sumd.cpp

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/*
  SUMD decoder, based on PX4Firmware/src/rc/lib/rc/sumd.c from PX4Firmware
  modified for use in AP_HAL_* by Andrew Tridgell
 */
/****************************************************************************
 *
 *   Copyright (c) 2015 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.
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 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
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 *    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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/*
 * @file sumd.h
 *
 * RC protocol definition for Graupner HoTT transmitter (SUMD/SUMH Protocol)
 *
 * @author Marco Bauer <marco@wtns.de>
 */

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

#define SUMD_MAX_CHANNELS   32
#define SUMD_HEADER_LENGTH  3
#define SUMD_HEADER_ID      0xA8
#define SUMD_ID_SUMH        0x00
#define SUMD_ID_SUMD        0x01
#define SUMD_ID_FAILSAFE    0x81


#pragma pack(push, 1)
typedef struct {
    uint8_t header;                         
    uint8_t status;                         
    uint8_t length;                         
    uint8_t sumd_data[SUMD_MAX_CHANNELS * 2];   
    uint8_t crc16_high;                     
    uint8_t crc16_low;                      
    uint8_t telemetry;                      
    uint8_t crc8;                           
} ReceiverFcPacketHoTT;
#pragma pack(pop)


enum SUMD_DECODE_STATE {
    SUMD_DECODE_STATE_UNSYNCED = 0,
    SUMD_DECODE_STATE_GOT_HEADER,
    SUMD_DECODE_STATE_GOT_STATE,
    SUMD_DECODE_STATE_GOT_LEN,
    SUMD_DECODE_STATE_GOT_DATA,
    SUMD_DECODE_STATE_GOT_CRC,
    SUMD_DECODE_STATE_GOT_CRC16_BYTE_1,
    SUMD_DECODE_STATE_GOT_CRC16_BYTE_2
};

/*
const char *decode_states[] = {"UNSYNCED",
                   "GOT_HEADER",
                   "GOT_STATE",
                   "GOT_LEN",
                   "GOT_DATA",
                   "GOT_CRC",
                   "GOT_CRC16_BYTE_1",
                   "GOT_CRC16_BYTE_2"
                  };
*/

static uint8_t  _crc8   = 0x00;
static uint16_t     _crc16 = 0x0000;
static bool         _sumd   = true;
static bool     _crcOK  = false;
static bool     _debug  = false;


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

#define SUMD_TARGET_MIN 1000.0f
#define SUMD_TARGET_MAX 2000.0f

/* pre-calculate the floating point stuff as far as possible at compile time */
#define SUMD_SCALE_FACTOR ((SUMD_TARGET_MAX - SUMD_TARGET_MIN) / (SUMD_RANGE_MAX - SUMD_RANGE_MIN))
#define SUMD_SCALE_OFFSET (int)(SUMD_TARGET_MIN - (SUMD_SCALE_FACTOR * SUMD_RANGE_MIN + 0.5f))

static enum SUMD_DECODE_STATE _decode_state = SUMD_DECODE_STATE_UNSYNCED;
static uint8_t _rxlen;

static ReceiverFcPacketHoTT _rxpacket;


static uint16_t sumd_crc16(uint16_t crc, uint8_t value)
{
    int i;
    crc ^= (uint16_t)value << 8;

    for (i = 0; i < 8; i++) {
        crc = (crc & 0x8000) ? (crc << 1) ^ 0x1021 : (crc << 1);
    }

    return crc;
}

static uint8_t sumd_crc8(uint8_t crc, uint8_t value)
{
    crc += value;
    return crc;
}

int sumd_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 SUMD_DECODE_STATE_UNSYNCED:
        if (_debug) {
            printf(" SUMD_DECODE_STATE_UNSYNCED \n") ;
        }

        if (byte == SUMD_HEADER_ID) {
            _rxpacket.header = byte;
            _sumd = true;
            _rxlen = 0;
            _crc16 = 0x0000;
            _crc8 = 0x00;
            _crcOK = false;
            _crc16 = sumd_crc16(_crc16, byte);
            _crc8 = sumd_crc8(_crc8, byte);
            _decode_state = SUMD_DECODE_STATE_GOT_HEADER;

            if (_debug) {
                printf(" SUMD_DECODE_STATE_GOT_HEADER: %x \n", byte) ;
            }

        } else {
            ret = 3;
        }

        break;

    case SUMD_DECODE_STATE_GOT_HEADER:
        if (byte == SUMD_ID_SUMD || byte == SUMD_ID_SUMH) {
            _rxpacket.status = byte;

            if (byte == SUMD_ID_SUMH) {
                _sumd = false;
            }

            if (_sumd) {
                _crc16 = sumd_crc16(_crc16, byte);

            } else {
                _crc8 = sumd_crc8(_crc8, byte);
            }

            _decode_state = SUMD_DECODE_STATE_GOT_STATE;

            if (_debug) {
                printf(" SUMD_DECODE_STATE_GOT_STATE: %x \n", byte) ;
            }

        } else {
            _decode_state = SUMD_DECODE_STATE_UNSYNCED;
        }

        break;

    case SUMD_DECODE_STATE_GOT_STATE:
        if (byte >= 2 && byte <= SUMD_MAX_CHANNELS) {
            _rxpacket.length = byte;

            if (_sumd) {
                _crc16 = sumd_crc16(_crc16, byte);

            } else {
                _crc8 = sumd_crc8(_crc8, byte);
            }

            _rxlen++;
            _decode_state = SUMD_DECODE_STATE_GOT_LEN;

            if (_debug) {
                printf(" SUMD_DECODE_STATE_GOT_LEN: %x (%d) \n", byte, byte) ;
            }

        } else {
            _decode_state = SUMD_DECODE_STATE_UNSYNCED;
        }

        break;

    case SUMD_DECODE_STATE_GOT_LEN:
        _rxpacket.sumd_data[_rxlen] = byte;

        if (_sumd) {
            _crc16 = sumd_crc16(_crc16, byte);

        } else {
            _crc8 = sumd_crc8(_crc8, byte);
        }

        _rxlen++;

        if (_rxlen <= ((_rxpacket.length * 2))) {
            if (_debug) {
                printf(" SUMD_DECODE_STATE_GOT_DATA[%d]: %x\n", _rxlen - 2, byte) ;
            }

        } else {
            _decode_state = SUMD_DECODE_STATE_GOT_DATA;

            if (_debug) {
                printf(" SUMD_DECODE_STATE_GOT_DATA -- finish --\n") ;
            }

        }

        break;

    case SUMD_DECODE_STATE_GOT_DATA:
        _rxpacket.crc16_high = byte;

        if (_debug) {
            printf(" SUMD_DECODE_STATE_GOT_CRC16[1]: %x   [%x]\n", byte, ((_crc16 >> 8) & 0xff)) ;
        }

        if (_sumd) {
            _decode_state = SUMD_DECODE_STATE_GOT_CRC;

        } else {
            _decode_state = SUMD_DECODE_STATE_GOT_CRC16_BYTE_1;
        }

        break;

    case SUMD_DECODE_STATE_GOT_CRC16_BYTE_1:
        _rxpacket.crc16_low = byte;

        if (_debug) {
            printf(" SUMD_DECODE_STATE_GOT_CRC16[2]: %x   [%x]\n", byte, (_crc16 & 0xff)) ;
        }

        _decode_state = SUMD_DECODE_STATE_GOT_CRC16_BYTE_2;

        break;

    case SUMD_DECODE_STATE_GOT_CRC16_BYTE_2:
        _rxpacket.telemetry = byte;

        if (_debug) {
            printf(" SUMD_DECODE_STATE_GOT_SUMH_TELEMETRY: %x\n", byte) ;
        }

        _decode_state = SUMD_DECODE_STATE_GOT_CRC;

        break;

    case SUMD_DECODE_STATE_GOT_CRC:
        if (_sumd) {
            _rxpacket.crc16_low = byte;

            if (_debug) {
                printf(" SUMD_DECODE_STATE_GOT_CRC[2]: %x   [%x]\n\n", byte, (_crc16 & 0xff)) ;
            }

            if (_crc16 == (uint16_t)(_rxpacket.crc16_high << 8) + _rxpacket.crc16_low) {
                _crcOK = true;
            }

        } else {
            _rxpacket.crc8 = byte;

            if (_debug) {
                printf(" SUMD_DECODE_STATE_GOT_CRC8_SUMH: %x   [%x]\n\n", byte, _crc8) ;
            }

            if (_crc8 == _rxpacket.crc8) {
                _crcOK = true;
            }
        }

        if (_crcOK) {
            if (_debug) {
                printf(" CRC - OK \n") ;
            }

            if (_sumd) {
                if (_debug) {
                    printf(" Got valid SUMD Packet\n") ;
                }

            } else {
                if (_debug) {
                    printf(" Got valid SUMH Packet\n") ;
                }

            }

            if (_debug) {
                printf(" RXLEN: %d  [Chans: %d] \n\n", _rxlen - 1, (_rxlen - 1) / 2) ;
            }

            ret = 0;
            unsigned i;
            uint8_t _cnt = *rx_count + 1;
            *rx_count = _cnt;

            *rssi = 100;

            /* received Channels */
            if ((uint16_t)_rxpacket.length > max_chan_count) {
                _rxpacket.length = (uint8_t) max_chan_count;
            }

            *channel_count = (uint16_t)_rxpacket.length;

            /* decode the actual packet */
            /* reorder first 4 channels */

            /* ch1 = roll -> sumd = ch2 */
            channels[0] = (uint16_t)((_rxpacket.sumd_data[1 * 2 + 1] << 8) | _rxpacket.sumd_data[1 * 2 + 2]) >> 3;
            /* ch2 = pitch -> sumd = ch2 */
            channels[1] = (uint16_t)((_rxpacket.sumd_data[2 * 2 + 1] << 8) | _rxpacket.sumd_data[2 * 2 + 2]) >> 3;
            /* ch3 = throttle -> sumd = ch2 */
            channels[2] = (uint16_t)((_rxpacket.sumd_data[0 * 2 + 1] << 8) | _rxpacket.sumd_data[0 * 2 + 2]) >> 3;
            /* ch4 = yaw -> sumd = ch2 */
            channels[3] = (uint16_t)((_rxpacket.sumd_data[3 * 2 + 1] << 8) | _rxpacket.sumd_data[3 * 2 + 2]) >> 3;

            /* we start at channel 5(index 4) */
            unsigned chan_index = 4;

            for (i = 4; i < _rxpacket.length; i++) {
                if (_debug) {
                    printf("ch[%d] : %x %x [ %x    %d ]\n", i + 1, _rxpacket.sumd_data[i * 2 + 1], _rxpacket.sumd_data[i * 2 + 2],
                           ((_rxpacket.sumd_data[i * 2 + 1] << 8) | _rxpacket.sumd_data[i * 2 + 2]) >> 3,
                           ((_rxpacket.sumd_data[i * 2 + 1] << 8) | _rxpacket.sumd_data[i * 2 + 2]) >> 3);
                }

                channels[chan_index] = (uint16_t)((_rxpacket.sumd_data[i * 2 + 1] << 8) | _rxpacket.sumd_data[i * 2 + 2]) >> 3;
                /* convert values to 1000-2000 ppm encoding in a not too sloppy fashion */
                //channels[chan_index] = (uint16_t)(channels[chan_index] * SUMD_SCALE_FACTOR + .5f) + SUMD_SCALE_OFFSET;

                chan_index++;
            }

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

            if (_debug) {
                printf(" CRC - fail \n") ;
            }

        }

        _decode_state = SUMD_DECODE_STATE_UNSYNCED;
        break;
    }

    return ret;
}