB1


#include <avr/io.h>
#include <avr/interrupt.h>

// Used to control the behaviour of COMPB_vect
volatile char long_timer_active = 0;


int main(void)
{
    // Set Output pin as output
    DDRB |= _BV(OUTPUT_PIN);

    // Set up the interrupt on pin 5
    GIMSK |= _BV(PCIE); // pin change interrupt enable
    PCMSK |= _BV(PCINT0); // only react to changes on PCINT0, ie PB0, pin 5

    // Timer0 interrupt settings to limit the pulse width
    TCCR0A = _BV(WGM01); // ctc mode
    TIMSK |= _BV(OCIE0A) | _BV(OCIE0B); // Enable Timer 0 COMP A & B interrupt vectors on match
    // Not setting this here as it starts the timer. That will be done by the timer 1 interrupt.
    //TCCR0B|= _BV(CS01); // prescaler 8, so OCR0A is in µs @ 8MHz
    OCR0A = MAX_PULSE_WIDTH; // should be close to microseconds with prescalar 8 and 8MHz clock
    OCR0B = MIN_GUARD_TIMER; // minimum time between pulses

    sei(); // enable global interrupts

    while(1)
    {
        //TODO:: Please write your application code
    }
}


ISR(PCINT0_vect) {
// React to changes on pin 5. Logic is reversed as the fibre optic receiver
// holds the pin high until it receives a signal.
    if ((PINB & _BV(PB0)) == 0) { // pin is now low, so rising edge of original pulse
        PORTB |= _BV(OUTPUT_PIN); // turn it on
        TCNT0 = 0; // reset the counter
        TCCR0B|= _BV(CS01); // start the max pulse width timer with prescaler 8
        long_timer_active = 1;
    } else { // pin now high, so falling edge of original pulse
        PORTB &= ~_BV(OUTPUT_PIN);
        TCNT0 = 0;
        PCMSK = 0; // disable pin change interrupt
        long_timer_active = 0;
    }
}


ISR(TIMER0_COMPA_vect)
// Max pulse width timer. If we get to here then a pulse was received that exceeded the max
// allowable. This could cause an over-current condition and damage to the IGBTs.
{
    // Turn off the pin.
    PORTB &= ~_BV(PB1);
    PCMSK = 0; // disable pin change interrupt
    // set the prescalar to 1024 to really delay the next possible pulse.
    TCCR0B = _BV(CS00) | _BV(CS02);
    long_timer_active = 0;
}

ISR(TIMER0_COMPB_vect)
// Guard timer. No pin change interrupts are allowed until this has expired.
{
    if (long_timer_active == 0) {
        // Turn the timer off, we're done with it for now
        TCCR0B = 0;
    }
    // Reenable pin change interrupts on the input pin
    PCMSK |= _BV(PCINT0);
}