Arduino not reacting sometimes when reading serial commands

Question

When I am sending hex numbers via computer to the arduino leonardo, it sometimes won't react, but the serial monitor says that it was sent. My code;

    void setup()
{
  Serial.begin(9600);
  pinMode(9, OUTPUT);
  pinMode(A0, OUTPUT);
  pinMode(10, OUTPUT);
  pinMode(11, OUTPUT);
}

void loop()
{
  delay(100); // Wait for 100 millisecond(s)
  Serial.println(Serial.read());
  if (Serial.read() == 49) {
    Serial.println("left");
    analogWrite(9, 1023);
    digitalWrite(A0, HIGH);
    analogWrite(10, 1023);
    digitalWrite(11, LOW);
  }
  if (Serial.read() == 50) {
    Serial.println("right");
    analogWrite(9, 1023);
    digitalWrite(A0, HIGH);
    analogWrite(10, 1023);
    digitalWrite(11, LOW);
  }
  if (Serial.read() == 51) {
    Serial.println("backwards");
    analogWrite(9, 1023);
    digitalWrite(A0, LOW);
    analogWrite(10, 1023);
    digitalWrite(11, LOW);
  }
  if (Serial.read() == 52) {
    Serial.println("forwards");
    analogWrite(9, 1023);
    digitalWrite(A0, HIGH);
    analogWrite(10, 1023);
    digitalWrite(11, HIGH);
  }
  if (Serial.read() == 48) {
    Serial.println("stop");
    analogWrite(9, 0);
    digitalWrite(A0, LOW);
    analogWrite(10, 0);
    digitalWrite(11, LOW);
  }
}

When I send the numbers 1,2,3,4, and 0, it reads what the command does. Pins A0 and D11 are direction control for the motor controller, and pins D9 and D10 are speed.

the serial monitor prints a different byte from the one that controls the motors — jsotola
– jsotola, Commented Nov 2, 2023 at 4:01

Nick Gammon · Accepted Answer · 2023-11-03 06:53:58Z

3

Serial.println(Serial.read());
if (Serial.read() == 49) {

You are doing two reads here, not one. So you are reading and displaying, and then reading a second byte (not the one you displayed).

Much better would be:

int cmd = Serial.read();
Serial.println(cmd);
if (cmd == 49)  {
  ... do something ...
} else if (cmd == 50)  {
  .. do something else ...
}

And so on.

Also, why use numbers when you can show the ASCII characters? eg.

int cmd = Serial.read();
Serial.println(cmd);
if (cmd == '1')  {
  ... do something ...
} else if (cmd == '2')  {
  .. do something else ...
}

You may also want to consider using:

if (!Serial.available())
    return;

That way you don't test for serial data that does not exist.

Why the delay?

edited Nov 3, 2023 at 6:53

answered Nov 2, 2023 at 6:22

Nick Gammon♦

38.9k13 gold badges70 silver badges126 bronze badges

When I made it delay, it seemed to work better.

Kyle_3_1415
– Kyle_3_1415

2023-11-03 15:36:11 +00:00
Commented Nov 3, 2023 at 15:36

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