const int outPin = 8;                 // digital pin 8

#define rpm_to_us(rpm) (1000000UL / (rpm / 60))

void setup() {  
  Serial.begin(115200UL);  //シリアル通信開始  
  Serial.print("Begin\n");
  
  pinMode(outPin, OUTPUT);
  digitalWrite(outPin, HIGH);
}

void loop() {
  digitalWrite(outPin, LOW);
  delayMicroseconds(100);
  digitalWrite(outPin, HIGH);
  delayMicroseconds(rpm_to_us(20000) - 100);
}