I2C Protocol Specification

Specification for the micro:bit I2C Protocol

micro:bit I2C Protocol Specification

This is version 0.9.1 of the specification.


Term Definition
DAPLink Interface firmware providing USB and programming capabilities
I2C Inter-Integrated Circuit bus
I2C main I2C node in control of the clock and initiating transactions
I2C secondary I2C peripheral that responds to the I2C main
Storage Flash available in the KL27 for micro:bit data and config storage
SWD Serial Wire Debug
UART Universal asynchronous receiver-transmitter


The micro:bit contains two microcontrollers, the Interface MCU which provides the USB functionality, and the Target MCU where the user code runs. More information can be found in the Tech Site DAPLink page.

In micro:bit V1 there are UART and SWD signals connecting the Interface MCU (KL26) and the Target MCU (nRF51). These are used to program the Target MCU (nRF51) and to provide serial communication between the Target (nRF51) and the computer.

The micro:bit V2 adds an internal I2C bus connected to the Interface MCU (KL27), the Target MCU (nRF52), and the motion sensors (in V1 the motions sensors are connected to the external I2C bus, connected only to the Target MCU (nRF51), more info in the Tech Site I2C page). This new I2C bus allows the Interface (KL27) to provide additional features to the Target (nRF52), and to co-operate to set the board into different power modes (more info in the Power Management Spec).

I2C Diagram

The additional features provided by the Interface (KL27) via I2C are:

  • Device Information
    • Board ID, DAPLink version, and more
  • Power Management
  • I2C Flash Storage
    • The Interface (KL27) flash is 256 KBs, where 128KBs are reserved for non-volatile storage accessible to the Target (nRF52)

I2C Secondary addresses

I2C Secondary 7-bit address
KL27 (I2C nRF – KL27 config/comms interface) 0x70
KL27 (I2C USB/HID interface) 0x71
KL27 (I2C Flash interface) 0x72
FXOS8700CQ 0x1F
LSM303AGR Accelerometer 0x19
LSM303AGR Magnetometer 0x1E

I2C nRF – KL27 config/comms interface

Types of commands

Command CMD ID Used by
nop_cmd 0x00 main only
read_request 0x10 main only
read_response 0x11 secondary only
write_request 0x12 main & secondary
write_response 0x13 main & secondary
error_response 0x20 main & secondary

Packet format for each command




CMD ID (1B) Property ID (1B)


CMD ID (1B) Property ID (1B) Data size (1B) Data (NB)


CMD ID (1B) Property ID (1B) Data size (1B) Data (NB)


CMD ID (1B) Property ID (1B)


CMD ID (1B) Error code (1B)


Property Property ID Data
DAPLink Board version (R) 0x01 Size: 2B e.g. 0x9904
I2C protocol version (R) 0x02 Size: 2B e.g. 0x0001
DAPLink version (R) 0x03 Size 2B e.g. 0x00FD
Power state (R) 0x04 Size 1B e.g. 0x01
typedef enum {
} power_source_t;
Power consumption (R) 0x05 Size 8B
4B for bat_sense voltage
4B for vin voltage
(units in microvolts uint32)
USB enumeration state (R) 0x06 Size 1B e.g. 0x02
typedef enum main_usb_connect {
} main_usb_connect_t;
KL27 Power mode (W) 0x07 Size 1B e.g. 0x08
kAPP_PowerModeVlls0 = 0x08
Power LED Sleep state (W) 0x08 Size 1B e.g. 0x01
0x00 -> OFF
0x01-0xFF -> ON
User event (R asynch) 0x09 Size 1B e.g. 0x01
0x01 -> Wakeup from reset button (not used)
0x02 -> Wakeup from WAKE_ON_EDGE
0x03 -> reset button long press
Automatic sleep (W) 0x0A Size 1B e.g. 0x01
0x00 -> OFF
0x01-0xFF -> ON

Error codes

Error Error Code
Success 0x30
Incomplete command 0x31
Unknown command 0x32
Command disallowed 0x33
Unknown property (for both read and write requests) 0x34
Wrong size for property (for write requests) 0x35
Read property disallowed 0x36
Write property disallowed 0x37
Write fail 0x38


  • Read DAPLink Board version (nRF I2C main)
    1. read_request (cmd id + property) I2C Write: 0x10 0x01
    2. (KL27 processes cmd and asserts COMBINED_SENSOR_INT signal when response is ready)
    3. read_response (cmd id + property + size + data) I2C Read: 0x11 0x01 0x02 0x04 0x99
    4. (KL27 releases COMBINED_SENSOR_INT signal)


  • read_request can only be sent by the I2C main (nrf)
    • The main (nRF) must wait for the COMBINED_SENSOR_INT signal to be asserted by the secondary (KL27)
  • write_request can be sent by both secondary and main.
    • For the secondary to initiate this, it must assert the interrupt signal first and then the main must poll (i2c read) the device for data.

I2C Flash interface

KL27 storage memory layout:

↓ KL27 flash address 0x20000                                ↓ KL27 flash address0x40000
↓         ↓ KL27 flash address 0x20400                      ↓
|[config] |[data                           [ DAL’s config ]]|
          ↑ storage address 0x0000         ↑ storage address set by file size
  • config
    • Controlled by KL27
    • nRF reads/write data fields via I2C commands
  • data
    • Controlled by nRF
    • nRF reads/write bytes via I2C commands
    • Storage address range: 0x00000-max_size


  • Word (4 Bytes) align transactions
  • All transactions start with:
    • 1 byte command ID
    • N byte data depending on command
  • For KL27 responses, KL27 to hold interrupt until success/error messages have been served


  • Read/Write config data (stored in RAM)
    • File name (includes extension) (cmd id 0x01)
      • 11B uppercase characters following 8.3 format (e.g. "DATA BIN")
      • default → DATA.BIN
    • File size (from start of data section) (0x02)
      • 4B size in bytes (MSB First)
      • default → 126 KBs
      • max size is 126 KBs (128KB - 1KB for flash interface config and 1KB for DAL’s config)
    • Enable file visible in MSD (0x03)
      • 1B visibility
      • default → false
  • Write config data to flash (0x04)
    • Handles sector erase if RAM config data is different than Flash config data
  • Erase all config (0x05)
    • Config data will be in its default state
  • Get available storage size (0x06)
    • 1B size in KB
  • Get flash sector size (0x07)
    • 2B sector size in bytes (MSB first)
  • Re-mount MSD (0x08)

Flash Operations

Reading/writing config data

  • For writing the config data (file name, file size and file visibility), the I2C main has to send the 1B command ID followed by the corresponding config data. If the data size is unexpected, an error will be returned in the I2C response.
  • The I2C Response after a successful write operation will contain the corresponding command ID followed by the written config data which can be used as confirmation by the I2C main.
  • For reading the config data, the I2C main has to send only the 1B command ID with no further data. The I2C response will then contain the config data.

Write storage data

  • 1 byte write command (cmd id 0x0B)
  • 3 bytes address (MSB First)
  • 4 bytes for length (MSB First)
  • data to write
  • Address and size check
    • Size has to be a multiple of 4 bytes
    • Address has to be aligned to a 4 byte boundary
    • Bound check
  • KL27 can clock stretch

Read storage data

  • 1 bytes read command (0x0A)
  • 3 bytes address
  • 4 bytes for length

Erase storage data

  • 1 bytes erase command (0x0C)
  • 3 bytes start address
  • 1 unused
  • 3 byte end address (inclusive)
    • For a single sector erase Start address == End address
  • Address checks
    • Addresses have to be sector align
    • End address >= Start address
    • Bound check
  • On errors KL27 to trigger interrupt and send error code
  • On success KL27 to trigger interrupt and send success message
  • KL to provide some kind of status read command

KL27 behaviour

  • Before enumeration it checks if it should show the file on the MSD
    • It sets the file name from the config
    • It sets the file size from the config
  • KL27 I2C buffer size: 1KB + 4 bytes
  • Storage writes should not trigger “hidden” sector erases, the nRF is responsible to write and erase
  • When writing to the config data first check if the data to be written is different than present, avoid an erase-and-write operation if it’s the same
  • USB should have higher priority than any I2C transaction


Storage Write

  • Write 4 bytes “1234” to storage address 0x000010
    I2C Write:  [0x72,    0x0B,  0x00,0x00,0x10,  0x00,0x00,0x00,0x04,  0x31,0x32,0x33,0x34]
         I2C Addr ↑  cmd id ↑    address ↑        length ↑              data ↑
    I2C Read:   [0x72,    0x0B,  0x00,0x00,0x10,  0x00,0x00,0x00,0x04,  0x31,0x32,0x33,0x34]
         I2C Addr ↑  cmd id ↑    address ↑        length ↑              data ↑

Storage Read

  • Read 4 bytes at storage address 0x000010
    I2C Write:  [0x72,    0x0A,  0x00,0x00,0x10,  0x00,0x00,0x00,0x04]
         I2C Addr ↑  cmd id ↑    address ↑        length ↑
    I2C Read:   [0x72,    0x0A,  0x00,0x00,0x10,  0x00,0x00,0x00,0x04,  0x31,0x32,0x33,0x34]
         I2C Addr ↑  cmd id ↑    address ↑        length ↑              data ↑

Config Update, enable file visibility and remount MSD drive

  • Change filename to LOG.TXT
    I2C Write:  [0x72,    0x01,   0x4c,0x4f,0x47,0x20,0x20,0x20,0x20,0x20,0x54,0x58,0x54]
         I2C Addr ↑  cmd id ↑    filename "LOG     TXT" ↑
    I2C Read:   [0x72,    0x01,   0x4c,0x4f,0x47,0x20,0x20,0x20,0x20,0x20,0x54,0x58,0x54]
         I2C Addr ↑  cmd id ↑    filename "LOG     TXT" ↑
  • Enable file visibility
    I2C Write:  [0x72,    0x03,           0x01]
         I2C Addr ↑  cmd id ↑    visibility ↑
    I2C Read:   [0x72,    0x03,           0x01]
         I2C Addr ↑  cmd id ↑    visibility ↑
  • Remount MSD
    I2C Write:  [0x72,    0x08]
         I2C Addr ↑  cmd id ↑
    I2C Read:   [0x72,    0x08]
         I2C Addr ↑  cmd id ↑

Universal Hex

KL27 storage area should be writeable via Universal Hex.

HID Interface

All I2C features should be available via HID interface, so that it can be accessed via WebUSB.