PX4-Autopilot/src/lib/rc/dsm.cpp

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/**
 * @file dsm.cpp
 *
 * Serial protocol decoder for the Spektrum DSM* family of protocols.
 *
 * Decodes into the global PPM buffer and updates accordingly.
 */

#include <px4_platform_common/px4_config.h>
#include <board_config.h>
#include <px4_platform_common/defines.h>

#include <fcntl.h>
#include <math.h>
#include <unistd.h>
#include <termios.h>
#include <string.h>

#include "dsm.h"
#include "spektrum_rssi.h"
#include "common_rc.h"
#include <drivers/drv_hrt.h>

#include <include/containers/Bitset.hpp>

#if defined(__PX4_NUTTX)
#include <nuttx/arch.h>
#define dsm_udelay(arg)    up_udelay(arg)
#else
#define dsm_udelay(arg) px4_usleep(arg)
#endif

// #define DSM_DEBUG

static enum DSM_DECODE_STATE {
	DSM_DECODE_STATE_DESYNC = 0,
	DSM_DECODE_STATE_SYNC
} dsm_decode_state = DSM_DECODE_STATE_DESYNC;

static int dsm_fd = -1;						/**< File handle to the DSM UART */
static hrt_abstime dsm_last_rx_time;            /**< Timestamp when we last received data */
static hrt_abstime dsm_last_frame_time;		/**< Timestamp for start of last valid dsm frame */
static dsm_frame_t &dsm_frame = rc_decode_buf.dsm.frame;	/**< DSM_BUFFER_SIZE DSM dsm frame receive buffer */
static dsm_buf_t &dsm_buf = rc_decode_buf.dsm.buf;	/**< DSM_BUFFER_SIZE DSM dsm frame receive buffer */

static uint16_t dsm_chan_buf[DSM_MAX_CHANNEL_COUNT];
static unsigned dsm_partial_frame_count;	/**< Count of bytes received for current dsm frame */
static unsigned dsm_channel_shift = 0;			/**< Channel resolution, 0=unknown, 10=10 bit (1024), 11=11 bit (2048) */
static unsigned dsm_frame_drops = 0;			/**< Count of incomplete DSM frames */
static uint16_t dsm_chan_count = 0;         /**< DSM channel count */
static uint16_t dsm_chan_count_prev = 0;    /**< last valid DSM channel count */

/**
 * Attempt to decode a single channel raw channel datum
 *
 * The DSM* protocol doesn't provide any explicit framing,
 * so we detect dsm frame boundaries by the inter-dsm frame delay.
 *
 * The minimum dsm frame spacing is 11ms; with 16 bytes at 115200bps
 * dsm frame transmission time is ~1.4ms.
 *
 * We expect to only be called when bytes arrive for processing,
 * and if an interval of more than 5ms passes between calls,
 * the first byte we read will be the first byte of a dsm frame.
 *
 * In the case where byte(s) are dropped from a dsm frame, this also
 * provides a degree of protection. Of course, it would be better
 * if we didn't drop bytes...
 *
 * Upon receiving a full dsm frame we attempt to decode it
 *
 * @param[in] raw 16 bit raw channel value from dsm frame
 * @param[in] shift position of channel number in raw data
 * @param[out] channel pointer to returned channel number
 * @param[out] value pointer to returned channel value
 * @return true=raw value successfully decoded
 */
static bool dsm_decode_channel(uint16_t raw, unsigned shift, uint8_t &channel, uint16_t &value)
{
	if (raw == 0 || raw == 0xffff) {
		return false;
	}

	if (shift == 10) {
		// 1024 Mode: This format is used only by DSM2/22ms mode. All other modes use 2048 data.
		//  Bits 15-10 Channel ID
		//  Bits 9-0   Servo Position
		static constexpr uint16_t MASK_1024_CHANID = 0xFC00;
		static constexpr uint16_t MASK_1024_SXPOS = 0x03FF;

		channel = (raw & MASK_1024_CHANID) >> 10; // 6 bits

		const uint16_t servo_position = (raw & MASK_1024_SXPOS); // 10 bits

		if (channel > DSM_MAX_CHANNEL_COUNT) {
			PX4_DEBUG("invalid channel: %d\n", channel);
			return false;
		}

		// PWM_OUT = (ServoPosition x 1.166μs) + Offset
		static constexpr uint16_t offset = 903; // microseconds
		value = roundf(servo_position * 1.166f) + offset;

		// Spektrum range is 903μs to 2097μs (Specification for Spektrum Remote Receiver Interfacing Rev G 9.1)
		//  ±100% travel is 1102µs to 1898 µs
		if (value < 903 || value > 2097) {
			// if the value is unrealistic, fail the parsing entirely
			PX4_DEBUG("channel %d invalid range %d", channel, value);
			return false;
		}

		return true;

	} else if (shift == 11) {
		// 2048 Mode
		//  Bits 15    Servo Phase
		//  Bits 14-11 Channel ID
		//  Bits 10-0  Servo Position

		uint16_t servo_position = 0;

		// from Spektrum Remote Receiver Interfacing Rev G Page 6
		const bool phase = raw & (2 >> 15); // the phase is part of the X-Plus address (bit 15)
		uint8_t chan = (raw >> 11) & 0x0F;

		if (chan < 12) {
			// Normal channels
			static constexpr uint16_t MASK_2048_SXPOS = 0x07FF;
			servo_position = (raw & MASK_2048_SXPOS);

		} else if (chan == 12) {
			// XPlus channels
			chan += ((raw >> 9) & 0x03);

			if (phase) {
				chan += 4;
			}

			if (chan > DSM_MAX_CHANNEL_COUNT) {
				PX4_DEBUG("invalid channel: %d\n", chan);
				return false;
			}

			servo_position = (raw & 0x01FF) << 2;

			channel = chan;

		} else {
			PX4_DEBUG("invalid channel: %d\n", chan);
			return false;
		}

		channel = chan;

		// PWM_OUT = (ServoPosition x 0.583μs) + Offset
		static constexpr uint16_t offset = 903; // microseconds
		value = roundf(servo_position * 0.583f) + offset;

		// Spektrum range is 903μs to 2097μs (Specification for Spektrum Remote Receiver Interfacing Rev G 9.1)
		//  ±100% travel is 1102µs to 1898 µs
		if (value < 903 || value > 2097) {
			// if the value is unrealistic, fail the parsing entirely
			PX4_DEBUG("channel %d invalid range %d", channel, value);
			return false;
		}

		PX4_DEBUG(stderr, "CH%d=%d(0x%02x), ", channel, value, raw);

		return true;
	}

	return false;
}

/**
 * Attempt to guess if receiving 10 or 11 bit channel values
 *
 * @param[in] reset true=reset the 10/11 bit state to unknown
 */
static bool dsm_guess_format(bool reset)
{
	static uint32_t	cs10 = 0;
	static uint32_t	cs11 = 0;
	static unsigned samples = 0;

	/* reset the 10/11 bit sniffed channel masks */
	if (reset) {
		PX4_DEBUG("dsm_guess_format reset");
		cs10 = 0;
		cs11 = 0;
		samples = 0;
		dsm_channel_shift = 0;
		return false;
	}

	px4::Bitset<DSM_MAX_CHANNEL_COUNT> channels_found_10;
	px4::Bitset<DSM_MAX_CHANNEL_COUNT> channels_found_11;

	bool cs10_frame_valid = true;
	bool cs11_frame_valid = true;

	/* scan the channels in the current dsm_frame in both 10- and 11-bit mode */
	for (unsigned i = 0; i < DSM_FRAME_CHANNELS; i++) {

		uint8_t *dp = &dsm_frame[2 + (2 * i)];
		uint16_t raw = (dp[0] << 8) | dp[1];

		uint8_t channel = 0;
		uint16_t value = 0;

		/* if the channel decodes, remember the assigned number */
		if (dsm_decode_channel(raw, 10, channel, value)) {
			// invalidate entire frame (for 1024) if channel already found, no duplicate channels per DSM frame
			if (channels_found_10[channel]) {
				cs10_frame_valid = false;

			} else {
				channels_found_10.set(channel);
			}
		}

		if (dsm_decode_channel(raw, 11, channel, value)) {
			// invalidate entire frame (for 2048) if channel already found, no duplicate channels per DSM frame
			if (channels_found_11[channel]) {
				cs11_frame_valid = false;

			} else {
				channels_found_11.set(channel);
			}
		}
	}

	// add valid cs10 channels
	if (cs10_frame_valid) {
		for (unsigned channel = 0; channel < DSM_FRAME_CHANNELS; channel++) {
			if (channels_found_10[channel]) {
				cs10 |= 1 << channel;
			}
		}
	}

	// add valid cs11 channels
	if (cs11_frame_valid) {
		for (unsigned channel = 0; channel < DSM_FRAME_CHANNELS; channel++) {
			if (channels_found_11[channel]) {
				cs11 |= 1 << channel;
			}
		}
	}

	samples++;

#ifdef DSM_DEBUG
	printf("dsm guess format: samples: %d %s\n", samples, (reset) ? "RESET" : "");
#endif

	/* wait until we have seen plenty of frames */
	if (samples < 10) {
		return false;
	}

	/*
	 * Iterate the set of sensible sniffed channel sets and see whether
	 * decoding in 10 or 11-bit mode has yielded anything we recognize.
	 *
	 * XXX Note that due to what seem to be bugs in the DSM2 high-resolution
	 *     stream, we may want to sniff for longer in some cases when we think we
	 *     are talking to a DSM2 receiver in high-resolution mode (so that we can
	 *     reject it, ideally).
	 */
	static uint32_t masks[] = {
		0x3f,	/* 6 channels (DX6) */
		0x7f,	/* 7 channels (DX7) */
		0xff,	/* 8 channels (DX8) */
		0x1ff,	/* 9 channels (DX9, etc.) */
		0x3ff,	/* 10 channels (DX10) */
		0x1fff,	/* 13 channels (DX10t) */
		0xffff,	/* 16 channels */
		0x3ffff,/* 18 channels (DX10) */
	};

	unsigned votes10 = 0;
	unsigned votes11 = 0;

	for (unsigned i = 0; i < (sizeof(masks) / sizeof(masks[0])); i++) {

		if (cs10 == masks[i]) {
			votes10++;
		}

		if (cs11 == masks[i]) {
			votes11++;
		}
	}

	if ((votes11 == 1) && (votes10 == 0)) {
		dsm_channel_shift = 11;
#ifdef DSM_DEBUG
		printf("DSM: 11-bit format\n");
#endif
		return true;
	}

	if ((votes10 == 1) && (votes11 == 0)) {
		dsm_channel_shift = 10;
#ifdef DSM_DEBUG
		printf("DSM: 10-bit format\n");
#endif
		return true;
	}

	/* call ourselves to reset our state ... we have to try again */
#ifdef DSM_DEBUG
	printf("DSM: format detect fail, 10: 0x%08x %d 11: 0x%08x %d\n", cs10, votes10, cs11, votes11);
#endif
	dsm_guess_format(true);
	return false;
}

int dsm_config(int fd)
{
#ifdef SPEKTRUM_POWER_CONFIG
	// Enable power controls for Spektrum receiver
	SPEKTRUM_POWER_CONFIG();
#endif
#ifdef SPEKTRUM_POWER
	// enable power on DSM connector
	SPEKTRUM_POWER(true);
#endif

	int ret = -1;

	if (fd >= 0) {

		struct termios t;

		/* 115200bps, no parity, one stop bit */
		tcgetattr(fd, &t);
		cfsetspeed(&t, 115200);
		t.c_cflag &= ~(CSTOPB | PARENB);
		tcsetattr(fd, TCSANOW, &t);

		/* initialise the decoder */
		dsm_partial_frame_count = 0;
		dsm_last_rx_time = hrt_absolute_time();

		/* reset the format detector */
		dsm_guess_format(true);

		ret = 0;
	}

	return ret;
}

void dsm_proto_init()
{
	dsm_channel_shift = 0;
	dsm_frame_drops = 0;
	dsm_chan_count = 0;
	dsm_decode_state = DSM_DECODE_STATE_DESYNC;

	for (unsigned i = 0; i < DSM_MAX_CHANNEL_COUNT; i++) {
		dsm_chan_buf[i] = 0;
	}
}

/**
 * Initialize the DSM receive functionality
 *
 * Open the UART for receiving DSM frames and configure it appropriately
 *
 * @param[in] device Device name of DSM UART
 */
int dsm_init(const char *device)
{
	if (dsm_fd < 0) {
		dsm_fd = open(device, O_RDWR | O_NONBLOCK);
	}

	dsm_proto_init();

	int ret = dsm_config(dsm_fd);

	if (!ret) {
		return dsm_fd;

	} else {
		return -1;
	}
}

void dsm_deinit()
{
#ifdef SPEKTRUM_POWER_PASSIVE
	// Turn power controls to passive
	SPEKTRUM_POWER_PASSIVE();
#endif

	if (dsm_fd >= 0) {
		close(dsm_fd);
	}

	dsm_fd = -1;
}

#if defined(SPEKTRUM_POWER)
/**
 * Handle DSM satellite receiver bind mode handler
 *
 * @param[in] cmd commands - dsm_bind_power_down, dsm_bind_power_up, dsm_bind_set_rx_out, dsm_bind_send_pulses, dsm_bind_reinit_uart
 * @param[in] pulses Number of pulses for dsm_bind_send_pulses command
 */
void dsm_bind(uint16_t cmd, int pulses)
{
	if (dsm_fd < 0) {
		return;
	}

	switch (cmd) {
	case DSM_CMD_BIND_POWER_DOWN:
		// power down DSM satellite
#if defined(DSM_DEBUG)
		printf("DSM: DSM_CMD_BIND_POWER_DOWN\n");
#endif
		SPEKTRUM_POWER(false);
		break;

	case DSM_CMD_BIND_POWER_UP:
		// power up DSM satellite
#if defined(DSM_DEBUG)
		printf("DSM: DSM_CMD_BIND_POWER_UP\n");
#endif
		SPEKTRUM_POWER(true);
		dsm_guess_format(true);
		break;

	case DSM_CMD_BIND_SET_RX_OUT:
		// Set UART RX pin to active output mode
#if defined(DSM_DEBUG)
		printf("DSM: DSM_CMD_BIND_SET_RX_OUT\n");
#endif
		SPEKTRUM_RX_AS_GPIO_OUTPUT();
		break;

	case DSM_CMD_BIND_SEND_PULSES:
		// Pulse RX pin a number of times
#if defined(DSM_DEBUG)
		printf("DSM: DSM_CMD_BIND_SEND_PULSES\n");
#endif

		for (int i = 0; i < pulses; i++) {
			dsm_udelay(120);
			SPEKTRUM_OUT(false);
			dsm_udelay(120);
			SPEKTRUM_OUT(true);
		}

		break;

	case DSM_CMD_BIND_REINIT_UART:
		// Restore USART RX pin to RS232 receive mode
#if defined(DSM_DEBUG)
		printf("DSM: DSM_CMD_BIND_REINIT_UART\n");
#endif
		SPEKTRUM_RX_AS_UART();
		break;

	}
}
#endif

/**
 * Decode the entire dsm frame (all contained channels)
 *
 * @param[in] frame_time timestamp when this dsm frame was received. Used to detect RX loss in order to reset 10/11 bit guess.
 * @param[out] values pointer to per channel array of decoded values
 * @param[out] num_values pointer to number of raw channel values returned
 * @return true=DSM frame successfully decoded, false=no update
 */
bool dsm_decode(hrt_abstime frame_time, uint16_t *values, uint16_t *num_values, bool *dsm_11_bit, unsigned max_values,
		int8_t *rssi_percent)
{
	/*
	debug("DSM dsm_frame %02x%02x %02x%02x %02x%02x %02x%02x %02x%02x %02x%02x %02x%02x %02x%02x",
		dsm_frame[0], dsm_frame[1], dsm_frame[2], dsm_frame[3], dsm_frame[4], dsm_frame[5], dsm_frame[6], dsm_frame[7],
		dsm_frame[8], dsm_frame[9], dsm_frame[10], dsm_frame[11], dsm_frame[12], dsm_frame[13], dsm_frame[14], dsm_frame[15]);
	*/
	/*
	 * If we have lost signal for at least a second, reset the
	 * format guessing heuristic.
	 */
	if (((frame_time - dsm_last_frame_time) > 1000000) && (dsm_channel_shift != 0)) {
		dsm_guess_format(true);
	}

	/* if we don't know the dsm_frame format, update the guessing state machine */
	if (dsm_channel_shift == 0) {
		if (!dsm_guess_format(false)) {
			return false;
		}
	}

	/*
	 * The first byte represents the rssi in dBm on telemetry receivers with updated
	 * firmware, or fades on others. If the value is less than zero, it's rssi.
	 * We have other ways to detect bad link metrics, so ignore positive values,
	 * but rssi dBm is a useful value.
	 */

	// The SPM4649T with firmware version 1.1RC9 or later will have RSSI in place of fades
	if (rssi_percent) {
		if (((int8_t *)dsm_frame)[0] < 0) {
			/*
			 * RSSI is a signed integer between -42dBm and -92dBm
			 * If signal is lost, the value is -128
			 */
			int8_t dbm = (int8_t)dsm_frame[0];

			if (dbm == -128) {
				*rssi_percent = 0;

			} else {
				*rssi_percent = spek_dbm_to_percent(dbm);
			}

		} else {
			/* if we don't know the rssi, anything over 100 will invalidate it */
			*rssi_percent = 127;
		}
	}

	/*
	 * Each channel is a 16-bit unsigned value containing either a 10-
	 * or 11-bit channel value and a 4-bit channel number, shifted
	 * either 10 or 11 bits. The MSB may also be set to indicate the
	 * second dsm_frame in variants of the protocol where more than
	 * seven channels are being transmitted.
	 */

	px4::Bitset<DSM_MAX_CHANNEL_COUNT> channels_found;

	unsigned channel_decode_failures = 0;

	for (unsigned i = 0; i < DSM_FRAME_CHANNELS; i++) {

		uint8_t *dp = &dsm_frame[2 + (2 * i)];
		uint16_t raw = (dp[0] << 8) | dp[1];

		// ignore
		if (raw == 0 || raw == 0xffff) {
			continue;
		}

		uint8_t channel = 0;
		uint16_t value = 0;

		if (!dsm_decode_channel(raw, dsm_channel_shift, channel, value)) {
			channel_decode_failures++;
			continue;
		}

		// discard entire frame if at least half of it (4 channels) failed to decode
		if (channel_decode_failures >= 4) {
			return false;
		}

		// abort if channel already found, no duplicate channels per DSM frame
		if (channels_found[channel]) {
			PX4_DEBUG("duplicate channel %d\n\n", channel);
			dsm_guess_format(true);
			return false;

		} else {
			channels_found.set(channel);
		}

		/* reset bit guessing state machine if the channel index is out of bounds */
		if (channel > DSM_MAX_CHANNEL_COUNT) {
			PX4_DEBUG("channel %d > %d (DSM_MAX_CHANNEL_COUNT)", channel, DSM_MAX_CHANNEL_COUNT);
			dsm_guess_format(true);
			return false;
		}

		/* ignore channels out of range */
		if (channel >= max_values) {
			continue;
		}

		/* update the decoded channel count */
		if (channel >= *num_values) {
			*num_values = channel + 1;
		}

		/*
		 * Store the decoded channel into the R/C input buffer, taking into
		 * account the different ideas about channel assignement that we have.
		 *
		 * Specifically, the first four channels in rc_channel_data are roll, pitch, thrust, yaw,
		 * but the first four channels from the DSM receiver are thrust, roll, pitch, yaw.
		 */
		switch (channel) {
		case 0:
			channel = 2; // Spektrum Throttle (0) -> 2
			break;

		case 1:
			channel = 0; // Spektrum Aileron (1) -> 0
			break;

		case 2:
			channel = 1; // Spektrum Elevator (2) -> 1

		default:
			break;
		}

		values[channel] = value;
	}

	/* Set the 11-bit data indicator */
	*dsm_11_bit = (dsm_channel_shift == 11);

	/* we have received something we think is a dsm_frame */
	dsm_last_frame_time = frame_time;

	/*
	 * XXX Note that we may be in failsafe here; we need to work out how to detect that.
	 */

#ifdef DSM_DEBUG
	printf("PARSED PACKET\n");
#endif

	/* check all values */
	for (unsigned i = 0; i < *num_values; i++) {
		// Spektrum range is 903μs to 2097μs (Specification for Spektrum Remote Receiver Interfacing Rev G 9.1)
		if (values[i] < 903 || values[i] > 2097) {
			// if the value is unrealistic, fail the parsing entirely
#ifdef DSM_DEBUG
			printf("DSM: VALUE RANGE FAIL: %d: %d\n", (int)i, (int)values[i]);
#endif
			*num_values = 0;
			return false;
		}
	}

	return true;
}

/**
 * Called periodically to check for input data from the DSM UART
 *
 * The DSM* protocol doesn't provide any explicit framing,
 * so we detect dsm frame boundaries by the inter-dsm frame delay.
 * The minimum dsm frame spacing is 11ms; with 16 bytes at 115200bps
 * dsm frame transmission time is ~1.4ms.
 * We expect to only be called when bytes arrive for processing,
 * and if an interval of more than 5ms passes between calls,
 * the first byte we read will be the first byte of a dsm frame.
 * In the case where byte(s) are dropped from a dsm frame, this also
 * provides a degree of protection. Of course, it would be better
 * if we didn't drop bytes...
 * Upon receiving a full dsm frame we attempt to decode it.
 *
 * @param[out] values pointer to per channel array of decoded values
 * @param[out] num_values pointer to number of raw channel values returned, high order bit 0:10 bit data, 1:11 bit data
 * @param[out] n_butes number of bytes read
 * @param[out] bytes pointer to the buffer of read bytes
 * @param[out] rssi value in percent, if supported, or 127
 * @param[out] frame_drops dropped frames (indication of an unstable link)
 * @param[in] max_values maximum number of channels the receiver can process
 * @return true=decoded raw channel values updated, false=no update
 */
bool dsm_input(int fd, uint16_t *values, uint16_t *num_values, bool *dsm_11_bit, uint8_t *n_bytes, uint8_t **bytes,
	       int8_t *rssi, unsigned *frame_drops, unsigned max_values)
{
	/*
	 * The S.BUS protocol doesn't provide reliable framing,
	 * so we detect frame boundaries by the inter-frame delay.
	 *
	 * The minimum frame spacing is 7ms; with 25 bytes at 100000bps
	 * frame transmission time is ~2ms.
	 *
	 * We expect to only be called when bytes arrive for processing,
	 * and if an interval of more than 3ms passes between calls,
	 * the first byte we read will be the first byte of a frame.
	 *
	 * In the case where byte(s) are dropped from a frame, this also
	 * provides a degree of protection. Of course, it would be better
	 * if we didn't drop bytes...
	 */
	const hrt_abstime now = hrt_absolute_time();

	/*
	 * Fetch bytes, but no more than we would need to complete
	 * a complete frame.
	 */

	int ret = read(fd, &dsm_buf[0], sizeof(dsm_buf) / sizeof(dsm_buf[0]));

	/* if the read failed for any reason, just give up here */
	if (ret < 1) {
		return false;

	} else {
		*n_bytes = ret;
		*bytes = &dsm_buf[0];
	}

	/*
	 * Try to decode something with what we got
	 */
	return dsm_parse(now, &dsm_buf[0], ret, values, num_values, dsm_11_bit, &dsm_frame_drops, rssi, max_values);
}

bool dsm_parse(const uint64_t now, const uint8_t *frame, const unsigned len, uint16_t *values,
	       uint16_t *num_values, bool *dsm_11_bit, unsigned *frame_drops, int8_t *rssi_percent, uint16_t max_channels)
{
	/* this is set by the decoding state machine and will default to false
	 * once everything that was decodable has been decoded.
	 */
	bool decode_ret = false;

	/* ensure there can be no overflows */
	if (max_channels > sizeof(dsm_chan_buf) / sizeof(dsm_chan_buf[0])) {
		max_channels = sizeof(dsm_chan_buf) / sizeof(dsm_chan_buf[0]);
	}

	/* keep decoding until we have consumed the buffer */
	for (unsigned d = 0; d < len; d++) {

		/* overflow check */
		if (dsm_partial_frame_count == sizeof(dsm_frame) / sizeof(dsm_frame[0])) {
			dsm_partial_frame_count = 0;
			dsm_decode_state = DSM_DECODE_STATE_DESYNC;
#ifdef DSM_DEBUG
			printf("DSM: RESET (BUF LIM)\n");
#endif
		}

		if (dsm_partial_frame_count == DSM_FRAME_SIZE) {
			dsm_partial_frame_count = 0;
			dsm_decode_state = DSM_DECODE_STATE_DESYNC;
#ifdef DSM_DEBUG
			printf("DSM: RESET (PACKET LIM)\n");
#endif
		}

#ifdef DSM_DEBUG
#if 1
		printf("dsm state: %s%s, count: %d, val: %02x\n",
		       (dsm_decode_state == DSM_DECODE_STATE_DESYNC) ? "DSM_DECODE_STATE_DESYNC" : "",
		       (dsm_decode_state == DSM_DECODE_STATE_SYNC) ? "DSM_DECODE_STATE_SYNC" : "",
		       dsm_partial_frame_count,
		       (unsigned)frame[d]);
#endif
#endif

		switch (dsm_decode_state) {
		case DSM_DECODE_STATE_DESYNC:

			/* we are de-synced and only interested in the frame marker */
			if ((now - dsm_last_rx_time) > 5000) {
				dsm_decode_state = DSM_DECODE_STATE_SYNC;
				dsm_partial_frame_count = 0;
				dsm_chan_count = 0;
				dsm_frame[dsm_partial_frame_count++] = frame[d];
			}

			break;

		case DSM_DECODE_STATE_SYNC: {
				dsm_frame[dsm_partial_frame_count++] = frame[d];

				/* decode whatever we got and expect */
				if (dsm_partial_frame_count < DSM_FRAME_SIZE) {
					break;
				}

				/*
				 * Great, it looks like we might have a frame.  Go ahead and
				 * decode it.
				 */
				decode_ret = dsm_decode(now, &dsm_chan_buf[0], &dsm_chan_count, dsm_11_bit, max_channels, rssi_percent);

				/* we consumed the partial frame, reset */
				dsm_partial_frame_count = 0;

				/* if decoding failed, set proto to desync */
				if (!decode_ret) {
					dsm_decode_state = DSM_DECODE_STATE_DESYNC;
					dsm_frame_drops++;
				}
			}
			break;

		default:
#ifdef DSM_DEBUG
			printf("UNKNOWN PROTO STATE");
#endif
			decode_ret = false;
		}
	}

	if (frame_drops) {
		*frame_drops = dsm_frame_drops;
	}

	if (decode_ret) {
		// require stable channel count (dsm_chan_count == dsm_chan_count_prev) before considering the decode valid
		if ((dsm_chan_count > 0) && (dsm_chan_count <= DSM_MAX_CHANNEL_COUNT) && (dsm_chan_count == dsm_chan_count_prev)) {
			*num_values = dsm_chan_count;
			memcpy(&values[0], &dsm_chan_buf[0], dsm_chan_count * sizeof(dsm_chan_buf[0]));

		} else {
			decode_ret = false;
		}

		dsm_chan_count_prev = dsm_chan_count;

#ifdef DSM_DEBUG
		printf("PACKET ---------\n");
		printf("frame drops: %u, chan #: %u\n", dsm_frame_drops, dsm_chan_count);

		for (unsigned i = 0; i < dsm_chan_count; i++) {
			printf("dsm_decode: #CH %02u: %u\n", i + 1, values[i]);
		}

#endif
	}

	dsm_last_rx_time = now;

	/* return false as default */
	return decode_ret;
}