st24: fix RC lost detection based on error count

The packet_count was actually an error_count, therefore we should
process RC input only when the error_count since the last packet is 0.

Also, this commit fixes the RSSI scaling for st24.

src/drivers/px4fmu/fmu.cpp

@@ -1358,18 +1358,21 @@ PX4FMU::cycle()
 
 			if (newBytes > 0) {
 				// parse new data
-				uint8_t st24_rssi, rx_count;
+				uint8_t st24_rssi, lost_count;
 
 				rc_updated = false;
 
 				for (unsigned i = 0; i < (unsigned)newBytes; i++) {
 					/* set updated flag if one complete packet was parsed */
 					st24_rssi = RC_INPUT_RSSI_MAX;
-					rc_updated = (OK == st24_decode(_rcs_buf[i], &st24_rssi, &rx_count,
+					rc_updated = (OK == st24_decode(_rcs_buf[i], &st24_rssi, &lost_count,
 									&raw_rc_count, raw_rc_values, input_rc_s::RC_INPUT_MAX_CHANNELS));
 				}
 
-				if (rc_updated) {
+				// The st24 will keep outputting RC channels and RSSI even if RC has been lost.
+				// The only way to detect RC loss is therefore to look at the lost_count.
+
+				if (rc_updated && lost_count == 0) {
 					// we have a new ST24 frame. Publish it.
 					_rc_in.input_source = input_rc_s::RC_INPUT_SOURCE_PX4FMU_ST24;
 					fill_rc_in(raw_rc_count, raw_rc_values, _cycle_timestamp,

src/lib/rc/st24.c

@@ -103,10 +103,9 @@ uint8_t st24_common_crc8(uint8_t *ptr, uint8_t len)
 }
 
 
-int st24_decode(uint8_t byte, uint8_t *rssi, uint8_t *rx_count, uint16_t *channel_count, uint16_t *channels,
+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) {
@@ -175,8 +174,9 @@ int st24_decode(uint8_t byte, uint8_t *rssi, uint8_t *rx_count, uint16_t *channe
 			case ST24_PACKET_TYPE_CHANNELDATA12: {
 					ChannelData12 *d = (ChannelData12 *)_rxpacket.st24_data;
 
-					*rssi = d->rssi;
-					*rx_count = d->packet_count;
+					// 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;
@@ -203,8 +203,9 @@ int st24_decode(uint8_t byte, uint8_t *rssi, uint8_t *rx_count, uint16_t *channe
 			case ST24_PACKET_TYPE_CHANNELDATA24: {
 					ChannelData24 *d = (ChannelData24 *)&_rxpacket.st24_data;
 
-					*rssi = d->rssi;
-					*rx_count = d->packet_count;
+					// 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;

src/lib/rc/st24.h

@@ -73,7 +73,7 @@ typedef struct {
 typedef struct {
 	uint16_t t;			///< packet counter or clock
 	uint8_t	rssi;			///< signal strength
-	uint8_t	packet_count;		///< Number of UART packets sent since reception of last RF frame (this tells something about age / rate)
+	uint8_t	lost_count;		///< Number of UART packets sent since reception of last RF frame (100 frame means RC timeout of 1s)
 	uint8_t	channel[18];		///< channel data, 12 channels (12 bit numbers)
 } ChannelData12;
 
@@ -84,7 +84,7 @@ typedef struct {
 typedef struct {
 	uint16_t t;			///< packet counter or clock
 	uint8_t	rssi;			///< signal strength
-	uint8_t	packet_count;		///< Number of UART packets sent since reception of last RF frame (this tells something about age / rate)
+	uint8_t	lost_count;		///< Number of UART packets sent since reception of last RF frame (100 frame means RC timeout of 1s)
 	uint8_t	channel[36];		///< channel data, 24 channels (12 bit numbers)
 } ChannelData24;
 
@@ -152,12 +152,12 @@ uint8_t st24_common_crc8(uint8_t *ptr, uint8_t len);
  *
  * @param byte current char to read
  * @param rssi pointer to a byte where the RSSI value is written back to
- * @param rx_count pointer to a byte where the receive count of packets signce last wireless frame is written back to
+ * @param lost_count pointer to a byte where the receive count of packets since last wireless frame is written back to ( > 0 if RC is lost)
  * @param channels pointer to a datastructure of size max_chan_count where channel values (12 bit) are written back to
  * @param max_chan_count maximum channels to decode - if more channels are decoded, the last n are skipped and success (0) is returned
  * @return 0 for success (a decoded packet), 1 for no packet yet (accumulating), 2 for unknown packet, 3 for out of sync, 4 for checksum error
  */
-__EXPORT int st24_decode(uint8_t byte, uint8_t *rssi, uint8_t *rx_count, uint16_t *channel_count,
+__EXPORT 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);
 
 __END_DECLS

src/modules/px4iofirmware/controls.c

@@ -96,7 +96,7 @@ bool dsm_port_input(uint16_t *rssi, bool *dsm_updated, bool *st24_updated, bool
 	perf_end(c_gather_dsm);
 
 	/* get data from FD and attempt to parse with DSM and ST24 libs */
-	uint8_t st24_rssi, rx_count;
+	uint8_t st24_rssi, lost_count;
 	uint16_t st24_channel_count = 0;
 
 	*st24_updated = false;
@@ -104,11 +104,11 @@ bool dsm_port_input(uint16_t *rssi, bool *dsm_updated, bool *st24_updated, bool
 	for (unsigned i = 0; i < n_bytes; i++) {
 		/* set updated flag if one complete packet was parsed */
 		st24_rssi = RC_INPUT_RSSI_MAX;
-		*st24_updated |= (OK == st24_decode(bytes[i], &st24_rssi, &rx_count,
+		*st24_updated |= (OK == st24_decode(bytes[i], &st24_rssi, &lost_count,
 						    &st24_channel_count, r_raw_rc_values, PX4IO_RC_INPUT_CHANNELS));
 	}
 
-	if (*st24_updated) {
+	if (*st24_updated && lost_count == 0) {
 
 		/* ensure ADC RSSI is disabled */
 		r_setup_features &= ~(PX4IO_P_SETUP_FEATURES_ADC_RSSI);