sbus_futaba: Add SBUS2 telemetry decoding and optimize performance

[edwin-oetelaar]

This commit adds full support for Futaba SBUS2 telemetry frames while significantly improving the decoder's execution performance and memory footprint.

Background:
Standard Futaba SBUS frames end with a 0x00 footer byte. SBUS2 receivers use non-zero footers (e.g. 0x04, 0x14, 0x24, 0x34) to request telemetry slot data (slots 0-7, 8-15, 16-23, 24-31). Following an inter-frame gap (~1960 µs), sensors and receivers respond with 3-byte telemetry slots. Previously, non-zero footers produced warnings and subsequent telemetry bytes triggered "excess data" warnings, causing frame decoding to fail.

Key Changes:

• SBUS2 Telemetry Support:

  • Recognize valid SBUS2 telemetry footers and indicate requested telemetry slot groups in UI annotations.
  • Added a state machine transition to decode 3-byte telemetry packets (1-byte Slot ID + 2-byte 16-bit payload value) after inter-frame gaps.
  • Handled UART IDLE gaps between main servo frames and telemetry bursts seamlessly.

• High-Performance Byte-Buffer Architecture:

  • Replaced bit-by-bit accumulator tuple parsing with byte-level buffering, eliminating thousands of micro-allocations per capture.
  • Implemented fast 11-bit channel extraction directly from the 22-byte payload using bitwise shifts.

• Precise Sample Alignment & Visual Usability:

  • Added bit-level UART sample interpolation for Byte 23 (Digital Ch 17, 18, Frame Lost, Failsafe) and Channels 1-16. This prevents annotation overlapping in PulseView and ensures clean, distinct visual rendering.

• Code Quality & Documentation:

  • Restructured state machine representation to use integer constants.
  • Added standardized docstrings (Brief, Params, Invariants, Output) to all methods.

Tested against live Futaba SBUS2 receiver captures containing receiver battery voltage telemetry slots in PulseView.

[edwin-oetelaar]

Hi everyone,

I noticed that the current sbus_futaba decoder has trouble decoding frames from Futaba SBUS2 receivers when telemetry is enabled, as non-zero footers and telemetry response bytes trigger unexpected warnings.

I have updated the decoder to support SBUS2 telemetry decoding while also refactoring the internal bit-accumulator logic into a high-performance byte-buffer architecture for significantly faster decoding in PulseView.

Summary of Changes

• SBUS2 Telemetry: Added support for non-zero footers (0x04, 0x14, etc.) and decoding of 3-byte telemetry slots (Slot ID + 16-bit payload).
• Performance: Replaced bit-by-bit tuple slicing with direct byte-buffer bitwise unpacking.
• UI & Alignment: Added bit-accurate sample timing interpolation for Digital channels (Ch 17, 18), flags (Frame Lost, Failsafe), and Proportional channels so annotations render cleanly without overlapping in PulseView.
• Code Quality: Added docstrings (Brief, Params, Invariants, Output) to all methods and converted state flags to integer constants.

Tested on a physical SBUS2 receiver trace with receiver battery voltage telemetry.

Feedback and suggestions are very welcome!

Best regards,
Edwin van den Oetelaar

[edwin-oetelaar]

added some more decoding of telemetry :

SBUS2 telemetry decoding now recognizes sensor type flags:
0xC0: voltage, decoded in 0.1 V steps
0xC4: current, decoded in mA
Other flags: retained as raw 16-bit telemetry values
Decoded values are available as graphical annotations and Python output packets.