If RSSI Channel is set to Disabled when using S.Bus then generate RSS… (#5090)

* If RSSI Channel is set to Disabled when using S.Bus then generate RSSI signal using frame drop flags from the rx

* Set RSSI max level for S.Bus to 1024 so OSD defaults can be used

* Failsfafe must be detected rather than just reporting dropped frames

* Failsafe implies dropped frames

* Remove failsafe debug

* Use RSSI_SOURCE_RX_PROTOCOL

* Add rssi_from_rx_protocol to enable siqnal quality from rx to be processed as RSSI

* Use RSSI_MAX_VALUE definition

* Use rssi_from_rx_protocol flag for fport rx

* Update serialpassthrough help text

* Revert erroneous commit

* Use rssi_src_frame_errors boolean

* rssi_src_frame_errors = ON | OFF

* Moved rssi_src_frame_errors to end of rxConfig_t struct

* Add documentation of rssi_src_frame_errors

* Synthesise RX_FRAME_FAILSAFE flag to protect from bad implementation in receivers

* Match rx failsafe behaviour exactly

* Only set RX_FRAME_COMPLETE if valid frame is received

* RSSI_SOURCE_FRAME_ERRORS moved to end of rssiSource_e enum

* Removed superfluous else if clause

* Restore debug code

* Restore stateFlags

* Set RX_FRAME_DROPPED flag when failsafe is triggered

docs/Rssi.md

@@ -28,6 +28,16 @@ Default is set to "OFF" for normal operation ( 100 = Full signal / 0 = Lost sign
 
 Connect the RSSI signal to any PWM input channel then set the RSSI channel as you would for RSSI via PPM
 
+## RSSI from Futaba S.Bus receiver
+
+The S.Bus serial protocol includes detection of dropped frames. These may be monitored and reported as RSSI by using the following command.
+
+```
+set rssi_src_frame_errors = ON
+```
+
+Note that RSSI stands for Received Signal Strength Indicator; the detection of S.Bus dropped frames is really a signal quality, not strength indication. Consequently you may experience a more rapid drop in reported RSSI at the extremes of range when using this facility than when using RSSI reporting signal strength.
+
 ## RSSI ADC
 
 Connect the RSSI signal to the RC2/CH2 input. The signal must be between 0v and 3.3v.

src/main/interface/settings.c

@@ -440,6 +440,7 @@ const clivalue_t valueTable[] = {
     { "min_check",                  VAR_UINT16 | MASTER_VALUE, .config.minmax = { PWM_PULSE_MIN, PWM_PULSE_MAX }, PG_RX_CONFIG, offsetof(rxConfig_t, mincheck) },
     { "max_check",                  VAR_UINT16 | MASTER_VALUE, .config.minmax = { PWM_PULSE_MIN, PWM_PULSE_MAX }, PG_RX_CONFIG, offsetof(rxConfig_t, maxcheck) },
     { "rssi_channel",               VAR_INT8   | MASTER_VALUE, .config.minmax = { 0, MAX_SUPPORTED_RC_CHANNEL_COUNT }, PG_RX_CONFIG, offsetof(rxConfig_t, rssi_channel) },
+    { "rssi_src_frame_errors",      VAR_INT8   | MASTER_VALUE | MODE_LOOKUP, .config.lookup = { TABLE_OFF_ON }, PG_RX_CONFIG, offsetof(rxConfig_t, rssi_src_frame_errors) },
     { "rssi_scale",                 VAR_UINT8  | MASTER_VALUE, .config.minmax = { RSSI_SCALE_MIN, RSSI_SCALE_MAX }, PG_RX_CONFIG, offsetof(rxConfig_t, rssi_scale) },
     { "rssi_invert",                VAR_INT8   | MASTER_VALUE | MODE_LOOKUP, .config.lookup = { TABLE_OFF_ON }, PG_RX_CONFIG, offsetof(rxConfig_t, rssi_invert) },
     { "rc_interp",                  VAR_UINT8  | MASTER_VALUE | MODE_LOOKUP, .config.lookup = { TABLE_RC_INTERPOLATION }, PG_RX_CONFIG, offsetof(rxConfig_t, rcInterpolation) },

src/main/rx/rx.c

@@ -141,6 +141,7 @@ void pgResetFn_rxConfig(rxConfig_t *rxConfig)
         .maxcheck = 1900,
         .rx_min_usec = RX_MIN_USEC,          // any of first 4 channels below this value will trigger rx loss detection
         .rx_max_usec = RX_MAX_USEC,         // any of first 4 channels above this value will trigger rx loss detection
+        .rssi_src_frame_errors = true,
         .rssi_channel = 0,
         .rssi_scale = RSSI_SCALE_DEFAULT,
         .rssi_invert = 0,
@@ -416,11 +417,19 @@ bool rxUpdateCheck(timeUs_t currentTimeUs, timeDelta_t currentDeltaTime)
 #endif
     {
         const uint8_t frameStatus = rxRuntimeConfig.rcFrameStatusFn(&rxRuntimeConfig);
-        if (frameStatus & RX_FRAME_COMPLETE) {
+        if (frameStatus & (RX_FRAME_COMPLETE | RX_FRAME_FAILSAFE | RX_FRAME_DROPPED)) {
             rxDataProcessingRequired = true;
             rxIsInFailsafeMode = (frameStatus & RX_FRAME_FAILSAFE) != 0;
-            rxSignalReceived = !rxIsInFailsafeMode;
+            rxSignalReceived = (frameStatus & RX_FRAME_COMPLETE) != 0;
             needRxSignalBefore = currentTimeUs + needRxSignalMaxDelayUs;
+
+            if (frameStatus & RX_FRAME_DROPPED) {
+            	// No (0%) signal
+            	setRssiUnfiltered(0, RSSI_SOURCE_FRAME_ERRORS);
+            } else {
+            	// Valid (100%) signal
+            	setRssiUnfiltered(RSSI_MAX_VALUE, RSSI_SOURCE_FRAME_ERRORS);
+            }
         }
 
         if (frameStatus & RX_FRAME_PROCESSING_REQUIRED) {
@@ -624,7 +633,6 @@ void setRssiFiltered(uint16_t newRssi, rssiSource_e source)
 }
 
 #define RSSI_SAMPLE_COUNT 16
-#define RSSI_MAX_VALUE 1023
 
 void setRssiUnfiltered(uint16_t rssiValue, rssiSource_e source)
 {

src/main/rx/rx.h

@@ -45,6 +45,7 @@ typedef enum {
     RX_FRAME_COMPLETE = (1 << 0),
     RX_FRAME_FAILSAFE = (1 << 1),
     RX_FRAME_PROCESSING_REQUIRED = (1 << 2),
+    RX_FRAME_DROPPED = (1 << 3)
 } rxFrameState_e;
 
 typedef enum {
@@ -144,6 +145,7 @@ typedef struct rxConfig_s {
     uint16_t rx_min_usec;
     uint16_t rx_max_usec;
     uint8_t max_aux_channel;
+    uint8_t rssi_src_frame_errors;          // true to use frame drop flags in the rx protocol
 } rxConfig_t;
 
 PG_DECLARE(rxConfig_t, rxConfig);
@@ -169,6 +171,7 @@ typedef enum {
     RSSI_SOURCE_RX_CHANNEL,
     RSSI_SOURCE_RX_PROTOCOL,
     RSSI_SOURCE_MSP,
+    RSSI_SOURCE_FRAME_ERRORS,
 } rssiSource_e;
 
 extern rssiSource_e rssiSource;
@@ -183,6 +186,8 @@ bool calculateRxChannelsAndUpdateFailsafe(timeUs_t currentTimeUs);
 
 void parseRcChannels(const char *input, rxConfig_t *rxConfig);
 
+#define RSSI_MAX_VALUE 1023
+
 void setRssiFiltered(uint16_t newRssi, rssiSource_e source);
 void setRssiUnfiltered(uint16_t rssiValue, rssiSource_e source);
 void setRssiMsp(uint8_t newMspRssi);

src/main/rx/sbus.c

@@ -192,6 +192,10 @@ bool sbusInit(const rxConfig_t *rxConfig, rxRuntimeConfig_t *rxRuntimeConfig)
         SBUS_PORT_OPTIONS | (rxConfig->serialrx_inverted ? 0 : SERIAL_INVERTED) | (rxConfig->halfDuplex ? SERIAL_BIDIR : 0)
         );
 
+    if (rxConfig->rssi_src_frame_errors) {
+        rssiSource = RSSI_SOURCE_FRAME_ERRORS;
+    }
+
 #ifdef USE_TELEMETRY
     if (portShared) {
         telemetrySharedPort = sBusPort;

src/main/rx/sbus_channels.c

@@ -68,12 +68,15 @@ uint8_t sbusChannelsDecode(rxRuntimeConfig_t *rxRuntimeConfig, const sbusChannel
         sbusChannelData[17] = SBUS_DIGITAL_CHANNEL_MIN;
     }
 
-    if (channels->flags & SBUS_FLAG_SIGNAL_LOSS) {
-    }
     if (channels->flags & SBUS_FLAG_FAILSAFE_ACTIVE) {
         // internal failsafe enabled and rx failsafe flag set
-        // RX *should* still be sending valid channel data, so use it.
+        // RX *should* still be sending valid channel data (repeated), so use it.
-        return RX_FRAME_COMPLETE | RX_FRAME_FAILSAFE;
+        return RX_FRAME_DROPPED | RX_FRAME_FAILSAFE;
+    }
+
+    if (channels->flags & SBUS_FLAG_SIGNAL_LOSS) {
+    	// The received data is a repeat of the last valid data so can be considered complete.
+    	return RX_FRAME_DROPPED;
     }
 
     return RX_FRAME_COMPLETE;