This library communicates with SBUS receivers and servos.

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SBUS is a bus protocol for receivers to send commands to servos. Unlike PWM, SBUS uses a bus architecture where a single serial line can be connected with up to 16 servos with each receiving a unique command.

The SBUS protocol uses an inverted serial logic with a baud rate of 100000, 8 data bits, even parity, and 2 stop bits. The SBUS packet is 25 bytes long consisting of:

• Byte[0]: SBUS header, 0x0F
• Byte[1 -22]: 16 servo channels, 11 bits each
• Byte[23]
  • Bit 7: channel 17 (0x80)
  • Bit 6: channel 18 (0x40)
  • Bit 5: frame lost (0x20)
  • Bit 4: failsafe activated (0x10)
• Byte[24]: SBUS footer

Note that lost frame is indicated when a frame is lost between the transmitter and receiver. Failsafe activation typically requires that several frames are lost in a row and indicates that the receiver has moved into failsafe mode. Packets are sent approximately every 10 ms or 20 ms.

Note on CH17 and CH18: Channel 17 and channel 18 are digital on/off channels. These are not universally available on all SBUS receivers and servos.

FrSky receivers will output a range of 172 - 1811 with channels set to a range of -100% to +100%. Using extended limits of -150% to +150% outputs a range of 0 to 2047, which is the maximum range acheivable with 11 bits of data.

Because SBUS is a digital bus format, it is an excellent means of receiving pilot commands from a transmitter and an SBUS capable receiver. If SBUS servos are used in the aircraft, SBUS is also an excellent means of sending actuator commands - servo commands can often be sent with lower latency and, by only using a single pin to command up to 16 servos, additional microcontroller pins are freed for other uses.

CMake is used to build this library, which is exported as a library target called sbus. The header is added as:

#include "sbus/sbus.h"

The library can be also be compiled stand-alone using the CMake idiom of creating a build directory and then, from within that directory issuing:

cmake .. -DMCU=MK66FX1M0
make

This will build the library and example executable called sbus_example. The example executable source file is located at examples/sbus_example.cc. Notice that the cmake command includes a define specifying the microcontroller the code is being compiled for. This is required to correctly configure the code, CPU frequency, and compile/linker options. The available MCUs are:

• MK20DX128
• MK20DX256
• MK64FX512
• MK66FX1M0
• MKL26Z64
• IMXRT1062_T40
• IMXRT1062_T41

These are known to work with the same packages used in Teensy products. Also switching packages is known to work well, as long as it's only a package change.

The sbus_example target creates an executable for communicating with sbus receivers and servos. This target also has a _hex for creating the hex file to upload to the microcontroller.

This library is within the namespace bfs

SbusRx(HardwareSerial *bus) Creates an SbusRx object. A pointer to the Serial object corresponding to the serial port used is passed. The RX pin of the serial port will receive SBUS packets.

bfs::SbusRx sbus(&Serial1);

void Begin() Initializes SBUS communication.

sbus.Begin();

bool Read() Parses SBUS packets, returns true on successfully receiving an SBUS packet.

if (sbus.Read()) {
   // Do something with the received data
}

std::array<uint16_t, 16> rx_channels() Returns the array of received channel data.

std::array<uint16_t, 16> sbus_data = sbus.rx_channels();

bool ch17() Returns the value of channel 17.

bool ch17 = sbus.ch17();

bool ch18() Returns the value of channel 18.

bool ch18 = sbus.ch18();

bool lost_frame() Returns true if a frame has been lost.

bool lost_frame = sbus.lost_frame();

bool failsafe() Returns true if the receiver has entered failsafe mode.

bool failsafe = sbus.failsafe();

SbusTx(HardwareSerial *bus) Creates an SbusTx object. A pointer to the Serial object corresponding to the serial port used is passed. The TX pin of the serial port will transmit SBUS packets.

bfs::SbusTx sbus(&Serial1);

void Begin() Initializes SBUS communication.

sbus.Begin();

void Write() Writes an SBUS packet. The packet is written immediately, you should regulate timing of sending packets to servos to maintain a frequency of approximately 100 Hz.

sbus.Write();

void tx_channels(const std::array<uint16_t, 16> &val) Sets the channel data to be transmitted.

sbus.tx_channels(sbus_tx_data);

void ch17(bool val) Sets the value of channel 17 to be transmitted.

sbus.ch17(true);

void ch18(bool val) Sets the value of channel 18 to be transmitted.

sbus.ch18(true);

void lost_frame(bool val) Sets whether to transmit the lost frame flag.

sbus.lost_frame(true);

void failsafe(bool val) Sets whether to transmit the failsafe flag.

sbus.failsafe(true);

std::array<uint16_t, 16> tx_channels() Returns the array of channel data to be transmitted.

std::array<uint16_t, 16> sbus_tx_data = sbus.tx_channels();

bool ch17() Returns the value of channel 17 to be transmitted.

bool ch17 = sbus.ch17();

bool ch18() Returns the value of channel 18 to be transmitted.

bool ch18 = sbus.ch18();

bool lost_frame() Returns the lost frame flag value to be transmitted.

bool lost_frame = sbus.lost_frame();

bool failsafe() Returns the failsafe flag value to be transmitted.

bool failsafe = sbus.failsafe();