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Power Management Specification

Specification for the micro:bit Power Management

Power Management Specification

This is version 1.0.3 of the specification.

Table of Contents

Introduction

The micro:bit contains two microcontrollers, the Interface MCU (KL27 in micro:bit V2.00 or nRF52 in micro:bit V2.2x) which provides the USB functionality, and the Target MCU (nRF52 in all micro:bit V2 versions) where the user code runs. More information can be found in the Tech Site DAPLink page.

In micro:bit V1 the Interface MCU (KL26) is not powered via batteries or the Edge Connector, so the sleep functionality is only implemented in the Target MCU (nRF51). The micro:bit V2 powers both MCUs with all power sources, so to set the board into a sleep mode some co-operation via the I2C protocol is needed.

The micro:bit V2 has four power modes described in this document:

Terminology

Term Definition
MCU Microcontroller
Interface MCU The microcontroller providing USB (DAPLink) functionality
Target MCU The microcontroller where the user code runs
Sleep A sleep mode for the individual MCUs (Interface or Target) where they can wake up and continue operation
Power Down The lowest power state mode for individual MCUs (Interface or Target)
DAPLink Interface firmware providing USB and programming capabilities
PC Connected When DAPLink on the Interface MCU is USB enumerated and/or has USB activity with a PC
On-board components Motion sensors, speaker, microphone
I2C (Inter-Integrated Circuit bus
VBUS_ABSENT Signal connected to the Interface MCU indicate USB voltage presence (previously named WAKE_ON_EDGE)
COMBINED_SENSOR_INT Interrupt signal shared between all the internal I2C devices in the micro:bit board

micro:bit Power Modes

We want to define 4 user-facing power modes for the micro:bit board. These board-level modes will be achieved via a combination of different subsystem power modes.

  • On Mode: Normal running mode.
    • As far as the user is concerned, everything is running
    • The software in the Target MCU and Interface MCU can decide to go into Sleep if they are idle
      • If the Interface MCU is not PC Connected and is idle it must go into Sleep
        • When the Interface MCU is in Sleep it must automatically wake up when it receives I2C transactions from the Target MCU
      • If the Target MCU is idle it can automatically go into Sleep
        • When the Target MCU is in Sleep it must to wake up on the ticker to run background operations
        • When the Target MCU is in Sleep it must not drop any input data (buses, radio, gpio, etc)
      • When the Interface MCU or the Target MCU wake up they must resume where they left off
    • All events and background tasks must to be running during this mode
    • The Power (red) LED must be ON
    • If the board is PC connected the USB (orange) LED must be ON
    • If the board is not PC connected the USB (orange) LED must be OFF
  • Deep Sleep Mode: A mode triggered by the user code to go into a low-power state
    • The Interface MCU does not do anything different to the On Mode
    • The Target MCU must turn off the on-board components
    • The Target MCU must go into Sleep
    • When the Target MCU wakes up it must restore the on-board components to run again
    • When the Target MCU wakes up its software must resume where it left off
    • During the time the board is in Deep Sleep mode, it will not react to anything until it wakes up
      • Any data received via radio or other buses will be ignored and dropped
    • Entered via:
      • User code calling a CODAL uBit method
    • Woken up via:
      • Timer on the Target MCU
        • The user can configure the elapsed time to wake-up
      • Any pin in Target MCU
        • Edge connector pin
        • A or B buttons
        • The Interface MCU can wake up the Target MCU via COMBINED_SENSOR_INT
        • USB cable insertion (VBUS_ABSENT signal)
      • Pressing the reset button
        • The Interface MCU must halt the Target MCU while the button is pressed down
        • The Interface MCU must reset the Target MCU when the reset button is released
    • If the board is PC Connected the USB (orange) and Power (red) LEDs must be ON
    • If the board is not PC Connected the USB (orange) and Power (red) LEDs must be OFF
  • Off Mode: Lowest possible power state for all components on the board
    • The board can be considered “off”, although it will still consume power
    • This mode can only be reached if the micro:bit is powered via battery or USB bank (not USB Connected)
    • The Target MCU must turn off the on-board components
    • The Target MCU must go into Power Down
    • The Interface MCU must go into Power Down
    • The board will not react to anything except the wake up sources
    • Both microcontrollers must reset on wake
    • Entered via:
      • Long press of the reset button
      • User code calling a CODAL uBit method
    • Woken up via:
      • USB cable insertion (VBUS_ABSENT signal)
      • Pressing the reset button
    • The Power (red) LED must be OFF
    • The USB (orange) LED must be OFF
  • Stand-by Mode: User long presses the reset button, while the micro:bit is PC Connected, to stop their Target MCU programme
    • This is the same as the Off Mode, but this mode is activated when the micro:bit is PC Connected
    • The Interface MCU must not go into any sleep in this case, the rest behaves the same
    • The Power (red) LED must be blinking
    • The USB (orange) LED must be ON

Interface MCU Power Modes

There is a large selection of sleep modes available in the Interface MCU KL27 microcontroller, two have been selected for the micro:bit requirements.

1) Power Down: VLLS0, Very Low-Leakage Stop 0 - Lowest power mode available in the KL27 - Wakes up in reset mode - Woken up via LLWU pins: - VBUS_ABSENT triggered by inserting the USB cable - BTN_NOT_PRESSED triggered by pressing the reset button 2) Sleep: VLPS0, Very Low Power Stop 0 - The deepest sleep mode that can be woken up via I2C - On wake up it resumes where it left off - Can wake up from same sources as Deep Sleep, plus: - I2C transactions

Other notes:

  • The Interface MCU must not enter either of these two modes when it is PC Connected
    • As both modes are not PC connected, the Interface MCU won’t respond to any UART activity during Sleep or Power Down
  • The Interface MCU must automatically go into Sleep mode when it’s idle
    • This power mode is managed exclusively by the Interface MCU
  • The Interface MCU must go into Power Down mode when requested by the Target MCU via I2C
    • If the Interface MCU is PC Connected it must ignore the request to go into Power Down

LED behaviour

  • In both modes the USB (orange) LED must be OFF
    • This is because this LED is only ON when the board is PC connected
  • In both modes the Power (red) LED state will depend on a setting configured by the Target MCU via I2C
    • When the Power LED Sleep state setting is ON, the Interface MCU must set the Power (red) LED ON when it goes into Sleep or Power Down mode
      • In Sleep mode the Interface MCU is able to keep the LED brightness PWM controlled, so the LED must be dimmed
      • In Power Down mode the Interface MCU can only configure the pin high or low, so the LED will be in full brightness
    • When the Power LED Sleep state setting is OFF, the Interface MCU must turn OFF the Power (red) LED when it goes into Sleep or Power Down mode
    • On Interface MCU reset, the default Power LED Sleep state setting value must be set to ON
    • When the reset button is pressed, the Interface MCU must reset the Power LED Sleep state value to the default ON
    • When the Interface MCU wakes up the Target MCU, the Interface MCU must reset the Power LED Sleep state value to the default ON
    • When the Target MCU goes into Sleep or Power Down mode it must set the Power LED Sleep state setting to OFF
    • When the Target MCU wakes up from Sleep mode it must set the Power LED Sleep state setting to ON

Waking Up The Interface MCU

Waking up via Target MCU:

  • The Target MCU does not know the Interface MCU current power mode, but it needs the Interface MCU to always be responsive to I2C comms
  • To wake up the Interface MCU the Target MCU can send an I2C message
    • When the Interface MCU for V2.00 (KL27) is woken up via I2C, it is unable to process the data from the transaction that woke it (e8777 errata)
    • Errata e8777: Address match wake-up from low-power mode cannot receive data
      • https://www.nxp.com/docs/en/errata/KINETIS_L_1N71K.pdf
      • So the data in the I2C transmission will be lost if it wakes up the Interface MCU
    • The Target MCU could use this workaround from e8777 errata: Send only the matching secondary address followed by a repeated start and then resend the matching secondary address including any subsequent data
      • At the time of writing the nrfx drivers used in CODAL makes this very difficult, so instead the wake up workaround is to start all I2C transactions with a “nop” I2C command that does nothing, so it doesn’t matter if its lost and if it is received the Interface MCU will ignore it

Waking up from VBUS_ABSENT signal:

  • A USB cable insertion will set the VBUS_ABSENT signal active (low)
  • When the VBUS_ABSENT transitions from inactive to active the Interface MCU must wake up from any sleep mode

Waking up from the reset button:

  • Pressing the reset button sets the BTN_RST signal active (low)
  • When the BTN_RST signal transitions to active the Interface MCU must wake up from any sleep mode
  • The Interface MCU must halt the Target MCU while the button is pressed down
  • When the Interface MCU wakes up from Power Down it must start from a reset state
  • When the Interface MCU wakes up from Power Down the DAPLink bootloader must not enter MAINTENANCE mode
  • When the Interface MCU wakes up from Sleep the DAPLink interface must continue operation where it left off
  • When the button is released the Interface MCU must reset the Target MCU

Setting up the COMBINED_SENSOR_INT signal:

  • When the Interface MCU wakes up from any mode it must set the COMBINED_SENSOR_INT signal active
    • This signal is used to wake up the Target MCU
    • This signal is open drain and active low
      • To set the signal activate the Interface MCU must pull down its pin connected to this signal
      • To set the signal inactive the Interface MCU must set the pin to high impedance
  • If the signal was already in active state the Interface MCU will pulse the signal
  • The Interface MCU must be ready for the Target MCU to query via I2C the wake up source
    • If the Target MCU is in Power Down it might take a considerable amount of time for CODAL to start-up and service the Interface MCU interrupt
    • More information about the I2C protocol can be found in the I2C Protocol Spec
  • When the wake up source has been read by the Target MCU, the Interface MCU must release the COMBINED_SENSOR_INT to the inactive state

Target MCU Power Modes

The nRF52833 just has two power modes (section 5.3 of the datasheet): System Off, where everything is off; and System On, where everything is running but individual components can be configured into a low power state.

1) Power Down: System Off - Lowest power mode available in the nRF52 - The CPU stops and all peripherals are disabled - Wakes up in reset mode - Woken up via: - COMBINED_SENSOR_INT - SWD 2) Sleep: System On with the CPU in low power mode - The System On mode is the normal running mode, but we can configure components, like the CPU, in low power modes - On wake up it resumes where it left of - Can wake up from the same sources as Power Down, plus: - Internal timer - Microphone - Motion sensor events - Any user accessible nRF52 pin: - Edge connector pins - Face logo - A and B buttons

Other notes:

  • The Target MCU will enter Sleep when:
    • Software is idle
    • User code calls a CODAL uBit method to sleep
  • During Sleep the Target MCU must still be responsive to events, incoming data and run background tasks
  • The Target MCU will enter Power Down when:
    • The Interface MCU indicates the user has long-pressed the reset button
    • User code calls a CODAL uBit method to go into the board Off mode

Waking Up The Target MCU

  • When the COMBINED_SENSOR_INT signal transitions from inactive to active the Target MCU must wake up from Sleep or Power Down
  • When the Target MCU wakes up it must ensure the on-board components are enabled and running

Wake up from the reset button:

  • The reset button is only connected to the Interface MCU
  • The Interface MCU will set the COMBINED_SENSOR_INT signal active
  • The specific Interface MCU behaviour when the reset button is pressed can be found in the Waking Up The Interface MCU section

Wake up from USB cable insertion:

  • The USB cable insertion signal is only connected to the Interface MCU
  • The Interface MCU will set the COMBINED_SENSOR_INT active
  • The specific Interface MCU behaviour when the USB cable is inserted can be found in the Waking Up The Interface MCU section

Responding to the COMBINED_SENSOR_INT signal:

  • The COMBINED_SENSOR_INT signal is open drain and active low
    • The Target MCU must configure its internal pull up in the pin connected to this signal
    • The Target MCU interrupt must be configured to wake up when the signal transitions from high to low
  • The Target MCU must query the wake up source to the Interface MCU via I2C

Power Mode Transitions

On -> Deep Sleep

  • Target MCU turns off all on-board components
  • Target MCU sends the I2C command to the Interface MCU to set the power-led-sleep-state to OFF
  • Target MCU sets itself into Sleep mode
  • Target MCU does not need to set the Interface MCU into a sleep mode, the Interface will manage its Sleep mode independently

Deep Sleep -> On

  • One of these events activates this transition:
    • Target MCU wakes up
    • USB insertion wakes up the Interface MCU (if asleep) and the Interface wakes up the Target via COMBINED_SENSOR_INT
    • Reset button wakes up the Interface MCU (if asleep) and the Interface wakes up the Target MCU via SWD reset
  • Target MCU sends the I2C command to the Interface MCU to set the power-led-sleep-state to ON
  • Target MCU turns-on all the required on-board components
  • Target MCU does not need to wake up Interface MCU, as the Interface manages its own power state in Sleep mode

On -> Off or On ->Stand-by

  • User long-presses reset button
    • While the button is pressed down the Interface MCU halts the Target MCU
    • When the button is released the Interface MCU resets the the Target MCU
  • Interface MCU sets the interrupt signal active
    • Or it pulses it, if it was already active
  • Target MCU queries the Interface MCU status
  • Interface MCU responds to the Target MCU that the reset button has been long-pressed
  • Target MCU sends the command to the Interface MCU to go to Power Down
    • If the board is not PC connected, the Interface MCU goes into Deep Sleep
    • If the board is PC connected, the Interface MCU ignores the command and starts blinking the Power (red) LED
  • Target MCU turns off all on-board components
  • Target MCU sets itself to Deep Sleep

Off -> On or Stand-by -> On

  • One of these events activates this transition:
    • (Off mode only) USB insertion wakes up the Interface MCU (if asleep) and the Interface wakes up the Target MCU via COMBINED_SENSOR_INT
    • Reset button wakes up the Interface MCU (if asleep) and the Interface wakes up the Target MCU via SWD reset
  • (Off mode only) Interface MCU wakes up in a reset state
  • Interface MCU detects the wake up source
  • If the wake up source is the USB insertion the Interface MCU sets the COMBINED_SENSOR_INT signal active (or pulses it if already active) to wake up the Target MCU
  • If the wake up source is the reset button the Interface MCU halts the Target MCU while button is pressed, and resets the Target MCU when the button is released
  • Target MCU starts running from reset
  • The Target MCU software on start-up should configure all the on-board components to the expected “run mode” and not depend on the component default on-power-up state

Deep Sleep -> Off or Stand-by

The only way to transition from Sleep to Stand-by is by the user pressing the reset button.

  • User starts pressing the reset button
  • Interface MCU wakes up not on reset mode
  • Interface MCU detects the wake up source
  • While the button is pressed down the Interface MCU halts Target MCU
  • When the button is released the Interface MCU resets the Target MCU
  • If the user releases the button before the long-press state is reached the Interface MCU continues with the reset process
  • If the user releases the button after the long-press state is reached, it continues with the “On -> Off / Stand-by” process

Off or Stand-by -> Deep Sleep

Not possible.

Changelog

Version Changes
1.0.0 Initial release.
1.0.1 Added Table of Contents.
  Reorganised sections for better presentation.
  Added links to the power modes to the intro
  Added markdown frontmatter metadata for tech.microbit.org website rendering.
1.0.2 Reformatted changelog
  Fixed table of contents links
1.0.3 Replace references to “KL27” with “Interface MCU”, and “nRF52” with “Target MCU”