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pybricks-ble-broadcast-observe.md

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Pybricks Bluetooth Low Energy Broadcast/Observe

This document describes the format of messages used by the Pybricks BLE broadcast() and observe() APIs to send data between hubs without establishing a connection.

Transport

Messages are sent as Bluetooth Low Energy Advertisement Data. This is detailed in the Bluetooth Core Specification v4.0 or later.

Application

This describes how advertisements should be configured for sending data (broadcasting) and how they should interpret the received data (observing).

As per the Bluetooth spec, all multi-byte numeric values are little-endian.

Broadcasting

  • Advertisements should be sent using a 100ms interval.
  • Advertisements should be sent on all 3 radio channels.
  • Advertisements should be sent using advertising data type of ADV_NONCONN_IND (undirected, not scannable, not connectable).
  • Payload must contain only one advertisement data structure with type of Manufacturer Specific Data (0xFF).
  • Manufacturer Specific Data must use LEGO company identifier (0x0397).

Observing

  • Should use passive scanning.
  • Should scan on all 3 radio channels.
  • Should use scan interval of 100ms.
  • Should allow advertisement type other than ADV_NONCONN_IND (for City hub support).

Data format

The data portion of the Manufacturer Specific Data is encoded as follows:

  • First byte is the "channel" number (0 to 255).
  • This is followed by 0 or more values encoded as follows:
    • A one byte header that indicates the type and size of the value.
      • Types are one of:
        • SINGLE_OBJECT = 0, Indicator that the next value is the one and only value (instead of a tuple).
        • TRUE = 1, the Python True value.
        • FALSE = 2, the Python False value.
        • INT = 3, the Python int type.
        • FLOAT = 4, the Python float type.
        • STR = 5, the Python str type.
        • BYTES = 6, the Python bytes type.
      • The length is the number of bytes that follow that contain the value:
        • For SINGLE_OBJECT, TRUE, and FALSE, the length is always 0.
        • For INT, the length is 1 if the value is an 8-bit signed int (-128 to 127), 2 if the value is a 16-bit signed int (-32768 and 32767) or 4 if the value is a 32-bit signed int.
        • For FLOAT the length is always 4.
        • For STR and BYTES, the length can be 0 or greater.
      • The type and length are packed into one byte using (type << 5) | (length & 0x1F).
    • The value encoding depends on the type.
      • SINGLE_OBJECT, TRUE, and FALSE have no value.
      • INT can be an 8-bit signed int, a 16-bit signed int or a 32-bit signed int depending on the given length.
      • FLOAT is a 32-bit IEEE 754 floating point value.
      • STR must be valid UTF-8 (without zero-termination).
      • BYTES has no restrictions.
    • Since advertisements payloads are limited to 31 bytes by the Bluetooth spec and there are 5 bytes of overhead, the combined size of all headers and values is limited to 26 bytes.

Example payload

Tuple data:

0x0F, 0xFF, 0x97, 0x03, 0x01, 0x61, 0x64, 0x84, 0x00, 0x00, 0x80, 0x3f, 0xA2, 0x68, 0x69, 0x20
  ^     ^     ^     ^     ^     ^     ^     ^     ^     ^     ^     ^     ^     ^     ^     ^
  |     |     |_____|     |     |     |     |     |_____|_____|_____|     |     |_____|     |
  |     |     |           |     |     |     |     |                       |     |           Header indicating `TRUE` value.
  |     |     |           |     |     |     |     |                       |     `STR` value of "hi".
  |     |     |           |     |     |     |     |                       Header indicating `STR` value with length 2.
  |     |     |           |     |     |     |     `FLOAT` value of 1.0
  |     |     |           |     |     |     Header indicating `FLOAT` with length 4
  |     |     |           |     |     `INT` value of 100
  |     |     |           |     Header indicating `INT` with length 1
  |     |     |           Channel number
  |     |     LEGO Company Identifier
  |     Advertising data type indicating Manufacturer Data
  Length (number of bytes that follow, not including this one)

Single object:

0x07, 0xFF, 0x97, 0x03, 0x01, 0x00, 0x61, 0x64
  ^     ^     ^     ^     ^     ^     ^     ^
  |     |     |_____|     |     |     |     |
  |     |     |           |     |     |     |
  |     |     |           |     |     |     |
  |     |     |           |     |     |     |
  |     |     |           |     |     |     |
  |     |     |           |     |     |     `INT` value of 100
  |     |     |           |     |     Header indicating `INT` with length 1
  |     |     |           |     Header indicating `SINGLE_OBJECT`
  |     |     |           Channel number
  |     |     LEGO Company Identifier
  |     Advertising data type indicating Manufacturer Data
  Length (number of bytes that follow, not including this one)

Additional resources