Pocket-Size Programming

The idea of developing a microcontroller for schools dates back to 2012. In 2016, in cooperation with the University of Lancaster and several dozen industry partners, the British Broadcasting Corporation (BBC) delivered on this concept with the BBC micro:bit [1]. Although developed for seventh grade students, the BBC micro:bit offers an introduction to microcontroller programing for users of any age.

You can purchase the micro:bit individually for $14.95 or as the micro:bit Go Bundle (which includes batteries, a battery holder, and USB cable) for $17.50 from Adafruit [2].

The microcontroller and its components fit on a 5x4cm board (Figure 1). The 32-bit processor, an ARM Cortex-M0, runs at a clock speed of 16MHz. The micro:bit offers 16KB of RAM and a 256KB flash memory. In principle, a Bluetooth Low Energy (BLE) radio is also available. See Table 1 for more specifications.

Figure 1: The micro:bit with connecting cables, a breadboard, and components.

If you connect a micro:bit to a Raspberry Pi via a micro-USB cable, the Raspberry Pi will identify the micro:bit as a USB stick with a size of 64MB. Using the lsusb command will reveal that the micro:bit's USB interface component is an LPC1768 chip from NXP. Listing 1 reveals that the micro:bit has a device name of ttyACM3.

$ lsusb
[...]
Bus 002 Device 019: ID 0d28:0204 NXP LPC1768
[...]
$ dmesg | grep tty
[...]
[17860.723466] cdc_acm 2-1.2.1:1.1: ttyACM3: USB ACM device

The micro:bit can be powered via USB or battery, using the battery holder (shown in Figure 1 on the left) with two AAA zinc or alkaline batteries plugged into the top connector. Optionally, you can use the 3V pad at the bottom, but do so with caution. The micro:bit's voltage range is 1.8-3.6V.

The memory-hungry Bluetooth module cannot be addressed as a standard BLE in MicroPython, but a radio connection between modules should at least work.

MicroPython

While the micro:bit understands various programming languages, this article focuses on MicroPython, the version of Python optimized for running on microprocessors.

If you want to program microcontrollers on register level, you won't get far with MicroPython, but then neither is the micro:bit the tool for this task. Instead, an Arduino [3] would be the better choice, which, unlike the micro:bit, offers circuit diagrams and board layouts.

The micro:bit's operating system constantly monitors the USB memory. If it detects a hex file in USB memory, the system transfers it to the internal memory as a byte sequence and executes it.

MicroPython comes with a very simple filesystem that allows programs to write files to the microcontroller and read them from there. However, even attaching data to existing files is too much for the system. The size limit is 30KB. Since the files are stored in internal memory, they can only be addressed by the running program, but not via the USB interface.

Mu

To program the micro:bit, you use the Mu editor [4], a simple Python editor. Mu recognizes the micro:bit at startup if it is connected to a computer via a USB cable. Mu will prompt you to select the BBC micro:bit mode.

Due to the integrated libraries, the resulting programs are only executable on the micro:bit. This means that run-time errors can only be analyzed and output on the micro:bit. As soon as you press the Check button, the Mu editor searches for syntax errors. For example, in Listing 2, the word hello in the second line is syntactically incorrect. The solution would be to comment out the line or quote the string (Figure 2).

import microbit
print(hello)
print(1/0)

Figure 2: A MicroPython program with a run-time error message in the Mu editor.

Pressing the Flash button converts the program to hex code and transfers it to the micro:bit where it is executed immediately. You can restart the program via the reset button, which is just like switching on the micro:bit again by applying the supply voltage.

The code from Listing 2 terminates with a run-time error message in the Mu editor:

division by zero

The micro:bit outputs the error code as a ticker on its 5x5 LED matrix. You can look up the error code in the Mu editor by pressing the REPL (which stands for Read-Evaluate-Print-Loop) button (Figure 2).

A Simple Project

Listing 3 shows the code for a prank that delivers a high-pitched squeak when it's dark; if you turn on the light to find the sound source, the system remains silent.

01 import microbit
02 import music
03 import random
04
05 BrightTrig = 50
06
07 while True:
08   br = microbit.display.read_light_level()
09   if br < BrightTrig:
10     microbit.sleep(800 * (1+random.randrange(10)))
11     music.pitch(900, 80 * (1+random.randrange(5)))

Lines 1-3 import the required Python libraries. The micro:bit not only makes its LEDs light up, it can also use them like photodiodes. Depending on the brightness, the microbit.display.read_light_level() command returns a value between   and 255. At a value of 50, it is already quite dark (line 5).

The continuous loop introduced by while True stores the measured brightness in the variable br. If it is greater than the threshold value BrightTrig, nothing happens. Otherwise, the program calls microbit.sleep (a wait command) that lasts at least 800 milliseconds, depending on the random number random.randrange(10).

The command music.pitch generates a square wave signal of 900Hz at output  . Here too, a random generator controls the duration.

By connecting a small loudspeaker to the micro:bit contacts 0 and GND, the prank begins. Crocodile clips connect a jack plug to an amplifier's line-in input (Figure 3). The micro:bit's power is just enough to connect a ceramic speaker (the small black cylinder shown in Figure 1). You can also use a buzzer (i.e., a ceramic loudspeaker with a built-in tone generator). Then you just need to switch the output on and off again instead of modulating it at 900Hz.

Figure 3: Crocodile clips can be used to connect a jack plug to the BBC micro:bit.