srxl.cpp

Go to the documentation of this file.

/*
   This program 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 program 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/>.
 */
/*
  SRXL protocol decoder, tested against AR7700 SRXL port
  Andrew Tridgell, September 2016

  Co author: Roman Kirchner, September 2016
   - 2016.10.23: SRXL variant V1 sucessfully (Testbench and Pixhawk/MissionPlanner) tested with RX-9-DR M-LINK (SW v1.26)
 */

#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include "srxl.h"



/* SRXL datastream characteristics for all variants */
#define SRXL_MIN_FRAMESPACE_US 8000U    /* Minumum space between srxl frames in us (applies to all variants)  */
#define SRXL_MAX_CHANNELS 20U           /* Maximum number of channels from srxl datastream  */

/* decode progress states */
#define STATE_IDLE              0x00U       /* do nothing */
#define STATE_NEW               0x01U       /* get header of frame + prepare for frame reception + begin new crc cycle   */
#define STATE_COLLECT           0x02U       /* collect RC channel data from frame + concurrently calc crc over payload data + extract channel information */


/* Variant specific SRXL datastream characteristics */
/* Framelength in byte */
#define SRXL_FRAMELEN_V1    27U      /* Framelength with header in byte for:  Mpx SRXLv1 or XBUS Mode B */
#define SRXL_FRAMELEN_V2    35U      /* Framelength with header in byte for:  Mpx SRXLv2 */
#define SRXL_FRAMELEN_V5    18U      /* Framelength with header in byte for  Spk AR7700 etc. */
#define SRXL_FRAMELEN_MAX   35U      /* maximum possible framelengh  */

/* Headerbyte */
#define SRXL_HEADER_V1          0xA1U    /* Headerbyte for:  Mpx SRXLv1 or XBUS Mode B */
#define SRXL_HEADER_V2          0xA2U    /* Headerbyte for:  Mpx SRXLv2  */
#define SRXL_HEADER_V5          0xA5U    /* Headerbyte for:  Spk AR7700 etc. */
#define SRXL_HEADER_NOT_IMPL    0xFFU    /* Headerbyte for non impemented srxl header*/




static uint8_t buffer[SRXL_FRAMELEN_MAX];       /* buffer for raw srxl frame data in correct order --> buffer[0]=byte0  buffer[1]=byte1  */
static uint8_t buflen;                          /* length in number of bytes of received srxl dataframe in buffer  */
static uint64_t last_data_us;                   /* timespan since last received data in us   */
static uint16_t channels[SRXL_MAX_CHANNELS];    /* buffer for extracted RC channel data as pulsewidth in microseconds */

static uint8_t frame_header = 0U;                   /* Frame header from SRXL datastream    */
static uint8_t frame_len_full = 0U;                 /* Length in number of bytes of full srxl datastream */
static uint8_t decode_state = STATE_IDLE;           /* Current state of SRXL frame decoding */
static uint8_t decode_state_next = STATE_IDLE;      /* State of frame decoding thatwill be applied when the next byte from dataframe drops in  */
static uint16_t crc_fmu = 0U;                       /* CRC calculated over payload from srxl datastream on this machine */
static uint16_t crc_receiver = 0U;                  /* CRC extracted from srxl datastream  */


static uint16_t max_channels;




static uint16_t srxl_crc16 (uint16_t crc, uint8_t new_byte)
{
    uint8_t loop;
    crc = crc ^ (uint16_t)new_byte << 8;
    for(loop = 0; loop < 8; loop++) {
        crc = (crc & 0x8000) ? (crc << 1) ^ 0x1021 : (crc << 1);
    }
    return crc;
}


static int srxl_channels_get_v1v2(uint16_t max_values, uint8_t *num_values, uint16_t *values, bool *failsafe_state)
{
    uint8_t loop;
    uint32_t channel_raw_value;

    *num_values = (uint8_t)((frame_len_full - 3U)/2U);
    *failsafe_state = 0U;    /* this protocol version does not support failsafe information */

    /* get data channel data from frame */
    for (loop=0U; loop < *num_values; loop++) {
        channel_raw_value = ((((uint32_t)buffer[loop*2U+1U])& 0x0000000FU) << 8U)  | ((uint32_t)(buffer[loop*2U+2U]));   /* get 12bit channel raw value from srxl datastream (mask out unused bits with 0x0000000F)  */
        channels[loop] = (uint16_t)(((channel_raw_value * (uint32_t)1400U) >> 12U) + (uint32_t)800U);                    /* convert raw value to servo/esc signal pulsewidth in us */
    }

    /* provide channel data to FMU */
    if ( (uint16_t)*num_values > max_values) {
        *num_values = (uint8_t)max_values;
    }
    memcpy(values, channels, (*num_values)*2);

    return 0;   /* for srxl protocol version 1 and 2 it is not expected, that any error happen during decode process  */
    }


static int srxl_channels_get_v5(uint16_t max_values, uint8_t *num_values, uint16_t *values, bool *failsafe_state)
{
    // up to 7 channel values per packet. Each channel value is 16
    // bits, with 11 bits of data and 4 bits of channel number. The
    // top bit indicates a special X-Plus channel
    for (uint8_t i=0; i<7; i++) {
        uint16_t b = buffer[i*2+2] << 8 | buffer[i*2+3];
        uint16_t c = b >> 11; // channel number
        int32_t v = b & 0x7FF;
        if (b & 0x8000) {
            continue;
        } 
        if (c == 12) {
            // special handling for channel 12
            // see http://www.deviationtx.com/forum/protocol-development/2088-18-channels-for-dsm2-dsmx?start=40
            //printf("c12: 0x%x %02x %02x\n", (unsigned)(b>>9), (unsigned)buffer[0], (unsigned)buffer[1]);
            v = (b & 0x1FF) << 2;
            c = 10 + ((b >> 9) & 0x7);
            if (buffer[1] & 1) {
                c += 4;
            }
#if 0
            printf("b=0x%04x v=%u c=%u b[1]=0x%02x\n",
                   (unsigned)b, (unsigned)v, (unsigned)c, (unsigned)buffer[1]);
#endif
        } else if (c > 12) {
            // invalid
            v = 0;
        }

        // if channel number if greater than 16 then it is a X-Plus
        // channel. We don't yet know how to decode those. There is some information here:
        // http://www.deviationtx.com/forum/protocol-development/2088-18-channels-for-dsm2-dsmx?start=40
        // but we really need some sample data to confirm
        if (c < SRXL_MAX_CHANNELS) {
            v = (((v - 0x400) * 500) / 876) + 1500;
            channels[c] = v;
            if (c >= max_channels) {
                max_channels = c+1;
            }
        }
        
        //printf("%u:%u ", (unsigned)c, (unsigned)v);
    }
    //printf("\n");

    *num_values = max_channels;
    if (*num_values > max_values) {
        *num_values = max_values;
    }
    memcpy(values, channels, (*num_values)*2);

    // check failsafe bit, this goes low when connection to the
    // transmitter is lost
    *failsafe_state = ((buffer[1] & 2) == 0);
    
    // success
    return 0;
    }




int srxl_decode(uint64_t timestamp_us, uint8_t byte, uint8_t *num_values, uint16_t *values, uint16_t max_values, bool *failsafe_state)
{
    int ret = 1;

    /*----------------------------------------distinguish different srxl variants at the beginning of each frame---------------------------------------------- */
    /* Check if we have a new begin of a frame --> indicators: Time gap in datastream + SRXL header 0xA<VARIANT>*/
    if ( (timestamp_us - last_data_us) >= SRXL_MIN_FRAMESPACE_US) {
        /* Now detect SRXL variant based on header */
        switch(byte) {
        case SRXL_HEADER_V1:
            frame_len_full = SRXL_FRAMELEN_V1;
            frame_header = SRXL_HEADER_V1;
            decode_state = STATE_NEW;
            break;
        case SRXL_HEADER_V2:
            frame_len_full = SRXL_FRAMELEN_V2;
            frame_header = SRXL_HEADER_V2;
            decode_state = STATE_NEW;
            break;
        case SRXL_HEADER_V5:
            frame_len_full = SRXL_FRAMELEN_V5;
            frame_header = SRXL_HEADER_V5;
            decode_state = STATE_NEW;
            break;
        default:
            frame_len_full = 0U;
            frame_header = SRXL_HEADER_NOT_IMPL;
            decode_state = STATE_IDLE;
            buflen = 0;
            return 2; /* protocol version not implemented --> no channel data --> unknown packet  */
        }
    }


    /*--------------------------------------------collect all data from stream and decode-------------------------------------------------------*/
    switch (decode_state) {
    case STATE_NEW:   /* buffer header byte and prepare for frame reception and decoding */
        buffer[0U]=byte;
        crc_fmu = srxl_crc16(0U,byte);
        buflen = 1U;
        decode_state_next = STATE_COLLECT;
        break;

    case STATE_COLLECT: /* receive all bytes. After reception decode frame and provide rc channel information to FMU   */
        if (buflen >= frame_len_full) {
            // a logic bug in the state machine, this shouldn't happen
            decode_state = STATE_IDLE;
            buflen = 0;
            frame_len_full = 0;
            frame_header = SRXL_HEADER_NOT_IMPL;
            return 2;
        }
        buffer[buflen] = byte;
        buflen++;
        /* CRC not over last 2 frame bytes as these bytes inhabitate the crc */
        if (buflen <= (frame_len_full-2)) {
           crc_fmu = srxl_crc16(crc_fmu,byte);
        }
        if(  buflen == frame_len_full ) {
            /* CRC check here */
            crc_receiver =  ((uint16_t)buffer[buflen-2] << 8U) | ((uint16_t)buffer[buflen-1]);
            if (crc_receiver == crc_fmu) {
                /* at this point buffer contains all frame data and crc is valid --> extract channel info according to SRXL variant */
                switch (frame_header) {
                case SRXL_HEADER_V1:
                    ret = srxl_channels_get_v1v2(max_values, num_values, values, failsafe_state);
                    break;
                case SRXL_HEADER_V2:
                    ret = srxl_channels_get_v1v2(max_values, num_values, values, failsafe_state);
                    break;
                case SRXL_HEADER_V5:
                    ret = srxl_channels_get_v5(max_values, num_values, values, failsafe_state);
                    break;
                default:
                    ret = 2; /* protocol version not implemented --> no channel data  */
                    break;
                }
            }
            else {
                ret = 4; /* CRC fail --> no channel data */
            }
            decode_state_next = STATE_IDLE; /* frame data buffering and decoding finished --> statemachine not in use until new header drops is */
        }
        else {
            /* frame not completely received --> frame data buffering still ongoing  */
            decode_state_next = STATE_COLLECT;
        }
        break;

    default:
        ret = 1; /* STATE_IDLE --> do nothing */
        break;
    } /* switch (decode_state) */

   decode_state = decode_state_next;
   last_data_us = timestamp_us;
   return ret;
}


#ifdef TEST_MAIN_PROGRAM
/*
  test harness for use under Linux with USB serial adapter
 */
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <time.h>
#include <unistd.h>
#include <stdlib.h>
#include <termios.h>

static uint64_t micros64(void)
{
    struct timespec ts;
    clock_gettime(CLOCK_MONOTONIC, &ts);
    return 1.0e6*((ts.tv_sec + (ts.tv_nsec*1.0e-9)));
}

int main(int argc, const char *argv[])
{
    int fd = open(argv[1], O_RDONLY|O_CLOEXEC);
    if (fd == -1) {
        perror(argv[1]);
        exit(1);
    }

    struct termios options;

    tcgetattr(fd, &options);

    cfsetispeed(&options, B115200);
    cfsetospeed(&options, B115200);

    options.c_cflag &= ~(PARENB|CSTOPB|CSIZE);
    options.c_cflag |= CS8;

    options.c_lflag &= ~(ICANON|ECHO|ECHOE|ISIG);
    options.c_iflag &= ~(IXON|IXOFF|IXANY);
    options.c_oflag &= ~OPOST;

    if (tcsetattr(fd, TCSANOW, &options) != 0) {
        perror("tcsetattr");
        exit(1);
    }
    tcflush(fd, TCIOFLUSH);
    
    while (true) {
        uint8_t b;
        uint8_t num_values = 0;
        uint16_t values[20];
        bool failsafe_state;
        fd_set fds;
        struct timeval tv;
    
        FD_ZERO(&fds);
        FD_SET(fd, &fds);

        tv.tv_sec = 1;
        tv.tv_usec = 0;

        // check if any bytes are available
        if (select(fd+1, &fds, NULL, NULL, &tv) != 1) {
            break;
        }
        
        if (read(fd, &b, 1) != 1) {
            break;
        }

        if (srxl_decode(micros64(), b, &num_values, values, sizeof(values)/sizeof(values[0]), &failsafe_state) == 0) {
#if 1
            printf("%u: ", num_values);
            for (uint8_t i=0; i<num_values; i++) {
                printf("%u:%4u ", i+1, values[i]);
            }
            printf("%s\n", failsafe_state?"FAIL":"OK");
#endif
        }
    }
}
#endif


#ifdef TEST_BIN_PROGRAM
/*
  test harness for use under Linux with hex dump in a file
 */
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <time.h>
#include <unistd.h>
#include <stdlib.h>
#include <termios.h>

int main(int argc, const char *argv[])
{
    FILE *f = fopen(argv[1], "r");
    if (!f) {
        perror(argv[1]);
        exit(1);
    }

    uint64_t t=0;
    while (true) {
        uint8_t b;
        uint8_t num_values = 0;
        uint16_t values[20];
        bool failsafe_state;


        uint8_t header;
        if (fread(&header, 1, 1, f) != 1) {
            break;
        }

        uint8_t frame_size = 0;
        switch (header) {
        case SRXL_HEADER_V1:
            frame_size = SRXL_FRAMELEN_V1;
            break;
        case SRXL_HEADER_V2:
            frame_size = SRXL_FRAMELEN_V2;
            break;
        case SRXL_HEADER_V5:
            frame_size = SRXL_FRAMELEN_V5;
            break;
        }
        if (frame_size == 0) {
            continue;
        }
        uint8_t u[frame_size];
        u[0] = header;
        if (fread(&u[1], 1, sizeof(u)-1, f) != sizeof(u)-1) {
            break;
        }
        t += 11000;

        for (uint8_t i=0; i<sizeof(u); i++) {
            b = u[i];

            if (srxl_decode(t, b, &num_values, values, sizeof(values)/sizeof(values[0]), &failsafe_state) == 0) {
                printf("%u: ", num_values);
                for (uint8_t i=0; i<num_values; i++) {
                    printf("%u:%4u ", i+1, values[i]);
                }
                printf("%s\n", failsafe_state?"FAIL":"OK");
            }
        }
    }
}
#endif