synchronizing arduino timer with external trigger

the output of the electric generator fed to the op amp output of the square pulse is 50 Hz and 50% duty cycle. on the other hand the arduino I have set is set to generate 100 Hz CTC mode. I want to synchronize the two between arduino and the output of the op amp so that the division of 4 quadrants of high accuracy pulse output from arduino. I think to trigger an arduino timer on pin 2 which is attached from the op amp output as external interrupt, each time> if positive high pulse input then start timer if low input pulse, stop and reset timer.

#define ext_interrupt 2

void setup() {
pinMode(ext_interrupt, INPUT);
pinMode(3, OUTPUT);
pinMode(4, INPUT);
cli();
TCCR1A = 0;
TCCR1B = 0;
TCNT1 = 0;
OCR1A = 2499; //set pin dutty on time
OCR1B = 1249; //set pin dutty off time
TCCR1A |= (1<<COM1A1) | (1<<COM1B1);
TCCR1B |= (1<<CS10) |(1<<CS11) | (1<<WGM12);
TIMSK1 |= (1<<OCIE1A) | (1<<OCIE1B);
sei();
}

ISR(TIMER1_COMPA_vect){ //controll OCR1A
if (digitalRead(ext_interrupt, HIGH)){
//how to start timer programaticaly ??
digitalWrite(3,HIGH);
digitalWrite(4,LOW);
}
}

ISR(TIMER1_COMPB_vect){ //controll OCR1B
if (digitalRead(ext_interrupt, HIGH)){
digitalWrite(3,LOW);
digitalWrite(4,HIGH);
//how to reset timer ??
//then reset all parameter back to 0 ??
}
}

void blink() {
state = !state;
}
void loop() {
// put your main code here, to run repeatedly:

}

how can i do this? pls help :smiley:

Why is it that people don’t read this before posting a programming question and follow the advice given ?

Not too bad in this case because at least there are no italics or smileys in the code, but it would still be better to use code tags to make copying to an editor easier