DC Motors Control

hey, so were working on a project where we control track wheels with an app on our phone through a bt module. we need to implement an option on one of the buttons for the DC motors to rotate 360 untill we press a button again to stop it from rotating 360, so we can carrry on driving to a different position. here is our code were using. does anyone know how to add this idea into our “z” button

int E1 = 5; //M1 Speed Control
int E2 = 6; //M2 Speed Control
int M1 = 4; //M1 Direction Control
int M2 = 7; //M1 Direction Control

void stop(void) //Stop
{
digitalWrite(E1,0);
digitalWrite(M1,LOW);
digitalWrite(E2,0);
digitalWrite(M2,LOW);
}
void advance(char a,char b) //Move forward
{
analogWrite (E1,a); //PWM Speed Control
digitalWrite(M1,HIGH);
analogWrite (E2,b);
digitalWrite(M2,HIGH);
}
void back_off (char a,char b) //Move backward
{
analogWrite (E1,a);
digitalWrite(M1,LOW);
analogWrite (E2,b);
digitalWrite(M2,LOW);
}
void turn_L (char a,char b) //Turn Left
{
analogWrite (E1,a);
digitalWrite(M1,LOW);
analogWrite (E2,b);
digitalWrite(M2,HIGH);
}
void turn_R (char a,char b) //Turn Right
{
analogWrite (E1,a);
digitalWrite(M1,HIGH);
analogWrite (E2,b);
digitalWrite(M2,LOW);
}

void setup(void)
{
int i;
for(i=4;i<=7;i++)
pinMode(i, OUTPUT);
Serial.begin(19200); //Set Baud Rate
Serial.println(“Run keyboard control”);
digitalWrite(E1,LOW);
digitalWrite(E2,LOW);

}

void loop(void)
{
if(Serial.available()){
char val = Serial.read();
if(val != -1)
{
switch(val)
{
case ‘w’://Move Forward
advance (255,255); //move forward in max speed
break;
case ‘s’://Move Backward
back_off (255,255); //move back in max speed
break;
case ‘a’://Turn Left
turn_L (100,100);
break;
case ‘d’://Turn Right
turn_R (100,100);
break;
case ‘z’:
Serial.println(“Hello”);
break;
case ‘x’:
stop();
break;
}
}
else stop();
}

}

  if(Serial.available()){
    char val = Serial.read();
    if(val != -1)

Since there IS serial data to read, the value read will never be -1. So, that test is useless.

    else stop();

So is this.

If you send an 'a', the device should start moving forward. It should not stop until you send something else. If you are seeing different behavior, you have a problem. You need to describe that problem in better detail.