st24.cpp

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/*
 * @file st24.h
 *
 * RC protocol implementation for Yuneec ST24 transmitter.
 *
 * @author Lorenz Meier <lm@inf.ethz.ch>
 */

#include <stdbool.h>
#include <stdio.h>
#include "st24.h"
#include "common_rc.h"

const char *decode_states[] = {"UNSYNCED",
                   "GOT_STX1",
                   "GOT_STX2",
                   "GOT_LEN",
                   "GOT_TYPE",
                   "GOT_DATA"
                  };

/* define range mapping here, -+100% -> 1000..2000 */
#define ST24_RANGE_MIN 500.0f
#define ST24_RANGE_MAX 3500.0f

#define ST24_TARGET_MIN 1000.0f
#define ST24_TARGET_MAX 2000.0f

/* pre-calculate the floating point stuff as far as possible at compile time */
#define ST24_SCALE_FACTOR ((ST24_TARGET_MAX - ST24_TARGET_MIN) / (ST24_RANGE_MAX - ST24_RANGE_MIN))
#define ST24_SCALE_OFFSET (int)(ST24_TARGET_MIN - (ST24_SCALE_FACTOR * ST24_RANGE_MIN + 0.5f))

static enum ST24_DECODE_STATE _decode_state = ST24_DECODE_STATE_UNSYNCED;
static uint8_t _rxlen;

static ReceiverFcPacket &_rxpacket = rc_decode_buf._strxpacket;

uint8_t st24_common_crc8(uint8_t *ptr, uint8_t len)
{
    uint8_t i, crc ;
    crc = 0;

    while (len--) {
        for (i = 0x80; i != 0; i >>= 1) {
            if ((crc & 0x80) != 0) {
                crc <<= 1;
                crc ^= 0x07;

            } else {
                crc <<= 1;
            }

            if ((*ptr & i) != 0) {
                crc ^= 0x07;
            }
        }

        ptr++;
    }

    return (crc);
}


int st24_decode(uint8_t byte, uint8_t *rssi, uint8_t *lost_count, uint16_t *channel_count, uint16_t *channels,
        uint16_t max_chan_count)
{
    int ret = 1;

    switch (_decode_state) {
    case ST24_DECODE_STATE_UNSYNCED:
        if (byte == ST24_STX1) {
            _decode_state = ST24_DECODE_STATE_GOT_STX1;

        } else {
            ret = 3;
        }

        break;

    case ST24_DECODE_STATE_GOT_STX1:
        if (byte == ST24_STX2) {
            _decode_state = ST24_DECODE_STATE_GOT_STX2;

        } else {
            _decode_state = ST24_DECODE_STATE_UNSYNCED;
        }

        break;

    case ST24_DECODE_STATE_GOT_STX2:

        /* ensure no data overflow failure or hack is possible */
        if ((unsigned)byte <= sizeof(_rxpacket.length) + sizeof(_rxpacket.type) + sizeof(_rxpacket.st24_data)) {
            _rxpacket.length = byte;
            _rxlen = 0;
            _decode_state = ST24_DECODE_STATE_GOT_LEN;

        } else {
            _decode_state = ST24_DECODE_STATE_UNSYNCED;
        }

        break;

    case ST24_DECODE_STATE_GOT_LEN:
        _rxpacket.type = byte;
        _rxlen++;
        _decode_state = ST24_DECODE_STATE_GOT_TYPE;
        break;

    case ST24_DECODE_STATE_GOT_TYPE:
        _rxpacket.st24_data[_rxlen - 1] = byte;
        _rxlen++;

        if (_rxlen == (_rxpacket.length - 1)) {
            _decode_state = ST24_DECODE_STATE_GOT_DATA;
        }

        break;

    case ST24_DECODE_STATE_GOT_DATA:
        _rxpacket.crc8 = byte;
        _rxlen++;

        if (st24_common_crc8((uint8_t *) & (_rxpacket.length), _rxlen) == _rxpacket.crc8) {

            ret = 0;

            /* decode the actual packet */

            switch (_rxpacket.type) {

            case ST24_PACKET_TYPE_CHANNELDATA12: {
                    ChannelData12 *d = (ChannelData12 *)_rxpacket.st24_data;

                    // Scale from 0..255 to 100%.
                    *rssi = d->rssi * (100.0f / 255.0f);
                    *lost_count = d->lost_count;

                    /* this can lead to rounding of the strides */
                    *channel_count = (max_chan_count < 12) ? max_chan_count : 12;

                    unsigned stride_count = (*channel_count * 3) / 2;
                    unsigned chan_index = 0;

                    for (unsigned i = 0; i < stride_count; i += 3) {
                        channels[chan_index] = ((uint16_t)d->channel[i] << 4);
                        channels[chan_index] |= ((uint16_t)(0xF0 & d->channel[i + 1]) >> 4);
                        /* convert values to 1000-2000 ppm encoding in a not too sloppy fashion */
                        channels[chan_index] = (uint16_t)(channels[chan_index] * ST24_SCALE_FACTOR + .5f) + ST24_SCALE_OFFSET;
                        chan_index++;

                        channels[chan_index] = ((uint16_t)d->channel[i + 2]);
                        channels[chan_index] |= (((uint16_t)(0x0F & d->channel[i + 1])) << 8);
                        /* convert values to 1000-2000 ppm encoding in a not too sloppy fashion */
                        channels[chan_index] = (uint16_t)(channels[chan_index] * ST24_SCALE_FACTOR + .5f) + ST24_SCALE_OFFSET;
                        chan_index++;
                    }
                }
                break;

            case ST24_PACKET_TYPE_CHANNELDATA24: {
                    ChannelData24 *d = (ChannelData24 *)&_rxpacket.st24_data;

                    // Scale from 0..255 to 100%.
                    *rssi = d->rssi * (100.0f / 255.0f);
                    *lost_count = d->lost_count;

                    /* this can lead to rounding of the strides */
                    *channel_count = (max_chan_count < 24) ? max_chan_count : 24;

                    unsigned stride_count = (*channel_count * 3) / 2;
                    unsigned chan_index = 0;

                    for (unsigned i = 0; i < stride_count; i += 3) {
                        channels[chan_index] = ((uint16_t)d->channel[i] << 4);
                        channels[chan_index] |= ((uint16_t)(0xF0 & d->channel[i + 1]) >> 4);
                        /* convert values to 1000-2000 ppm encoding in a not too sloppy fashion */
                        channels[chan_index] = (uint16_t)(channels[chan_index] * ST24_SCALE_FACTOR + .5f) + ST24_SCALE_OFFSET;
                        chan_index++;

                        channels[chan_index] = ((uint16_t)d->channel[i + 2]);
                        channels[chan_index] |= (((uint16_t)(0x0F & d->channel[i + 1])) << 8);
                        /* convert values to 1000-2000 ppm encoding in a not too sloppy fashion */
                        channels[chan_index] = (uint16_t)(channels[chan_index] * ST24_SCALE_FACTOR + .5f) + ST24_SCALE_OFFSET;
                        chan_index++;
                    }
                }
                break;

            case ST24_PACKET_TYPE_TRANSMITTERGPSDATA: {

                    // ReceiverFcPacket* d = (ReceiverFcPacket*)&_rxpacket.st24_data;
                    /* we silently ignore this data for now, as it is unused */
                    ret = 5;
                }
                break;

            default:
                ret = 2;
                break;
            }

        } else {
            /* decoding failed */
            ret = 4;
        }

        _decode_state = ST24_DECODE_STATE_UNSYNCED;
        break;
    }

    return ret;
}