Arduino read string from Serial

Question

#include <stdio.h>

#define LED 13

void setup() {
  pinMode(LED, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  int i;
  char command[5];
  for (i = 0; i < 4; i++) {
    command[i] = Serial.read();
  }
  command[4] = '\0';

  Serial.println(command);

  if (strcmp(command, "AAAA") == 0) {
    digitalWrite(LED, HIGH);
    Serial.println("LED13 is ON");
  } else if (strcmp(command, "BBBB") == 0) {
    digitalWrite(LED, LOW);
    Serial.println("LED13 is OFF");
  }
}

I am trying to read a 4 characters long string with Arduino's Serial, and when it is AAAA turn on a LED, when it is BBBB turn off the serial.

However, when I enter "AAAA" it reads "AAAÿ" with lots of "ÿ"'s along the way.

I think I'm reading everything correctly, but it's not working so well, any idea of what I'm doing wrong?

Answer 1

String txtMsg = "";  
char s;

void loop() {
    while (serial.available() > 0) {
        s=(char)serial.read();
        if (s == '\n') {
            if(txtMsg=="HIGH") {  digitalWrite(13, HIGH);  }
            if(txtMsg=="LOW")  {  digitalWrite(13, LOW);   }
            // Serial.println(txtMsg); 
            txtMsg = "";  
        } else {  
            txtMsg +=s; 
        }
    }
}

Answer 2

You should check is there is something available to read. If not, then the read() will return -1. You can use Serial.available() to check the read buffer.

Answer 3

#define numberOfBytes 4
char command[numberOfBytes];

    void serialRX() {
      while (Serial.available() > numberOfBytes) {
        if (Serial.read() == 0x00) { //send a 0 before your string as a start byte
          for (byte i=0; i<numberOfBytes; i++)
            command[i] = Serial.read();
        }
      }
    }

Answer 4

It reads 'ÿ' because there is no char to read in the buffer. It takes some time for the others characters to unstack from the uart buffer. So, you cannot do a loop to read chars. You have to wait that another character is available before reading it.

Also, this way of waiting characters is not the best way because it blocks the main loop.

Here is what I do in my programs:

String command;

void loop()
{
    if(readCommand())
    {
        parseCommand();
        Serial.println(command);
        command = "";
    }
}

void parseCommand()
{
  //Parse command here
}

int readCommand() {
    char c;
    if(Serial.available() > 0)
    {
        c = Serial.read();
        if(c != '\n')
        {       
            command += c;
            return false;
        }
        else
            return true;

    }
}