//variables to keep track of the timing of recent interrupts
unsigned long trigger_time = 0;  
unsigned long last_trigger_time = 0;
unsigned long period = 1;
unsigned long idle = 1;
unsigned int rpm = 0;
const int pulsePerRev = 2;
int pulseDelay = 3;
unsigned int fireFlag = 0;


void IRAM_ATTR isr() {
    trigger_time = millis();
		period = trigger_time - last_trigger_time;
    last_trigger_time = trigger_time;
		fireFlag = 1;
		while (millis() < trigger_time + pulseDelay){}
		digitalWrite(19, LOW);
		while (millis() < trigger_time + pulseDelay +1){}
}

void fireCoil() {
	//Serial.printf( fireFlag ? "true" : "false"); // prints true
	//printf("\n");
	//Serial.printf("fire coil\n");
	delay(pulseDelay);
	digitalWrite(19, LOW);
	delayMicroseconds(500);
	digitalWrite(19, HIGH);
	fireFlag = 0;
}

void setup() {
	Serial.begin(115200);
	pinMode(18, INPUT_PULLUP);
	pinMode(19, OUTPUT);
	digitalWrite(19, HIGH);
	attachInterrupt(18, isr, FALLING);
}

void loop() {
	//Serial.print( fireFlag ); // prints true
	//Serial.printf("\n");
	if (fireFlag==true) {fireCoil();}
  rpm = 60000/(period*pulsePerRev);
	float deg_per_usec = 1000000.*60./360./rpm
	//Serial.printf("Period is %u msec\n", period);
	//Serial.printf("RPM is %u \n", rpm);
	Serial.printf("deg/usec is %.1 \n", deg_per_usec);

}