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Oct 8, 2018
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Motion sensor changes #88

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updated to include all info for previous revision
microbit-mark Oct 8, 2018
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Update hardware/index.md
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291 changes: 289 additions & 2 deletions hardware/1-3-revision.md
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Expand Up @@ -3,7 +3,8 @@ layout: page
order:
title: Previous micro:bit revision
heading: Previous micro:bit revision
description: Image of front and back of micro:bit with individual accelerometer and magnetometer
description: Details of the previous micro:bit revision, containing individual
accelerometer and magnetometers
permalink: /hardware/previous-revision/
ref: 1-3
lang: en
Expand All @@ -12,12 +13,142 @@ review-with: jonnya
---

# Overview

{:notoc}
This page details the hardware details of the previous micro:bit revision, showing the layout of the individual accelerometer and magnetometer components.

* TOC
{:toc}

![img](/docs/hardware/assets/microbit-overview.png)


# 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](./schematic), which shows the detailed component data and connectivity of the device.

* The [reference design](./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

## nRF51 Application Processor

The nRF51 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.

| item | details
| --- | ---
| Model | [Nordic nRF51822-QFAA-R rev 3](https://www.nordicsemi.com/eng/Products/Bluetooth-low-energy/nRF51822)
| Core variant | [ARM Cortex-M0 32 bit processor](https://www.arm.com/products/processors/cortex-m/cortex-m0.php)
| Flash ROM | 256KB
| RAM | 16KB
| Speed | 16MHz
| Debug | SWD, jlink/OB
| More Info | [here](../software)


## 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.

| item | details
| --- | ---
| Stack | Bluetooth 4.1 with Bluetooth low energy
| Band | 2.4GHz ISM (Industrial, Scientific and Medical) 2.4GHz..2.41GHz
| Channels | 50 2MHz channels, only 40 used (0 to 39), 3 advertising channels (37,38,39)
| Sensitivity | -93dBm in Bluetooth low energy mode
| Tx Power | -20dBM to 4dBm in 4 dB steps
| Role | [GAP Peripheral](http://bluetooth-mdw.blogspot.co.uk/2016/07/microbit-and-bluetooth-roles.html)
| Congestion avoidance | Adaptive Frequency Hopping
| Profiles | 1 [BBC micro:bit profile](https://lancaster-university.github.io/microbit-docs/ble/profile/)
| More Info | [here](../bluetooth)


## 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.

| item | details
| --- | ---
| Protocol | [Nordic Gazell](https://devzone.nordicsemi.com/documentation/nrf51/4.3.0/html/group__gzll__02__user__guide.html)
| Freq band | 2.4GHz
| Channel rate | 1Mbps or 2Mbps
| Encryption | None
| Channels | 101 (0..100)
| Group codes | 255
| Tx power | Eight user configurable settings from 0(-30dbm) to 7 (+4dbm)
| Payload size | 32 (standard) 255 (if reconfigured)
| More Info | [here](https://lancaster-university.github.io/microbit-docs/ubit/radio/)


## 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.

| item | details
| --- | ---
| Type | 2 tactile user buttons, 1 tactile system button
| Debounce | (A & B) software debounced, 54ms period
| Pullup | (A & B) external 4K7, (System) 10K


## Display

The display is a 5x5 array of LEDs.
It is connected to the micro:bit as a 3x9 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.

| item | details
| --- | ---
| Type | minature surface mount red LED
| Physical structure | 5x5 matrix
| Electrical structure | 3x9
| Intensity control | 10 steps
| Intensity range | TBC
| Sensing | ambient light estimation via software algorithm
| Sensing Range | TBC, 10 levels from off to full on
| Colour sensitivity | red centric, red is 700nm

## Accelerometer

Expand Down Expand Up @@ -51,3 +182,159 @@ This device is connected to the application processor via the I2C bus.
| Max update rate | 80Hz
| Full Scale range | 1000uT
| Sensitivity | 0.10uT


## 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.

| item | details
| --- | ---
| Type | on-core nRF51
| Sensing range | -25C .. 75C
| Resolution | 0.25C steps
| Accuracy | +/-4C (uncalibrated)
| More Info | [here](https://lancaster-university.github.io/microbit-docs/ubit/thermometer/)

## 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.

| item | details
| --- | ---
| Rings | 3 large IO rings and two large power rings, 4mm plug and crocodile clip compatible
| GPIO features | 19 assignable GPIO pins
|| 2 are assigned to the on board I2C interface
|| 6 are used for display or light sensing feature
|| 2 are used for on board button detection
|| 1 is reserved for an accessibility interface
|| 19 may be assigned as digital input or digital output
|| 19 may be assigned for up to 3 simultaneous PWM channels
|| 19 may be assigned for 1 serial transmit and 1 serial receive channel
|| 6 may be assigned as analog input pins
|| 3 may be assigned to an optional SPI communications interface
|| 3 may be assigned for up to 3 simultaneous touch sensing inputs
|ADC resolution | 10 bit (0..1023)
| Edge Connector| [Edge connector data sheet](/hardware/edgeconnector_ds/)
| Pitch | 1.27mm, 80 way double sided.
| Pads| 5 pads, with 4mm holes
| More Info | [here](/hardware/edgeconnector/)


## 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.

| item | details
| --- | ---
| Operating range | 1.8V .. 3.6V
| USB current | 120mA max
| Onboard Peripherals budget | 30mA
| Battery connector | JST X2B-PH-SM4-TB
| Battery current | TBC
| Max current provided via edge connector | 90mA
| More Info | [here](./powersupply)


## 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.

| item | details
| --- | ---
| Model |[Freescale MKL26Z128VFM4](http://www.nxp.com/webapp/search.partparamdetail.framework?PART_NUMBER=MKL26Z128VFM4)
| Core variant: | [ARM Cortex-M0+](https://www.arm.com/products/processors/cortex-m/cortex-m0plus.php)
| Flash ROM | 128KB
| RAM | 16KB
| Speed | 16MHz
| Debug capabilities | SWD
| More Info | [here](/software/daplink-interface/)


## 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.

| item | details
| --- | ---
| Connector | USB micro, MOLEX_47346-0001
| USB version | 1.1 Full Speed device
| Speed | 12Mbit/sec
| USB classes supported | [Mass Storage Class (MSC)](https://en.wikipedia.org/wiki/USB_mass_storage_device_class)
| | [Communications Device Class (CDC)](https://en.wikipedia.org/wiki/USB_communications_device_class)
| More Info | [here](/software/daplink-interface/)


## 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.

| item | details
| --- | ---
| Protocol | CMSIS-DAP
| Options | JLink/OB (via different firmware)
| More Info | [here](https://docs.mbed.com/docs/mbed-os-handbook/en/latest/debugging/debugging_microbit/)

## Mechanical

We have some [nice 2D and 3D CAD drawings and models of the micro:bit](https://github.com/microbit-foundation/microbit-reference-design) 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.

| item | details
| --- | ---
| Dimensions | 5cm(w) 4cm(h)
| Weight | 5g


# Links

[Schematic](./schematic)

[Reference Design](./reference-design)

[Nordic NRF51 datasheet](http://infocenter.nordicsemi.com/pdf/nRF51822_PS_v3.1.pdf)

[Freescale KL26 reference manual](http://cache.freescale.com/files/microcontrollers/doc/ref_manual/KL26P121M48SF4RM.pdf)

[Freescale KL26Z data sheet](http://www.nxp.com/docs/pcn_attachments/16440_KL26P64M48SF5_Rev.4.pdf)

[Freescale MMA8652 accelerometer datasheet](http://cache.freescale.com/files/sensors/doc/data_sheet/MMA8652FC.pdf)

[Freescale MAG3110 magnetometer datasheet](http://cache.freescale.com/files/sensors/doc/data_sheet/MAG3110.pdf)

[Technical Specifications 1](http://www.bbc.co.uk/mediacentre/mediapacks/microbit/specs)

[Technical Specifications 2](http://www.bbc.co.uk/mediacentre/mediapacks/microbit/thebbcmicrobit)

[Safety Advice](https://www.microbit.co.uk/safety-advice)

[I2C specification](http://cache.nxp.com/documents/user_manual/UM10204.pdf)

[SPI 'specification'](https://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus)

[ARM mbed overview](https://www.mbed.com/en/about-mbed/what-mbed/)

[Fritzing diagram, contributed by Kok Ho Huen](/docs/hardware/assets/Microbit.fzpz.zip)
7 changes: 0 additions & 7 deletions hardware/index.md
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![img](/docs/hardware/assets/microbit-overview-1-5.png)


See [details of the previous micro:bit revision](/hardware/previous-revision)


# Getting Started With the micro:bit Hardware

The micro:bit hardware is based on the ARM-mbed platform.
Expand Down Expand Up @@ -306,10 +303,6 @@ manufacture of attachments e.g. via 3D printing.

[Freescale KL26Z data sheet](http://www.nxp.com/docs/pcn_attachments/16440_KL26P64M48SF5_Rev.4.pdf)

[Freescale MMA8652 accelerometer datasheet](http://cache.freescale.com/files/sensors/doc/data_sheet/MMA8652FC.pdf)

[Freescale MAG3110 magnetometer datasheet](http://cache.freescale.com/files/sensors/doc/data_sheet/MAG3110.pdf)

[Technical Specifications 1](http://www.bbc.co.uk/mediacentre/mediapacks/microbit/specs)

[Technical Specifications 2](http://www.bbc.co.uk/mediacentre/mediapacks/microbit/thebbcmicrobit)
Expand Down