void setup() {
  // put your setup code here, to run once:
Serial.begin(9600);
Serial.println("Inicio");
}

void loop() {
  // put your main code here, to run repeatedly:
byte a=0xfa;
byte b=0x01;
byte c=123;
byte d='d';
if (a < 16) Serial.print("0");
Serial.print(a,HEX);
if (b < 16) Serial.print("0");
Serial.print(b,HEX);
if (c < 16) Serial.print("0");
Serial.print(c,HEX);
if (d < 16) Serial.print("0");
Serial.print(d,HEX);
while (1);
}