byte val = 0;
int speed1 = 9;
int speed2 = 10;
int g1a = 4;
int g1b = 5;
int g2a = 7;
int g2b = 6;

void stop()
{
  digitalWrite(g1a,  LOW);
  digitalWrite(g1b,  LOW);
  analogWrite(speed1, 0);
  digitalWrite(g2a,  LOW);
  digitalWrite(g2b,  LOW);
  analogWrite(speed2, 0);
}

void izquierda(int iz) 
{ 
  //stop();
  //delay(100);
  digitalWrite(g1a,  HIGH);
  digitalWrite(g1b,  LOW);
  //digitalWrite(g2a,  LOW);
  //digitalWrite(g2b,  LOW);
  analogWrite(speed2, 0);
  /*for (int i=64; i <= iz; i=i+10){
      analogWrite(speed1, i);
      delay(100);
  } */
  analogWrite(speed1, iz);
}

void derecha(int der) 
{ 
  //stop();
  //delay(100);
  digitalWrite(g2a,  HIGH);
  digitalWrite(g2b,  LOW);
  //digitalWrite(g1a,  LOW);
  //digitalWrite(g1b,  LOW);
  analogWrite(speed1, 0);
  /*for (int i=64; i <= der; i=i+10){
      analogWrite(speed2, i);
      delay(100);
  } */
  analogWrite(speed2, der);
}
 
void retrocede(int re)
{ 
  stop();
  //delay(100);
  digitalWrite(g2a,  HIGH);
  digitalWrite(g2b,  LOW);
  digitalWrite(g1a,  HIGH);
  digitalWrite(g1b,  LOW);
  /*for (int i=64; i <= re; i=i+10){
      analogWrite(speed1, i);
      analogWrite(speed2, i);
      delay(100);
  } */
  analogWrite(speed2, re);
  analogWrite(speed1, re);
} 

void avanza(int av) 
{ 
  stop();
  //delay(100);
  digitalWrite(g2a,  LOW);
  digitalWrite(g2b,  HIGH);
  digitalWrite(g1a,  LOW);
  digitalWrite(g1b,  HIGH);
  /*for (int i=64; i <= av; i=i+10){
      analogWrite(speed1, i);
      analogWrite(speed2, i);
      delay(100);
  } */
  analogWrite(speed2, av);
  analogWrite(speed1, av);
} 

void setup() 
{ 
  Serial.begin(9600); 
  delay(100); 
  pinMode(g2a, OUTPUT);
  pinMode(g2b, OUTPUT);
  pinMode(g1a, OUTPUT);
  pinMode(g1b, OUTPUT);
} 

void loop() 
{ 
  if (Serial.available()) {
    val = Serial.read();
    switch (val) {
    case 0x80:
      avanza(255);
    break;
    case 0x81:
      retrocede(255);
    break;
    case 0x82:
      izquierda(255);
    break;
    case 0x83:
      derecha(255);
    break;
    case 0x84:
      stop();
    break;
    }
  }  
}