ados-crsf-codec

no_std CRSF (Crossfire / ExpressLRS) codec: frames, packed RC channels, telemetry and parameter frames, axis scaling and link liveness.

CRSF is the serial protocol spoken by Crossfire and ExpressLRS radio modules, receivers and flight controllers. This crate encodes and decodes it: build an RC channels frame for a transmitter module, hunt frames out of a noisy byte stream, decode the telemetry a module sends back, and decide from link statistics whether the radio link is actually up. It is pure data handling with no I/O, so it runs the same on a microcontroller and on a Linux host.

use ados_crsf_codec::channels::{build_rc_frame, unpack_channels, CHANNEL_COUNT, CHANNEL_MID, PACKED_SIZE};
use ados_crsf_codec::frame::{Parser, ADDR_FLIGHT_CONTROLLER, TYPE_RC_CHANNELS_PACKED};

// 16 channels at center, with throttle low and one switch high.
let mut channels = [CHANNEL_MID; CHANNEL_COUNT];
channels[2] = 172;
channels[4] = 1811;

// One complete wire frame: sync, length, type, 22-byte payload, CRC-8.
let wire = build_rc_frame(&channels).unwrap();
assert_eq!(wire.len(), 26);

// Feed the bytes to the stream parser, split across two reads and
// preceded by line noise.
let mut parser = Parser::new();
assert!(parser.push(&[0x00, 0x42]).is_empty());
let mut frames = parser.push(&wire[..10]);
frames.extend(parser.push(&wire[10..]));
assert_eq!(frames.len(), 1);

let frame = &frames[0];
assert_eq!(frame.sync, ADDR_FLIGHT_CONTROLLER);
assert_eq!(frame.frame_type, TYPE_RC_CHANNELS_PACKED);
let payload: &[u8; PACKED_SIZE] = frame.payload.as_slice().try_into().unwrap();
assert_eq!(unpack_channels(payload), channels);

Highlights

• #![no_std] with alloc; zero dependencies. Builds for bare-metal targets such as thumbv7em-none-eabihf.
• The stream parser never panics on garbage: it resyncs byte by byte and counts the bytes it skipped.
• Codecs round-trip: every typed frame has an encoder and a decoder, and 61 unit tests check wire layouts and round trips.
• Payload sizes are validated against the frame type before decoding, so a short or long payload is an error, not a panic.
• Link liveness is a pure function: no clocks, no threads, just durations in and a state out.

How it works

The crate has seven modules.

• frame is the byte framing. Every frame is [sync] [len] [type] [payload...] [crc8]. len counts the bytes after it (type, payload and CRC, so 2..=62). The CRC-8 uses the DVB-S2 polynomial 0xD5 (init 0, no reflection, no xor-out) and covers the type and payload only. The sync byte is a device address: 0xC8 (flight controller), 0xEE (transmitter module) or 0xEA (radio handset); anything else is rejected. Parser buffers partial input and returns every complete, CRC-valid frame.
• channels packs 16 channels of 11 bits each, LSB first, into the 22-byte RC_CHANNELS_PACKED (0x16) payload. 172 is full low, 992 center and 1811 full high. The codec round-trips any 11-bit value; clamping to the usable range is the caller's choice.
• bank is a validated set of 16 channel values: writes outside 172..=1811 are rejected, and a bulk write is all or nothing.
• scale maps a signed 16-bit axis onto the channel range (with an exact 992 center) and an N-position switch onto evenly spaced channel values.
• telemetry decodes and encodes link statistics (0x14), GPS (0x02), battery (0x08), attitude (0x1E) and flight mode (0x21). Multi-byte fields are big-endian.
• params carries the parameter protocol: read (0x2E), write (0x2F) and settings entry (0x2D) frames.
• link derives a link state from what was observed. Bytes accepted by a serial driver prove nothing about the radio, so the verdict keys on fresh link-statistics telemetry and its uplink link quality. Transmitting with no fresh statistics past a grace window, or with uplink quality at zero, reads RfUnverified; only LinkOk and Degraded count as flyable.

Part of ADOS

Used in production by the ADOS Drone Agent, the onboard software of the ADOS software-defined drone platform. All ADOS crates · Docs · Discord

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License

Licensed under either of Apache License, Version 2.0 or MIT license at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.