BBC micro:bit V2 Battery Voltage Measurement

A micro:bit C++ project showing how to measure the BBC micro:bit V2 battery voltage.

This project measures the input voltage, in millivolts, of the nRF52833 microcontroller. It shows the value on the micro:bit display and also sends it via serial.

It's important to note that the battery voltage is not exactly the same as the microcontroller input voltage, as there is some safety circuitry between the battery and the microcontroller that will produce a small voltage drop.

A prebuilt hex file can be downloaded from the GitHub Releases page.

A similar example for micro:bit V1 can be found in: https://os.mbed.com/teams/microbit/code/microbit-battery-test/

Logging the voltage

If the button A is pressed (keep it pressed until the text stops scrolling and all display LEDs are on), the battery voltage will be logged into the data log.

The voltage is logged once per minute, and when the log is full (it should take over 5 days) a cross will be displayed and the programme will stop running.

The data-log can then be accessed by connecting the micro:bit to a computer and opening the MY_DATA.HTM file inside the MICROBIT USB drive.

How is the battery voltage measured

To measure the micro:bit battery voltage, the nRF52833 (the target MCU) microcontroller ADC is used to measure its Vdd input voltage.

All the ADC channels are currently used for the Analogue pins and the on-board microphone, so for this example we hijack the ADC channel used for P0 and re-route it to the internal Vdd (voltage from the microcontroller power input).

It's important to note that there are two protection diodes and a voltage regulator between the battery connector and the microcontroller Vdd pin, as can be seen in the micro:bit open source schematics.

So, there is a voltage drop on these components that will vary on different factors, like the current load and component temperature. Through some of the micro:bit testing we measured this voltage drop ranging from 210 to 300 mV (measured without any additional load on the micro:bit power supply). However, these values should not be considered definitive, as they may further fluctuate under different conditions.

Building the project

Dependencies

• GNU Arm Embedded Toolchain
• Git
• CMake
• Python 3
• Ninja Build (only needed on Windows)

For example, to install these dependencies on Ubuntu:

sudo apt install gcc
sudo apt install git
sudo apt install cmake
sudo apt install gcc-arm-none-eabi binutils-arm-none-eabi

Building

• Install the dependencies
• Clone this repository

  git clone https://github.com/microbit-foundation/microbit-v2-battery-voltage.git
  

• In the root of this repository run the build.py script

  cd microbit-v2-battery-voltage
  python build.py
  

• The MICROBIT.hex hex file will be built and placed in the root folder.
• Copy the MICROBIT.hex file into the MICORBIT USB drive to flash the micro:bit