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/*
* @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 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 unsigned _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);
}
uint8_t st24_decode(uint8_t byte, uint8_t *rssi, uint8_t* rx_count, uint16_t *channel_count, int16_t *channels, uint16_t max_chan_count)
{
bool ret = false;
switch (_decode_state) {
case ST24_DECODE_STATE_UNSYNCED:
if (byte == ST24_STX1) {
_decode_state = ST24_DECODE_STATE_GOT_STX1;
}
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 = true;
/* 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;
*channel_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;
*channel_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;
/* we silently ignore this data for now, as its not classic TX data */
ret = false;
}
break;
default:
ret = false;
break;
}
} else {
/* decoding failed */
}
_decode_state = ST24_DECODE_STATE_UNSYNCED;
break;
}
return !ret;
}
