Serial Library

The serial library supports serial communication between the BBC micro:bit and another computer. Basically, this allows you to send data from the micro:bit to your own computer. This is very useful for debugging purposes: you can add write line statements in your code and see them display on your computer as the program executes.
The code below shows a simple script that sends a line when the BBC micro:bit starts and another line each time the button A is pressed.
serial → write line("started...")
input → on button pressed(A) do
serial → write line("A pressed")
end function

How to read the micro:bit's serial output from your computer

Unfortunately, using the serial library requires quite a bit of a setup.


You must install a device driver (for the computer to recognize the serial interface of the micro:bit); then, you must also install a terminal emulator (which is going to connect to the micro:bit and read its output). Here's how to do it:
Once both the driver and the terminal emulator are installed, plug in the micro:bit and wait until the device is fully setup. Then, open TeraTerm.
  • Hit File > New Connection
  • Check "Serial"; in the dropdown menu, pick the COM port that says "mbed Serial Port". Hit Ok.
  • In the menus, hit Setup > Serial Port and set the baud rate to 115200.
You should be good. Feel free to hit Setup > Save Setup in the menus to erase the default configuration file with a new one so that you don't have to type in the settings again.
Please note that Windows will assign you a different COM port if you plug in another micro:bit. If you're juggling between micro:bits, you'll have to change the COM port every time.

Alternative Windows setup with Putty

If you prefer another terminal emulator (such as PuTTY), here are some instructions.
  • Open Windows's Device Manager; expand the section called "Ports (COM & LPT)"; write down the com number for "mbed Serial Port" (e.g. COM14)
  • Open PuTTY; on the main screen, use the following settings: Serial / COM14 / 115200. Replace COM14 with the COM port number you wrote down previously. Feel free to type in a name and hit "Save" to remember this configuration.
  • (optional): in the "Terminal" section, check "implicit cr in every lf"


  • Plug in the micro:bit
  • Open a terminal
  • dmesg | tail will show you which /dev/ node the micro:bit was assigned (e.g. /dev/ttyUSB0)
  • Then, do: screen /dev/ttyUSB0 115200 (install the screen program if you don't have it). To exit, run Ctrl-A Ctrl-D.
Alternative programs include minicom, etc.

Mac OS

  • Plug in the micro:bit
  • Open a terminal
  • ls /dev/cu.* will return to you a list of serial devices; one of them will look like /dev/cu.usbmodem1422 (the exact number depends on your computer)
  • screen /dev/cu.usbmodem1422 115200 will open up the micro:bit's serial output. To exit, hit Ctrl-A Ctrl-D.

Using the serial library in your programs

If the serial button is not available, you will need first to add the micro:bit serial library to your script:
  • tap on add new
  • tap on library
  • select micro:bit serial

Writing data

This is basically what you will use the serial library for: debugging purposes.
  • write a number
serial → write number(42)
  • write a string
serial → write string("hello")
  • write a line of text
serial → write line("this is a line")
Theoretically, you can dump more sophisticated data and then read it back in the event that two micro:bits should be connected to each other over serial. We have not tested this scenario yet as we have yet to expose functionality that allows re-routing the serial ports to the edge connector.
  • write an image
var img := image → create image()
serial → write image(img)
  • write the current screen LED status
serial → write screen

Reading data

This is useful if you have something connected at the other end. As explained above, this is not yet a scenario.
  • reads a line of text
var msg := serial → read string
  • reads an image
img := serial → read image
  • reads the state of the screen from serial
serial → read screen