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OSTC Bluetooth Import

The OSTC ("Open Source Tauchcomputer") family of dive computers by Heinrichs Weikamp uses the hwOS operating system. It allows transfering dive data via Bluetooth LE.

Bluetooth LE

Bluetooth LE endpoints are called characteristics (e.g. "firmware revision" or "battery level"). They are grouped into services (e.g. "device information"). Characteristics and services are identified by UUIDs.

Characteristics can allow:

  • Reading
  • Writing (with or without response, like TCP/UDP)
  • Notifications/Indications (app gets notified by device if a value changed, with our without acknowledgement)

A hwOS device has these endpoints:

  • Service: Generic Access (0x1800)
    • Characteristic: Device Name (0x2A00, READ) → "OSTCs 21876"
    • Characteristic: Appearance (0x2A01, READ)
  • Service: Generic Attribute (0x1801)
  • Service: Device Information (0x180A)
    • Characteristic: PnP ID Name (0x2A50, READ)
  • Service: Terminal IO (0xFEFB)
    • Characteristic: UART Data RX (00000001-0000-1000-8000-008025000000, WRITE - NO RESPONSE)
    • Characteristic: UART Data TX (00000002-0000-1000-8000-008025000000, NOTIFY)
    • Characteristic: UART Credits RX (00000003-0000-1000-8000-008025000000, WRITE)
    • Characteristic: UART Credits TX (00000004-0000-1000-8000-008025000000, INDICATE)

Communication Workflow

See Terminal I/O Profile Client Implementation Guide.

hwOS does not use Bluetooth LE characteristics directly as endpoints for fetching dives. Instead communication is done through a simulated virtual serial COM port over four specific BLE characteristics. This is based on the "Terminal I/O v2.0 protocol" by Telit Wireless Solutions (formerly Stollmann E+V GmbH).

  1. App connects to (previously bonded) BLE device
  2. App sets up indication to Credits TX
  3. App sets up notification to Data TX
  4. App sends credits (e.g. 0xFE as UInt8 for 254 credits) via Credits RX
  5. Device sends credits to app via Credits TX
  6. App can now:
    • Send commands via Data RX
    • Receive responses via Data TX

Package Format

See hwOS API specification.

Requests:

  • Byte 0: Command ID
  • Byte 1...n: Payload (variable length payloads like strings are 0x00 terminated)

Responses:

  • Byte 0: Command ID (echo)
  • Byte 1...(n-1): Payload
  • Byte n: 0x4D

Download Dives

See OstcConnection.kt for implementation details.

First we have to enable download mode by sending the "start communication" command (0xBB). Afterwards we can end communication by sending the "exit communication" command (0xFF). This will kill the bluetooth connection.

We have three options:

  • Send "get compact headers" command (0x6D):
    • Device will always send all 256 dive headers (4096 bytes total)
    • Unused headers consist of 0xFF and must be filtered
    • Includes date, duration, max depth and dive number
  • Send "get full headers" command (0x61):
    • 65536 bytes total
    • Includes more information than compact headers, like surface pressure, min. temperature, gas mix, deco model, etc.
  • Send "get dive profile" command (0x66) with dive number (0-255).
    • Sends full header and dive profile
    • Profile has variable length, e.g. ~21 kB for a one-hour dive
    • Profile contains depth and events like gas mix change