Overview

• Overview
• Getting Started With the micro:bit Hardware
• Hardware Description
  • nRF51 Application Processor
  • Bluetooth Wireless Communication
  • Low Level Radio Communications
  • Buttons
  • Display
  • Accelerometer
  • Magnetometer
  • Temperature Sensing
  • General Purpose Input/Output Pins
  • Power Supply
  • Interface
  • USB Communications
  • Debugging
  • Mechanical
• Links

Getting Started With the micro:bit Hardware

The micro:bit hardware is based on the ARM-mbed platform. It has an application processor with lots of on-chip perhipherals. Some off-chip peripherals are connected to this chip. There is an interface processor connected to the application processor, and it is the interface processors job to manage communications over the USB and to support the drag-and-drop code-flashing process. The interface processor does not connect to any of the micro:bit peripherals.

Two key pieces of information to help understand the internals of the micro:bit are:

• The schematics, which shows the detailed component data and connectivity of the device.
• The reference design, which is a complete module design of a compatible micro:bit, and is designed to be a starting point for anyone interested in understanding the micro:bit or designing their own variant.

Hardware Description

Nordic nRF52833 Application Processor

The nRF52833 application processor is where user programs run. A single, complete application including user code, runtime code and bluetooth stack is loaded and run directly from on chip flash memory. All user accessible GPIO pins are provided by this processor. There is an onboard 2.4GHz radio engine used to provide Bluetooth capabilities via an off-chip aerial.

Bluetooth Wireless Communication

The on board 2.4GHz transciever supports Bluetooth communications via the Nordic S110 SoftDevice, which provides a fully qualified Bluetooth low energy stack. This allows the micro:bit to communicate with a wide range of Bluetooth devices, including smartphones and tablets.

Low Level Radio Communications

The on board 2.4GHz transciever supports a number of other radio communications standards, including the proprietary Nordic Gazell protocol. This protocol provides a very simple small-packet broadcast radio interface between other devices that support this proprietary protocol, such as other micro:bit devices. The ‘radio’ interface that appears in a number of the languages on the micro:bit is built on top of this Gazell protocol. Additionally, the micro:bit runtime software adds a ‘group code’ to each data payload, allowing for simple user managed device addressing and filtering to take place.

Buttons

The two buttons on the front of the micro:bit, and the 1 button on the back, are tact momentary push to make buttons. The back button is connected to the KL26 interface processor and to the nRF51 processor for system reset purposes. This means that the application will reset regardless of if it is powered from USB or from battery.

Front buttons A and B can be programmed in the user application for any purpose. A and B are debounced by software, which also includes short press, long press, and ‘both A+B’ press detection. Buttons operate in a typical inverted electrical mode, where a pullup resistor ensures a logical ‘1’ when the button is released, and a logical ‘0’ when the button is pressed. Both A and B buttons are connected to GPIO pins that are also accessible on the micro:bit edge connector.

Display

The display is a 5×5 array of LEDs. It is connected to the micro:bit as a 3×9 matrix. Runtime software repeatedly refreshes this matrix at a high speed, such that it is within the user persistence of vision range, and no flicker is detected. This LED matrix is also used to sense ambient light, by repeatedly switching some of the LED drive pins into inputs and sampling the voltage decay time, which is roughly proportional to ambient light levels.

Accelerometer

The accelerometer is a separate chip that provides 3-axis sensing. It also includes some on board gesture detection (such as fall detection) in hardware, and additional gesture sensing (e.g. logo-up, logo-down, shake) via software algorithms. It is connected to the application processor via the I2C bus.

Magnetometer

The magnetometer is a separate chip that provides magnetic field strength sensing. A software algorithm in the standard runtime uses the on board accelerometer to turn these readings into a board orientation independent compass reading. The compass must be calibrated before use, and the calibration process is automatically initiated by the runtime software. This device is connected to the application processor via the I2C bus.

Temperature Sensing

The nRF51 application processor has an on board core temperature sensor. This is exposed via the standard runtime software, and provides an estimate of ambient temperature.

General Purpose Input/Output Pins

The edge connector brings out many of the GPIO circuits of the application processor. Some of these circuits are shared with other functions of the micro:bit, but many of these extra circuits can be re-allocated to general purpose use if some software features are turned off. Note: the nRF51 data sheet states that GPIO pins may be in std-drive (0.5mA) and high-drive (5mA) mode, with a maximum of 3 pins in high-drive mode at any one time.

Power Supply

Power to the micro:bit may be provided via the USB connection, via the interface chip (which has an on-board regulator), or via a battery plugged into the top connector. It is also possible (with care) to power the micro:bit from the 3V pad at the bottom. The 3V pad at the bottom can be used to supply a small amount of power external circuits.

Interface

The interface chip handles the USB connection, and is used for flashing new code to the micro:bit, sending and receiving serial data back and forth to your main computer.

USB Communications

The micro:bit has an on board USB communications stack, that is built into the firmware of the interface chip. This stack provides the ability to drag and drop files onto the MICROBIT drive in order to load code into the application processor. It also allows serial data to be streamed to and from the micro:bit application processor over USB to an external host computer, and supports the CMSIS-DAP protocol for host debugging of application programs.

Debugging

The interface processor can be used with special host tools to debug code that is running on the application processor. It connects to the application processor via 4 signal wires. The KL26 interface processor code can also be debuged via it’s integral SWD software debug interface, for example to load initial bootloader code into this processor at manufacturing time, or to recover a lost bootloader.

Mechanical

We have some nice 2D and 3D CAD drawings and models of the micro:bit including all the important dimensions. These models can be used as a basis for generating really nice marketing and project images of the micro:bit, but also as a basis for accurate manufacture of attachments e.g. via 3D printing.

Links

Schematic

Reference Design

Nordic NRF51 datasheet

Freescale KL26 reference manual

Freescale KL26Z data sheet

Freescale MMA8652 accelerometer datasheet

Freescale MAG3110 magnetometer datasheet

Technical Specifications 1

Technical Specifications 2

Safety Advice

I2C specification

SPI ‘specification’

ARM mbed overview

Fritzing diagram, contributed by Kok Ho Huen