/*

  Receive side of one way ultrasonic ranging using HC-SR04.

  Node sync is performed with 433 MHz OOK transmit receive pair via a sync sequence

 

  HC-SR04 Ping distance sensor:

  VCC to arduino 5v

  GND to arduino GND

  Echo to Arduino pin 7

  Trig to Arduino pin 8

 

  433 MHz Receiver:

  Data to Arduino pin 9

 

#define syncStateA 10

#define syncStateB 11

#define debugPinC 12

 

#define echoPin 7 // Echo Pin

#define trigPin 8 // Trigger Pin

#define rfRxPin 9 // rf Receive input pin

#define LEDPin 13 // Onboard LED

 

// Parameters for sync detection

#define minBitPeriod 450  // Low threshold for bit period (uS)

#define maxBitPeriod 550  // Max threshold for bit period (uS)

#define minSyncBits 8     // Min number of valid 1 plus 0 transitions before 1 plus 1

 

int maximumRange = 200; // Maximum range needed (cm)

int minimumRange = 0; // Minimum range needed

long duration, distance; // Duration used to calculate distance

long timeout = 50000;  // Ping duration timeout

 

void setup() {

  pinMode(syncStateA, OUTPUT) ;

  pinMode(syncStateB, OUTPUT) ;

  pinMode(debugPinC, OUTPUT) ;

 

  Serial.begin (9600);

  pinMode(trigPin, OUTPUT);

  pinMode(echoPin, INPUT);

  pinMode(rfRxPin, INPUT);

  pinMode(LEDPin, OUTPUT) ;

  digitalWrite(LEDPin, LOW) ;

  digitalWrite(trigPin, LOW) ;

}

 

void loop() {

  RxRFSync() ;    // Function blocks until sync sequence is detected

  digitalWrite(LEDPin,HIGH) ;

  digitalWrite(trigPin, HIGH);    // Start ping sequence

  delayMicroseconds(10);

  digitalWrite(trigPin, LOW);

  duration = pulseIn(echoPin, HIGH, timeout);

 

  //Calculate the distance (in cm) based on the speed of sound.

  distance = duration / 29.1 ;

 

  if (distance >= maximumRange || distance <= minimumRange) {

    /* Send a negative number to computer */

    Serial.println("-1");

  }

  else {

    /* Send the distance to the computer using Serial protocol */

    Serial.println(distance);

  }

  digitalWrite(LEDPin,LOW) ;

}

 

// Function to detect "syncSeq" bit sequence on RF receive port

void RxRFSync()

{

  long int lastBitTime = 0 ;    // holds time of last bit transition (uS)

  int BitCount = 0 ;            // counts number of valid bits detected

 

  boolean synced = false ;

  boolean lastBit = LOW ;  // holds last bit detected

 

  digitalWrite(syncStateA, LOW) ; digitalWrite(syncStateB, LOW) ; digitalWrite(debugPinC, LOW) ;

 

  while (!synced)

  {

    while (digitalRead(rfRxPin) == lastBit) { } // Block until bit transition

    int currBitTime = micros() ;

    int bitPeriod = currBitTime - lastBitTime ;

 

    if ((bitPeriod > (2 * minBitPeriod)) && (bitPeriod < (2 * maxBitPeriod)) && (lastBit == HIGH) && (BitCount > minSyncBits))

    {

      // Valid completion of sync sequence

      synced = true ;

      digitalWrite(syncStateA, HIGH) ; digitalWrite(syncStateB, HIGH) ; digitalWrite(debugPinC, lastBit) ;

    }

    else

    {

      lastBit = !lastBit ;

      lastBitTime = currBitTime ;

      if ((bitPeriod > minBitPeriod) && (bitPeriod < maxBitPeriod))

      {

        // Valid single bit detected, increment valid bit count and look for next bit

        BitCount += 1 ;     // increment valid bit count

        digitalWrite(syncStateA, LOW) ; digitalWrite(syncStateB, HIGH) ; digitalWrite(debugPinC, lastBit) ;

      }

      else

      {

        // Invalid bit detected, reset valid bit count and look for next bit

        BitCount = 0 ;

        digitalWrite(syncStateA, HIGH) ; digitalWrite(syncStateB, LOW) ; digitalWrite(debugPinC, lastBit) ;

      }

    }

  }

}