void setup() 
{
  pinMode(3,OUTPUT);
  digitalWrite(3, LOW);
  Serial.begin(9600);
  Serial.println("UART Teste 1");
}

int i = 0;
char rx[30];
int num;
void loop() 
{
  Serial.write(0x55);
  delay(100);

  i = (i+1)%2;
  digitalWrite(3, i);

  if (Serial.available() > 0)
  {
    char rx = Serial.read();
    
    Serial.print("Char: ");
    if(rx < ' ')
      Serial.println("não é um caractere");
    else
      Serial.println(rx);
    
    Serial.print("Hexa: ");
    Serial.println(rx, HEX);
  }
}