start.cpp

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#define F_CPU 1000000UL   //1 MHz
#include <Arduino.h>

//#define DEBUG_LCD
#include "_debug.h"


#define OUTPUT PORTC0
void setup()
{   
    DDRC = _BV(OUTPUT); //OUTPUT as output
    PORTC &= ~_BV(OUTPUT); // clear pin 
    
    TCCR2A = 0; TCCR2B=0; cli(); // remove arduion config for timer2
    //Timer2 im Compare Mode
    // Prescaler = 1024
    // external Input
    ASSR = _BV(EXCLK);  //switch to external clock instead of ext. 32kHz crystal
    ASSR |= AS2; // switch to clock signal from extern
    
    ASSR = 0; // for debug only
    
    TCCR2A = _BV(WGM21);  // Clear Timer on Compare Match MOde
    TIMSK2 |= _BV(OCIE2A); // Enable Compare Match Int. A
    TCCR2B |= _BV(CS22)|_BV(CS21)|_BV(CS20); // Prescal factor 1024; timer2 start

    sei(); 
}

void loop()
{
    uint8_t period = PINB; 
    period = 100; //debug only
    period = (period>>1)-1; // pin toggles, we need half the period (and a small correction)
    OCR2A = period; 
    
}
ISR(TIMER2_COMPA_vect){
    PORTC ^= _BV(OUTPUT); 
    T("Done");
}