#include <SoftwareSerial.h>

#define SENSOR_BAUD 9600
#define DEBUG_BAUD  9600

// Debug serial on pins 10 (RX), 11 (TX)
SoftwareSerial debugSerial(10, 11);

uint16_t distance = 0;

void setup()
{
  // Hardware Serial → Sensor
  Serial.begin(SENSOR_BAUD);

  // Software Serial → Debug terminal
  debugSerial.begin(DEBUG_BAUD);

  debugSerial.println("Ultrasonic AUTO Mode Receiver Started");
}

void loop()
{
  static uint8_t state = 0;
  static uint8_t H, L, CHK;

  while (Serial.available())
  {
    uint8_t b = Serial.read();

    switch (state)
    {
      case 0: // Wait for header
        if (b == 0xFF)
          state = 1;
        break;

      case 1: // High byte
        H = b;
        state = 2;
        break;

      case 2: // Low byte
        L = b;
        state = 3;
        break;

      case 3: // Checksum
        CHK = b;

        if (((0xFF + H + L) & 0xFF) == CHK)
        {
          uint16_t distance = ((uint16_t)H << 8) | L;

          debugSerial.print("Distance: ");
          debugSerial.print(distance);
          debugSerial.println(" cm");
        }
        else
        {
          debugSerial.println("Checksum Error");
        }

        state = 0;  // Always resync
        break;
    }
  }
}