arrow-firmware/components/aprs/aprs_decoder.c

/*
 * aprs_decoder.c
 *
 *  Created on: Aug 25, 2019
 *      Author: curiousmuch
 */

#include "aprs_decoder.h"

/* Public Variables */
decoder_varibles_t v;

/* Private Functions */
uint32_t fcs_calc(uint8_t *data, uint32_t len)
{
	uint32_t crc, aprs_polynomial, i, j, byte_lsb, crc_lsb;
	crc = 0xFFFF;
	aprs_polynomial = 0x8408;
	for (i=0;i<len;i++)
	{
		for (j=0;j<8;j++)
		{
			byte_lsb = (data[i] >> j) & 0x01;
			crc_lsb = crc & 0x0001;
			if (crc_lsb != byte_lsb)
				crc = (crc >> 1) ^ aprs_polynomial;
			else
				crc = crc >> 1;
		}
	}
	crc = crc ^ 0xFFFF;
	return crc & 0xFFFF;
}

void frame_buffer_init(uint8_t *buf, uint32_t len)
{
	v.frame_buffer = buf;
	v.frame_buffer_len = len;
}

void aprs_decoder_reset(void)
{
	v.decoder_state = FLAG_SEARCH;
	v.frame_buffer_index = 0;
	//v.frame_max_len = APRS_MAX_FRAME;
	v.flag_buffer = 0;
	v.flag_buffer_index = 0;
	v.byte_buffer = 0;
	v.byte_buffer_index = 0;
	v.current_nrzi_bit = 0;
	v.previous_nrzi_bit = 0;
	v.current_bit = 0;
	v.one_count = 0;
	v.skip_bit_flag = 0;
	v.packet_rx = 0;
}

uint8_t get_rx_status(void)
{
	if (v.packet_rx)
		return 1;
	else
		return 0;
}

uint8_t flag_found(void)
{
	if (v.flag_buffer == 0x7E)
	{
		return 1;
	}
	else
	{
		return 0;
	}
}

decoder_output_t ax25_decoder_feed_bit(uint8_t nrzi_bit)
{
	v.current_nrzi_bit = nrzi_bit;

	// decoder NRZI
	if (v.previous_nrzi_bit == v.current_nrzi_bit)
	{
		v.current_bit = 1;
		v.one_count += 1;
	}
	else
	{
		v.current_bit = 0;
		v.one_count = 0;
	}
	v.previous_nrzi_bit = v.current_nrzi_bit;

	// load bit into flag buffer
	v.flag_buffer = (v.flag_buffer >> 1) + (v.current_bit*0x80);

	switch(v.decoder_state)
	{
		case FLAG_SEARCH:
			if (flag_found())
			{
				// set state variable
				v.decoder_state = FLAG_FOUND;
				v.packet_rx = 1;

				// re-initialize buffer indexes
				v.flag_buffer_index = 0;
				v.frame_buffer_index = 0;
			}
			else
			{
				v.decoder_state = FLAG_SEARCH;
				v.packet_rx = 0;
			}
			break;
		case FLAG_FOUND:
			if (v.flag_buffer_index == 7)	// check every 8 bits for flag again
			{
				if (flag_found())	// if flag is found, payload hasn't started
				{
					v.decoder_state = FLAG_FOUND;
					v.flag_buffer_index = 0;
				}
				else
				{
					v.decoder_state = FRAME_START;

					// load current bits in byte buffer and remove 0 stuffing
					uint8_t i, bit;
					v.skip_bit_flag = 0;
					v.byte_buffer = 0;
					v.byte_buffer_index = 0;
					v.one_count = 0;

					for (i=0;i<7;i++)
					{
						bit = (v.flag_buffer << i) & 0x80; // load bit

						// count ones for to remove bit stuffing
						if (bit)
							v.one_count += 1;
						else
							v.one_count = 0;

						// skip bit or store in v.byte_buffer
						if (v.skip_bit_flag)
						{
							v.skip_bit_flag = 0;

							// if "0" is not stuffed packet is invalid
							if (bit != 0)
							{
								v.decoder_state = ABORT;
								break;
							}
						}
						else
						{
							v.byte_buffer |=  (bit) ? (0x80>>i): 0;
							v.byte_buffer_index += 1;
						}

						if (v.one_count == 5)
							v.skip_bit_flag = 1;
					}

					// check if byte buffer is full
					if (v.byte_buffer_index == 7)
					{
						v.frame_buffer[v.frame_buffer_index] = v.byte_buffer;
						v.byte_buffer = 0;
						v.byte_buffer_index = 0;
						v.frame_buffer_index += 1;
					}

					break;
				}
			}
			else
			{
				v.flag_buffer_index += 1;
			}
			break;
		case FRAME_START:
			// skip stuffed "0"
			if (v.skip_bit_flag == 1)
			{
				v.skip_bit_flag = 0;

				// bit will only not be stuffed properly if the HDLC flag is being received
				// indicating the end of the frame or the packet has become corrupted.
				if (v.current_bit != 0)
				{
					v.decoder_state = FRAME_BREAK;
					break;
				}
			}
			else
			{
				// load bit
				v.byte_buffer |= v.current_bit*(0x01<<v.byte_buffer_index);
				v.byte_buffer_index += 1;

				// check if byte buffer is full
				if (v.byte_buffer_index > 7)
				{
					v.frame_buffer[v.frame_buffer_index] = v.byte_buffer;
					v.byte_buffer = 0;
					v.byte_buffer_index = 0;
					v.frame_buffer_index += 1;

					// check for overflow
					if (v.frame_buffer_index >= v.frame_buffer_len)
					{
						v.decoder_state = ABORT;
						break;
					}
				}

			}

			if(v.one_count == 5)
				v.skip_bit_flag = 1;
			break;
		default:
			// we're in trouble.
			break;
	}

	if (v.decoder_state == FRAME_BREAK)
	{
		// state
		v.decoder_state = FLAG_SEARCH;

		// calculate CRC
		volatile uint32_t fcs, frame_fcs;

		if (v.frame_buffer_index < 10) // TODO: Determine minimum APRS packet
			return ERROR_PACKET_FORMAT;

		v.frame_len = v.frame_buffer_index - 2;
		fcs = fcs_calc(v.frame_buffer, v.frame_len);

		frame_fcs = (v.frame_buffer[v.frame_len+1]<<8) |
				v.frame_buffer[v.frame_len];

		if (fcs == frame_fcs)
			return FRAME_DECODED;
		else
			return ERROR_FCS_MISMATCH;
	}

	if (v.decoder_state == ABORT)
	{
		// re-initialize state machine
		aprs_decoder_init();
	}
	return NORMAL;
}