sbus.cpp

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/*
  SBUS decoder, based on src/modules/px4iofirmware/sbus.c from PX4Firmware
  modified for use in AP_HAL_* by Andrew Tridgell
 */
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#include <stdint.h>

#include "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} }
};


bool
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)) * 1000 + 998;
        /* channel 18 (index 17) */
        values[17] = (frame[SBUS_FLAGS_BYTE] & (1 << 1)) * 1000 + 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;
}