ardupilot/libraries/AP_RCProtocol/AP_RCProtocol_SBUS.cpp

/*
  SBUS decoder, based on src/modules/px4iofirmware/sbus.c from PX4Firmware
  modified for use in AP_HAL_* by Andrew Tridgell
 */
/****************************************************************************
 *
 *   Copyright (c) 2012-2014 PX4 Development Team. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
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 *    distribution.
 * 3. Neither the name PX4 nor the names of its contributors may be
 *    used to endorse or promote products derived from this software
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 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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/*
 * This file is free software: you can redistribute it and/or modify it
 * under the terms of the GNU General Public License as published by the
 * Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This file is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 * See the GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 * Code by Andrew Tridgell and Siddharth Bharat Purohit
 */

#include "AP_RCProtocol_SBUS.h"

#define SBUS_FRAME_SIZE		25
#define SBUS_INPUT_CHANNELS	16
#define SBUS_FLAGS_BYTE		23
#define SBUS_FAILSAFE_BIT	3
#define SBUS_FRAMELOST_BIT	2

/* define range mapping here, -+100% -> 1000..2000 */
#define SBUS_RANGE_MIN 200.0f
#define SBUS_RANGE_MAX 1800.0f

#define SBUS_TARGET_MIN 1000.0f
#define SBUS_TARGET_MAX 2000.0f

/* pre-calculate the floating point stuff as far as possible at compile time */
#define SBUS_SCALE_FACTOR ((SBUS_TARGET_MAX - SBUS_TARGET_MIN) / (SBUS_RANGE_MAX - SBUS_RANGE_MIN))
#define SBUS_SCALE_OFFSET (int)(SBUS_TARGET_MIN - (SBUS_SCALE_FACTOR * SBUS_RANGE_MIN + 0.5f))

/*
 * S.bus decoder matrix.
 *
 * Each channel value can come from up to 3 input bytes. Each row in the
 * matrix describes up to three bytes, and each entry gives:
 *
 * - byte offset in the data portion of the frame
 * - right shift applied to the data byte
 * - mask for the data byte
 * - left shift applied to the result into the channel value
 */
struct sbus_bit_pick {
    uint8_t byte;
    uint8_t rshift;
    uint8_t mask;
    uint8_t lshift;
};
static const struct sbus_bit_pick sbus_decoder[SBUS_INPUT_CHANNELS][3] = {
    /*  0 */ { { 0, 0, 0xff, 0}, { 1, 0, 0x07, 8}, { 0, 0, 0x00,  0} },
    /*  1 */ { { 1, 3, 0x1f, 0}, { 2, 0, 0x3f, 5}, { 0, 0, 0x00,  0} },
    /*  2 */ { { 2, 6, 0x03, 0}, { 3, 0, 0xff, 2}, { 4, 0, 0x01, 10} },
    /*  3 */ { { 4, 1, 0x7f, 0}, { 5, 0, 0x0f, 7}, { 0, 0, 0x00,  0} },
    /*  4 */ { { 5, 4, 0x0f, 0}, { 6, 0, 0x7f, 4}, { 0, 0, 0x00,  0} },
    /*  5 */ { { 6, 7, 0x01, 0}, { 7, 0, 0xff, 1}, { 8, 0, 0x03,  9} },
    /*  6 */ { { 8, 2, 0x3f, 0}, { 9, 0, 0x1f, 6}, { 0, 0, 0x00,  0} },
    /*  7 */ { { 9, 5, 0x07, 0}, {10, 0, 0xff, 3}, { 0, 0, 0x00,  0} },
    /*  8 */ { {11, 0, 0xff, 0}, {12, 0, 0x07, 8}, { 0, 0, 0x00,  0} },
    /*  9 */ { {12, 3, 0x1f, 0}, {13, 0, 0x3f, 5}, { 0, 0, 0x00,  0} },
    /* 10 */ { {13, 6, 0x03, 0}, {14, 0, 0xff, 2}, {15, 0, 0x01, 10} },
    /* 11 */ { {15, 1, 0x7f, 0}, {16, 0, 0x0f, 7}, { 0, 0, 0x00,  0} },
    /* 12 */ { {16, 4, 0x0f, 0}, {17, 0, 0x7f, 4}, { 0, 0, 0x00,  0} },
    /* 13 */ { {17, 7, 0x01, 0}, {18, 0, 0xff, 1}, {19, 0, 0x03,  9} },
    /* 14 */ { {19, 2, 0x3f, 0}, {20, 0, 0x1f, 6}, { 0, 0, 0x00,  0} },
    /* 15 */ { {20, 5, 0x07, 0}, {21, 0, 0xff, 3}, { 0, 0, 0x00,  0} }
};


// constructor
AP_RCProtocol_SBUS::AP_RCProtocol_SBUS(AP_RCProtocol &_frontend, bool _inverted) :
    AP_RCProtocol_Backend(_frontend),
    inverted(_inverted)
{}

// decode a full SBUS frame
bool AP_RCProtocol_SBUS::sbus_decode(const uint8_t frame[25], uint16_t *values, uint16_t *num_values,
                                     bool *sbus_failsafe, bool *sbus_frame_drop, uint16_t max_values)
{
    /* check frame boundary markers to avoid out-of-sync cases */
    if ((frame[0] != 0x0f)) {
        return false;
    }

    switch (frame[24]) {
    case 0x00:
        /* this is S.BUS 1 */
        break;
    case 0x03:
        /* S.BUS 2 SLOT0: RX battery and external voltage */
        break;
    case 0x83:
        /* S.BUS 2 SLOT1 */
        break;
    case 0x43:
    case 0xC3:
    case 0x23:
    case 0xA3:
    case 0x63:
    case 0xE3:
        break;
    default:
        /* we expect one of the bits above, but there are some we don't know yet */
        break;
    }

    unsigned chancount = (max_values > SBUS_INPUT_CHANNELS) ?
                         SBUS_INPUT_CHANNELS : max_values;

    /* use the decoder matrix to extract channel data */
    for (unsigned channel = 0; channel < chancount; channel++) {
        unsigned value = 0;

        for (unsigned pick = 0; pick < 3; pick++) {
            const struct sbus_bit_pick *decode = &sbus_decoder[channel][pick];

            if (decode->mask != 0) {
                unsigned piece = frame[1 + decode->byte];
                piece >>= decode->rshift;
                piece &= decode->mask;
                piece <<= decode->lshift;

                value |= piece;
            }
        }


        /* convert 0-2048 values to 1000-2000 ppm encoding in a not too sloppy fashion */
        values[channel] = (uint16_t)(value * SBUS_SCALE_FACTOR +.5f) + SBUS_SCALE_OFFSET;
    }

    /* decode switch channels if data fields are wide enough */
    if (max_values > 17 && chancount > 15) {
        chancount = 18;

        /* channel 17 (index 16) */
        values[16] = (frame[SBUS_FLAGS_BYTE] & (1 << 0))?1998:998;
        /* channel 18 (index 17) */
        values[17] = (frame[SBUS_FLAGS_BYTE] & (1 << 1))?1998:998;
    }

    /* note the number of channels decoded */
    *num_values = chancount;

    /* decode and handle failsafe and frame-lost flags */
    if (frame[SBUS_FLAGS_BYTE] & (1 << SBUS_FAILSAFE_BIT)) { /* failsafe */
        /* report that we failed to read anything valid off the receiver */
        *sbus_failsafe = true;
        *sbus_frame_drop = true;
    } else if (frame[SBUS_FLAGS_BYTE] & (1 << SBUS_FRAMELOST_BIT)) { /* a frame was lost */
        /* set a special warning flag
         *
         * Attention! This flag indicates a skipped frame only, not a total link loss! Handling this
         * condition as fail-safe greatly reduces the reliability and range of the radio link,
         * e.g. by prematurely issuing return-to-launch!!! */

        *sbus_failsafe = false;
        *sbus_frame_drop = true;
    } else {
        *sbus_failsafe = false;
        *sbus_frame_drop = false;
    }

    return true;
}


/*
  process a SBUS input pulse of the given width
 */
void AP_RCProtocol_SBUS::process_pulse(uint32_t width_s0, uint32_t width_s1)
{
    uint32_t w0 = width_s0;
    uint32_t w1 = width_s1;
    if (inverted) {
        w0 = saved_width;
        w1 = width_s0;
        saved_width = width_s1;
    }
    uint8_t b;
    if (ss.process_pulse(w0, w1, b)) {
        _process_byte(ss.get_byte_timestamp_us(), b);
    }
}

// support byte input
void AP_RCProtocol_SBUS::_process_byte(uint32_t timestamp_us, uint8_t b)
{
    const bool have_frame_gap = (timestamp_us - byte_input.last_byte_us >= 2000U);
    byte_input.last_byte_us = timestamp_us;

    if (have_frame_gap) {
        // if we have a frame gap then this must be the start of a new
        // frame
        byte_input.ofs = 0;
    }
    if (b != 0x0F && byte_input.ofs == 0) {
        // definately not SBUS, missing header byte
        return;
    }
    if (byte_input.ofs == 0 && !have_frame_gap) {
        // must have a frame gap before the start of a new SBUS frame
        return;
    }

    byte_input.buf[byte_input.ofs++] = b;

    if (byte_input.ofs == sizeof(byte_input.buf)) {
        uint16_t values[SBUS_INPUT_CHANNELS];
        uint16_t num_values=0;
        bool sbus_failsafe = false;
        bool sbus_frame_drop = false;
        if (sbus_decode(byte_input.buf, values, &num_values,
                        &sbus_failsafe, &sbus_frame_drop, SBUS_INPUT_CHANNELS) &&
            num_values >= MIN_RCIN_CHANNELS) {
            add_input(num_values, values, sbus_failsafe);
        }
        byte_input.ofs = 0;
    }
}

// support byte input
void AP_RCProtocol_SBUS::process_byte(uint8_t b, uint32_t baudrate)
{
    if (baudrate != 100000) {
        return;
    }
    _process_byte(AP_HAL::micros(), b);
}