Accessing the REPL

REPL (Read-Evaluate-Print-Loop) allows the micro:bit to read and evaluate code in real-time as you write it.

Using the micro:bit Python Editor

The browser-based Python editor has built-in REPL support, that can be accessed using WebUSB. You can read more about how WebUSB is used in the editors in this article on direct flashing from the browser in the micro:bit apps and editors.

To use WebUSB, you will need a Google Chrome based browser and a micro:bit with firmware at version 0249 or above.

To use the REPL:

  1. Flash a Python program to the micro:bit, if you have not done so already.
  2. Select Open Serial to open the REPL window.
  3. Click the blue bar to Send CTRL-C for REPL or press CTRL + C on your keyboard to enter the REPL.

Using a serial communication program

The Mu Editor has built-in support for REPL and even includes a real-time data plotter. Some other common options are picocom and screen. You will need to install a program and read the appropriate documentation to understand the basics of connecting to a device.

Determining the port

Accessing the REPL on the micro:bit will require you to:

  • Determine the communication port identifier for the micro:bit
  • Use a program to establish communication with the device

The micro:bit will have a port identifier (tty, usb) that can be used by the computer for communicating. Before connecting to the micro:bit we must determine the port identifier.

Windows

When you have installed the aforementioned drivers the micro:bit will appear in device-manager as a COM port.

Mac OS

Open Terminal and type ls /dev/cu.* to see a list of connected serial devices; one of them will look like /dev/cu.usbmodem1422 (the exact number will depend on your computer).

Linux

In terminal, type dmesg | tail which will show which /dev node the micro:bit was assigned (e.g. /dev/ttyUSB0).

Communicating with the micro:bit

Once you have found the port identifier you can use a serial terminal program to communicate with the micro:bit.

Windows

You may wish to use Tera Term, PuTTY, or another program.

In Tera Term:
  • Plug in the micro:bit and open Tera Term
  • Select Serial as the port
  • Go to Setup -> Serial port. Ensure the Port is the correct COM port.
  • Choose a baud rate of 115200, data 8 bits, parity none, stop 1 bit.
In PuTTY:
  • Plug in the micro:bit and open PuTTY
  • Switch the Connection Type to Serial
  • Ensure the Port is the correct COM port
  • Change the baud rate to 115200
  • Select ‘Serial’ on the menu on the left, then click ‘Open’

Mac OS

Open Terminal and type screen /dev/cu.usbmodem1422 115200, replacing /dev/cu.usbmodem1422 with the port you found earlier. This will open the micro:bit’s serial output and show all messages received from the device.

To exit, press Ctrl-A then Ctrl-\ and answer Yes to the question. There are many ways back to a command prompt including Ctrl-A then Ctrl-D, which will detach screen, but the serial port with still be locked, preventing other applications from accessing it. You can then restart screen by typing screen -r.

Linux

Using the screen program, type screen /dev/ttyUSB0 115200, replacing /dev/ttyUSB0 with the port you found earlier.

To exit, press Ctrl-A then \ and answer Yes to the question. There are many ways back to a command prompt including Ctrl-A then Ctrl-D, which will detach screen. All serial output from the micro:bit will still be received by screen, the serial port will be locked, preventing other applications from accessing it. You can restart screen by typing screen -r.

Using picocom, type picocom /dev/ttyACM0 -b 115200, again replacing /dev/ttyACM0 with the port you found earlier.

To exit, press Ctrl-A then Ctrl-Q.