void setup() {
  int val = 0xE5;
  char buffer[20];
  
  Serial.begin(115200);

  decimalToHex(val, buffer);

  Serial.print("decimal "); Serial.print(val); Serial.print(" in hex is: "); Serial.println(buffer);
  Serial.print("ascii:  "); Serial.println((char)val);
  Serial.print("strtol: "); Serial.println(strtol(buffer, 0, 16));
  
  Serial.print("First hex digit value: ");
  Serial.println(hexToDecimal(buffer[0]));
  Serial.print("Second hex digit value: ");
  Serial.println(hexToDecimal(buffer[1]));
}

void loop() {
}

/*****
  Function that converts decimal value to a hex string
  
  Argument list:
    int n;        the decimal value to convert
    char hex[]    the buffer to hold the conversion
    
  Return value:
    void
    
  Caution: This function assumes the buffer is large enough to hold
  the output.

*****/

void decimalToHex(int n, char hex[]) 
{
  int i = 0, rem;
  while (n > 0)
  {
    rem = n % 16;
    switch (rem)
    {
      case 10:
        hex[i] = 'A';
        break;
      case 11:
        hex[i] = 'B';
        break;
      case 12:
        hex[i] = 'C';
        break;
      case 13:
        hex[i] = 'D';
        break;
      case 14:
        hex[i] = 'E';
        break;
      case 15:
        hex[i] = 'F';
        break;
      default:
        hex[i] = rem + '0';
        break;
    }
    ++i;
    n /= 16;
  }
  hex[i] = '\0';
  strrev(hex);   /* Reverse string */
}

/*****
  Function that converts hex digit to a decimal value
  
  Argument list:
    char hex       the hex digit to convert
    
  Return value:
    int
*****/
int hexToDecimal(char hex)   /* Function to convert hexadecimal to decimal. */
{
  hex = toupper(hex);
  int temp = hex - '0';
  
  if (temp < 10)
    return temp;
  else
    return  hex - 'A' + 10;
  
}