Arduino – How to Get a String from Serial with readString()

In this tutorial I will show you how to use the Arduino readString() function, in order to receive a String (text) from Serial communication.

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Code example with Arduino readString() function

For this example, we will create a very simple application. Here we just want to receive some text (a String), and print it back with Serial. Then, once you know how to do that, you will be able to process the String in your code and do whatever you want.

Here is the code.

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void setup() {
  Serial.begin(9600);
  Serial.setTimeout(10);
}
void loop() {
  if (Serial.available() > 0) {
    String str = Serial.readString();
    str.trim();
    Serial.println(str);
  }
}

Code explained

Let’s break down the code line by line to understand what it does.

Setup – Init Serial and reading timeout

void setup() {
  Serial.begin(9600);

First things first, we start Serial communication in the void setup(). We need that in order to be able to receive any data from Serial.

Here I use 9600 for the baud rate, which is quite common, and pretty low. You could choose something like 115200 for better performance.

  Serial.setTimeout(10);
}

It’s quite important to understand why we need this line. The thing is, when you use the Arduino readString() function, it will block for a duration of time (a timeout) to make sure it has received everything.

The default timeout for reading with readString() is 1000 ms, or 1 second. Yes, one full second, which means that anytime you use readString(), it’s just like if you did a delay(1000), which is definitely not optimal.

Hence, we reduce the Serial reading timeout, to 10 milliseconds in this case. If we go lower we might have some issues with missed characters when we read from Serial.

I suggest you start with 10 ms, and then increase it if you see that you’re missing some characters in your code.

Loop – use readString()

That’s it for the setup, now let’s analyse the code that will run indefinitely on the Arduino loop() function.

void loop() {
  if (Serial.available() > 0) {

First we check if we have received any data from Serial. This line is quite classic and you’ll use that for anything you want to read from Serial (int, String, etc).

Serial.available() will tell how many bytes are ready to be read in the Serial buffer, so basically, if the number is potitive, we know we have some available data.

    String str = Serial.readString();

    str.trim();

    Serial.println(str)
  }
}