/*
*/
#define Sensor_IN 3

volatile uint16_t count; // 1ms = 8

//Timer2 Overflow Interrupt Vector
ISR(TIMER2_OVF_vect) 
{
  count++;               //Increments the interrupt counter
};

void setup() 
{
  pinMode(Sensor_IN, INPUT);
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(115200);
  
  TIMSK2 = 0x01;   //Overflow Interrupt Enable
  TCCR2A = 0x00;   //Set the CTC mode 
  TCCR2B = 0x02;   //Set the prescale 
}

// rate is 1000ms
// msg len is about 500 ms
#define tickperms 8
#define tickpermshalf tickperms/2

uint16_t oldcount; // 64 per ms
uint8_t sensorState,lastsensorState;   
uint16_t len; // 64 per ms
uint16_t synctime;

uint8_t bitarr[8*15];
uint8_t bitarrpt;
uint8_t bytearr[15];

void buildbyte(uint8_t pt)
{
  uint8_t mybyte = 0;
  for (int i=0;i<8;i++)
    mybyte |= bitarr[pt*8+i]<<i;
  bytearr[pt] = mybyte;
}

void loop() 
{
  sensorState = digitalRead(Sensor_IN);
  if (sensorState != lastsensorState)
  {
    lastsensorState = sensorState;
    len = count - oldcount;
    oldcount = count;
    
    //Serial.print(len); Serial.print("  ");
    if (sensorState == 0) // coming from high
    {
      if      (len < tickpermshalf) Serial.println("errlo ");
      else if (len < tickperms + tickpermshalf)
      {
        bitarr[bitarrpt++] = 1;
        //Serial.print("1");
      }
      else if (len < 3 * tickperms + tickpermshalf) 
      {
        bitarr[bitarrpt++] = 0;
        //Serial.print("0");
      }
      else if (len < 5 * tickperms + tickpermshalf) // sync
      {
        synctime = count+500*tickperms;        
        //Serial.println(bitarrpt);
        bitarrpt = 0;
      }
      else if (len > 300 * tickperms) // intermessage gap
      {
        count = 0;  // reset counter
        oldcount = 0;
      }
      
      if (bitarrpt > 8*15) 
      {
        bitarrpt = 0;
        Serial.println("too many bits");
      }
    } 
  }

  if (synctime != 0)
  {
    if (count >= synctime)
    {
      synctime = 0;
      if (bitarrpt != 8*15) 
      {
        Serial.print("wrong no of bits ");
        Serial.println(bitarrpt);
      }
    
      for (int i=0;i<15;i++) buildbyte(i);
        
      for (int i=0;i<15;i++) 
      {
        Serial.print(bytearr[i],HEX);
        Serial.print(" ");
      }
      Serial.println();
    }        
  }
}