I have learned Arduino for a month, and I want to bulid a car with 2 modes, Remote and Auto. Auto mode needs to have while() loop, so it can receive data from sensors. When I choose Auto mode via bluetooth, it keeps looping and it won't stop when I choose the Remote mode. Here's the part code:

char mode;

void loop()
{
    while(serial1.available())
    {
        char control = serial.read();
        if(control == 'r') mode = 'r';     // r stand for Remote Mode
        if(control == 'a') mode = 'a';     // a stand for Auto Mode

        if(mode == 'r') remoteMode(control);

        while(mode = 'a')
        {
            int IR = analogRead(A10);
            float U1cmMsec,U2cmMsec;
            long U1microsec = u1.timing();
            long U2microsec = u2.timing();
            U1cmMsec = u1.convert(U1microsec, Ultrasonic::CM);
            U2cmMsec = u2.convert(U2microsec, Ultrasonic::CM);

            robot(U1cmMsec, U2cmMsec,IR);
      
            /*
            Here is the code to stop while() that I don't know how
            to program.
            I have tried:
      
            if(serial1.read() != 'a') break; 
      
            But it will stop while(), because it will waits for
            bluetooth send data to arduino board. 
            And if I take this line off, it will keeps while() and
            nothing can stop it.
            So, what am I supposed to do?
            */
        }
    }
}