So, my main goal is to make a stopwatch with Tkinter, in which time is stoppped with an external pushbutton

My void loop function in Arduino Uno is

void loop() {

if(digitalRead(BUTTON) == HIGH){
digitalWrite(LED, HIGH);


 if(digitalRead(BUTTON) == HIGH){

if(digitalRead(BUTTON) == LOW){



Basically, whenever the pushbutton is pressed I send via serial the message "E\n" and the arduino enters in an infinte loop that only breaks when the pushbutton stops being pressed (to avoid sending more than one message).

Then I have my Python code that reads the serial through the serial.readline() function.

Part of my Python code is (remember that i'm making a stopwatch and time is appearing in the screen - variable timer[i] is what saves the second and the ms):

def update_timeText():
    if (state):
       global timer
       # Every time this function is called, 
       # it increments 1/100 of a second
       timer[1] += 1

      # Every 100 centisecond is equal to 1 second
       if (timer[1] >= 100):
           timer[1] = 0
           timer[0] += 1

       if arduino.readline() == 'E\n':

       # TIME STRING
       timeString = pattern.format(timer[0], timer[1])


   # Call the update_timeText() function after 1 centisecond
   root.after(10, update_timeText)

The variable state goes True whenever I start the stopwatch (via a Tkinter button, but that's irrelevant for this case).

My problem, is that the program stops in the line

if arduino.readline() == 'E\n':

i.e., it only continues if I press the pushbutton. What I think that is happening, is that the .readline() continues waiting and waiting until it receives the message 'E\n'... I've tried with the .readline.strip() and also only .read() but none of them worked.

It is though very strange, because if the condition in front of If is not verified, it should just continue...

Any suggestion?

  • It looks like your 'readline()` method blocks. You need a non-blocking version. It's a Python question, not an Arduino question, but if you edit into your question the Python library you are using (what is your arduino object?) I'll try to help.
    – Mark Smith
    Commented Feb 13, 2017 at 21:39

1 Answer 1


In the pyserial docs it states this:

Be careful when using readline(). Do specify a timeout when opening the serial port otherwise it could block forever if no newline character is received. Also note that readlines() only works with a timeout. readlines() depends on having a timeout and interprets that as EOF (end of file). It raises an exception if the port is not opened correctly.

You need to specify the timeout and the end of line string. Try using this modified line:

if arduino.readline(eol='\n') == 'E\n':

Also, add the timeout option where you define arduino as a pyserial object. For example:

arduino = pyserial.serial(port,baud,timeout=t,etc...)

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