st24.cpp

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/*
  st24 decoder, based on PX4Firmware/src/rc/lib/rc/st24.c from PX4Firmware
  modified for use in AP_HAL_* by Andrew Tridgell
 */
/****************************************************************************
 *
 *   Copyright (c) 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:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name PX4 nor the names of its contributors may be
 *    used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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/*
 * @file st24.cpp
 *
 * RC protocol implementation for Yuneec ST24 transmitter.
 *
 * @author Lorenz Meier <lm@inf.ethz.ch>
 */

#include <stdio.h>
#include <stdint.h>
#include "st24.h"

#define ST24_DATA_LEN_MAX   64
#define ST24_STX1       0x55
#define ST24_STX2       0x55

enum ST24_PACKET_TYPE {
    ST24_PACKET_TYPE_CHANNELDATA12 = 0,
    ST24_PACKET_TYPE_CHANNELDATA24,
    ST24_PACKET_TYPE_TRANSMITTERGPSDATA
};

#pragma pack(push, 1)
typedef struct {
    uint8_t header1;            
    uint8_t header2;            
    uint8_t length;             
    uint8_t type;               
    uint8_t st24_data[ST24_DATA_LEN_MAX];
    uint8_t crc8;               
} ReceiverFcPacket;

typedef struct {
    uint16_t t;         
    uint8_t rssi;           
    uint8_t packet_count;       
    uint8_t channel[18];        
} ChannelData12;

typedef struct {
    uint16_t t;         
    uint8_t rssi;           
    uint8_t packet_count;       
    uint8_t channel[36];        
} ChannelData24;

typedef struct {
    uint16_t t;         
    int32_t lat;            
    int32_t lon;            
    int32_t alt;            
    int16_t vx, vy, vz;         
    uint8_t nsat;           
    uint8_t voltage;        
    uint8_t current;        
    int16_t roll, pitch, yaw;   
    uint8_t motorStatus;        
    uint8_t imuStatus;      
    uint8_t pressCompassStatus; 
} TelemetryData;

#pragma pack(pop)

enum ST24_DECODE_STATE {
    ST24_DECODE_STATE_UNSYNCED = 0,
    ST24_DECODE_STATE_GOT_STX1,
    ST24_DECODE_STATE_GOT_STX2,
    ST24_DECODE_STATE_GOT_LEN,
    ST24_DECODE_STATE_GOT_TYPE,
    ST24_DECODE_STATE_GOT_DATA
};

/* define range mapping here, -+100% -> 1000..2000 */
#define ST24_RANGE_MIN 0.0f
#define ST24_RANGE_MAX 4096.0f

#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;

static 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 *rx_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;

                    *rssi = d->rssi;
                    *rx_count = d->packet_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;

                    *rssi = d->rssi;
                    *rx_count = d->packet_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 = 2;
                }
                break;

            default:
                ret = 2;
                break;
            }

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

        _decode_state = ST24_DECODE_STATE_UNSYNCED;
        break;
    }

    return ret;
}