Problem cleaning string read from serial buffer

Question

I am trying to move a stepper motor when a specific command is send to Arduino Mega 2560. This command is read character by character and stored in a string variable. Whenever a new command is sent, an act should be performed. However, it is not happening and I believe that the problem is in reading the sent command. I tried clearing the character and string variables, but to no avail. The sketch should run a loop that moves the motor clockwise or counterclockwise according to the command sent, which can be "crx*" for clockwise or "clx*" for anticlockwise, where "*" is the reading stop criterion of the characters. Any suggestions what to do? The script:

void(* resetFunc) (void) = 0;//Reset turn

// Stepper Motor X
const int stepPinX = 2; //X.STEP
const int dirPinX = 5; // X.DIR
const int pinEnable = 8;

// String read
char c;
String readString; //main captured String

// Unit steps
double av_len = 1.029; //medium length
double interval;
int cnt_steps = 0;
const int num_d = 200;//parameterized step
int fact = int(num_d / av_len);
int unit_len = fact * 1;//unit length (1mm)

void setup()
{
  // Sets pins as Outputs
  pinMode(stepPinX, OUTPUT);
  pinMode(dirPinX, OUTPUT);
  pinMode(pinEnable, OUTPUT);

  digitalWrite(pinEnable, HIGH);//lock driver on cnc shield 

  Serial.begin(115200);
  Serial.setTimeout(1);
  
  while (1) {
    
    while ( (Serial.available() == 0) )
    {

    }

    if (Serial.available())  {
      c = Serial.read();  //gets one byte from serial buffer
      //Serial.println(readString);
      if (c == '*') {
        Serial.println(readString);//test response
        if (readString == "crx") {
          cnt_steps = cnt_steps + 1;
          Serial.println(cnt_steps);
          digitalWrite(pinEnable, LOW);
          digitalWrite(dirPinX, HIGH);
          for (int x = 0; x < unit_len; x++) {
            digitalWrite(stepPinX, HIGH);
            delayMicroseconds(500);
            digitalWrite(stepPinX, LOW);
            delayMicroseconds(500);
          }
           
        }
        else if (readString == "clx") {
          cnt_steps += -1;
          Serial.println(cnt_steps);
          digitalWrite(pinEnable, LOW);
          digitalWrite(dirPinX, LOW);
          for (int x = 0; x < unit_len; x++) {
            digitalWrite(stepPinX, HIGH);
            delayMicroseconds(500);
            digitalWrite(stepPinX, LOW);
            delayMicroseconds(500);
          }
        }
         digitalWrite(pinEnable, HIGH);//lock driver on cnc shield    
         c = (char)0;
         Serial.flush();
         readString = "";
         //resetFunc();
      }

      else {
        readString += c; //makes the string readString

      }
    }
  }
}


void loop() {

}

Answer 1

When sending text commands to an Arduino, it is common to define a “line-oriented protocol”: each command is transmitted as a single line, with the line ending signifying the end of the command. Common line endings are carriage return (CR = '\r' = 0x0d = 13), line feed (LF = '\n' = 0x0a = 10) and the sequence CR+LF.

The Arduino serial monitor is designed to support this convention by automatically appending a line ending when you press “Send”. This line ending can be configured to one of the following options:

• No line ending
• Newline (meaning “line feed”)
• Carriage return
• Both NL & CR (but CR is actually sent before LF)

In your case, you have chosen to use '*' as a command termination character. For this to work, you have to choose “No line ending” and add the “*” explicitly to the command. Alternatively, you may consider changing your sketch so that it expects '\r' or '\n', which is a more standard way of ending a command.

Answer 2

In addition to the good responses you have already received, I offer this.

For the Arduino, know the differences between String string and char arrays - there is much on this e.g., https://forum.arduino.cc/t/string-vs-string-vs-char-arrays-correct-approach/537468 . Many do not want to deal with the vagaries of String, but their use can be convenient.

Run this code:

String readstring = "xxx";

void setup() {
  Serial.begin(115200);
}

void loop() {
  if (Serial.available()) {
    readstring = Serial.readStringUntil('/n');
    Serial.print(readstring);
    Serial.println(" received");
    if (readstring.equals("crx")) {
      // move counter clockwise
      Serial.println("moving counter clockwise");
      // counterclockwise() <-- your function
    }
    else if (readstring.equals("clx") ) {
      // move clockwise
      Serial.println("moving clockwise");
      // clockwise() <-- your function
    }
    else {
      // bad command
      Serial.println("Bad command received - no action");
      // badcommand() <-- your function
    }
  }
}

If you add functions for the stepper, it may do what you want within a limited context and with using the Arduino serial monitor for input. Take a look at the output.

Note that the serial monitor is set to "no line ending". The terminating character '/n' is provide by your input in the serial monitor. It will do what you want, but again, many advise you stay away from String.