text: /****************************************************************************
   *
   *   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
   * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
   * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
   * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
   * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
   * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
   * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   * POSSIBILITY OF SUCH DAMAGE.
   *
   ****************************************************************************/
  
  /*
   * @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"
  
  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
  };
  
  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;
  
  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;
  }
  