hcsr04-tiva-energia.c

#define HCSR04_TRIG PD_3
#define HCSR04_ECHO PD_2

void setup()
{
  Serial.begin(115200);
  pinMode(HCSR04_TRIG,OUTPUT);
  pinMode(HCSR04_ECHO,INPUT); 
}

void loop()
{
  digitalWrite(HCSR04_TRIG,0); //trig off for 2us
  delayMicroseconds(10);
  digitalWrite(HCSR04_TRIG,1); //trig on for 10us
  delayMicroseconds(10);
  digitalWrite(HCSR04_TRIG,0); //trig off 
  //150us=minimum echo
  //25ms=maximum echo
  //38ms=no-return echo
  long echo=pulseIn(HCSR04_ECHO,HIGH,40000);
  if(echo>250&&echo<25000){
      double distance_cm=echo/58.2;
      Serial.println(distance_cm);
      delayMicroseconds(40000-echo);
  }
}