Due: Serial.readString() doesn't work

The last few days i have been breaking my virtue on a problem I’m having with the serial library.

I started programming a piece of code where user input through the serial monitor is read in using Serial.readString() on an Arduino Uno. When the code became too long and the Uno’s memory too small I switched to an Arduino Due.
Oddly enough, the code stopped working, without any changes made.

Since then I’ve tried multiple possible solutions without any succes. I’ve listed what I’ve done below:

  • Used Serial.readStringUntil(’\n’); instead
  • Moved the String definition inside the loop()
  • Tried reading the input character per character using a loop function
  • Tested the Serial.readString() function in a piece of example code (Which worked fine) [Example code: see below

My code:

boolean switchState = 0;
boolean previous = 0;
//String spell ="";
char symbol[20];
int i = 0;
int l=0;

void setup() {
  // Define the switch pin
 pinMode(0, INPUT);
 // Set up serial communication to enable word entries
 Serial.println("Power On.");

void loop() {
  // Read Serial data when it's available
  Serial.println("Please enter a valid word to spell: "); //Prompt User for input
  while (Serial.available()==0) {             //Wait for user input
  String spell = Serial.readString();                 //Read user input into spell  
  Serial.print("The word you entered is: "); Serial.print(spell);
  l = spell.length(); 
  //Serial.println(l); //Debug
  spell.toCharArray(symbol,l); //Convert the string to an array of characters.
    Serial.print("Letter");Serial.print(i);Serial.print(": "); Serial.println(symbol[i]); Debug

Example code used:

  String indexOf() and lastIndexOf() functions
 Examples of how to evaluate, look for, and replace characters in a String
 created 27 July 2010
 by Tom Igoe
 This example code is in the public domain. 

void setup() {
  Serial.println("\n\nStream.readString() and readStringUntil()  functions:");

  // Set a long timeout so the user has chance to write things
  // before the calls to read them time out
  Serial.println("You will have up to 15 seconds to type your message before it times out.\n");

void loop() {
  Serial.println("Send a message over serial:");
  String stringOne = Serial.readString();
  Serial.print("Received the string >>");
  Serial.println("Now send a message over serial containing the letter 'g':");
  String stringTwo = Serial.readStringUntil('g');
  Serial.print("Received the string >>");
// do nothing while true:

Does any of you know how I can solve this issue ?

After some more debugging, the program miraculously started working again.
I started from sketch, adding every line of code and testing whether it worked or not.
In the end, the code worked without any significant changes compared to the original.


Hi Fierens,

Did you ever find out exactly why this didn't work? It seems like I am having a similar issue.


No, unfortunately I haven't found the exact cause of the problem.
After building up the program from scratch, and testing every step it eventually worked without a significant difference with the original.

If this shouldn't work, maybe you can post a piece of code ? Maybe I can help.


Thanks for your reply.

I went through my code line by line, and in the end I found out that one part somehow blocked the execution of the code. This part of the code worked on UNO in order to get rid of HTTP header. I have no clue why - but now it works on the DUE (with the HTTP header though).

My original thread: