sbus.h File Reference

Futaba S.BUS: sixteen channels in twenty-five bytes. More...

#include <stdbool.h>
#include <stdint.h>

Go to the source code of this file.

Data Structures
struct	sbus_frame_t
	One decoded S.BUS frame. More...
struct	sbus_decoder_t
	Decoder state for one S.BUS byte stream. More...

Macros
#define	SBUS_FRAME_BYTES   25u
	Frame layout: byte count, header byte and channel count.
#define	SBUS_HEADER   0x0Fu
#define	SBUS_CHANNELS   16u
#define	SBUS_RAW_MIN   0u
	Raw channel range: eleven bits.
#define	SBUS_RAW_MAX   2047u
#define	SBUS_RAW_1000US   172u
	The two ends of the range a transmitter at its endpoints actually sends, and what they mean in microseconds.
#define	SBUS_RAW_2000US   1811u
#define	SBUS_GAP_US   2000u
	Silence that means a frame boundary.
#define	SBUS_FLAG_CH17   0x01u
	Flag bits in byte 23.
#define	SBUS_FLAG_CH18   0x02u
#define	SBUS_FLAG_FRAME_LOST   0x04u
#define	SBUS_FLAG_FAILSAFE   0x08u

Functions
void	sbus_decoder_reset (sbus_decoder_t *d)
	Clear a decoder, including its diagnostic counters.
bool	sbus_decode_byte (sbus_decoder_t *d, uint8_t byte, uint32_t now_us, sbus_frame_t *out)
	Feed one byte with the time it arrived; true when out holds a frame.
uint16_t	sbus_to_us (uint16_t raw)
	Microseconds for a raw channel value, clamped to a sane pulse.

Detailed Description

Futaba S.BUS: sixteen channels in twenty-five bytes.

Implements the S.BUS receiver protocol of Futaba. Futaba and S.BUS are names or trademarks of their respective owners; they are used here only to say which receivers this module works with (NOTICE, "Trademarks").

The wire is an inverted 8E2 UART (universal asynchronous receiver-transmitter) at 100 kbaud, produced by the IOmcu's PIO (programmable input/output); this file decodes the bytes.

S.BUS has no checksum and no escaping. A frame is a header byte, a bit-packed payload and a footer, and the header value 0x0F is an ordinary value inside channel data, so the header alone cannot locate a frame start: a decoder that trusts it locks onto the middle of a frame and reports sixteen plausible channels that are all wrong.

The frame boundary is the gap. Frames arrive every 7ms or 14ms and take 3ms to send, so there are at least 4ms of silence between them, against 120 microseconds between bytes within one. The decoder is fed a timestamp per byte and frames on that gap; the header and footer confirm.

Two flags outrank the channels. failsafe means the receiver has lost the transmitter and is generating the values it sends; frame lost means this frame did not arrive intact. Anything driven from S.BUS input treats failsafe as a stop, not as sixteen valid channels.

SPDX-License-Identifier: MIT

Macro Definition Documentation

SBUS_CHANNELS

#define SBUS_CHANNELS   16u

channels per frame

SBUS_FLAG_CH17

#define SBUS_FLAG_CH17   0x01u

Flag bits in byte 23.

digital channel 17

SBUS_FLAG_CH18

#define SBUS_FLAG_CH18   0x02u

digital channel 18

SBUS_FLAG_FAILSAFE

#define SBUS_FLAG_FAILSAFE   0x08u

receiver is in failsafe

SBUS_FLAG_FRAME_LOST

#define SBUS_FLAG_FRAME_LOST   0x04u

frame did not arrive intact

SBUS_FRAME_BYTES

#define SBUS_FRAME_BYTES   25u

Frame layout: byte count, header byte and channel count.

bytes per frame, header to footer

SBUS_GAP_US

#define SBUS_GAP_US   2000u

Silence that means a frame boundary.

Bytes within a frame are 120 microseconds apart at 100 kbaud 8E2; frames are at least 4ms apart. Two milliseconds sits clear of both, so it cannot split a frame and cannot join two.

SBUS_HEADER

#define SBUS_HEADER   0x0Fu

first byte of every frame

SBUS_RAW_1000US

#define SBUS_RAW_1000US   172u

The two ends of the range a transmitter at its endpoints actually sends, and what they mean in microseconds.

Not the raw limits: 172 and 1811 are what the protocol settled on, and a receiver can send outside them. raw value of a 1000us pulse

SBUS_RAW_2000US

#define SBUS_RAW_2000US   1811u

raw value of a 2000us pulse

Function Documentation

sbus_decode_byte()

bool sbus_decode_byte	(	sbus_decoder_t *	d,
		uint8_t	byte,
		uint32_t	now_us,
		sbus_frame_t *	out )

Feed one byte with the time it arrived; true when out holds a frame.

now_us paces the framing. A gap of SBUS_GAP_US or more starts a new frame, whatever was half-collected before it; the gap is the only reliable boundary this protocol has.

Parameters

d	Decoder state. NULL returns false.
byte	Received byte.
now_us	Arrival time of the byte, in us.
out	Receives the frame; may be NULL.

Returns

true when a frame with a valid footer completes.

sbus_decoder_reset()

void sbus_decoder_reset

(

sbus_decoder_t *

d

)

Clear a decoder, including its diagnostic counters.

Parameters

d	Decoder to clear. NULL is ignored.

sbus_to_us()

uint16_t sbus_to_us

(

uint16_t

raw

)

Microseconds for a raw channel value, clamped to a sane pulse.

Parameters

raw	Raw channel value.

Returns

Pulse width in us, clamped to 800 to 2200.