Non-destructive handling of failsafe signals, distinction between frame loss and signal loss.

This kind of handling might be moved upstream into the application, once alarms are propagated by the ORB system.

This change is compatible with RX failsafe settings, but does not rely on it (uses SBus flags instead).

src/modules/px4iofirmware/sbus.c

@@ -54,6 +54,9 @@
 
 #define SBUS_FRAME_SIZE		25
 #define SBUS_INPUT_CHANNELS	16
+#define SBUS_FLAGS_BYTE		23
+#define SBUS_FAILSAFE_BIT	3
+#define SBUS_FRAMELOST_BIT	2
 
 /*
   Measured values with Futaba FX-30/R6108SB:
@@ -220,15 +223,6 @@ sbus_decode(hrt_abstime frame_time, uint16_t *values, uint16_t *num_values, uint
 		return false;
 	}
 
-	/* if the failsafe or connection lost bit is set, we consider the frame invalid */
-	if ((frame[23] & (1 << 2)) && /* signal lost */
-	    (frame[23] & (1 << 3))) { /* failsafe */
-
-		/* actively announce signal loss */
-		*values = 0;
-		return false;
-	}
-
 	/* we have received something we think is a frame */
 	last_frame_time = frame_time;
 
@@ -262,13 +256,28 @@ sbus_decode(hrt_abstime frame_time, uint16_t *values, uint16_t *num_values, uint
 		chancount = 18;
 
 		/* channel 17 (index 16) */
-		values[16] = (frame[23] & (1 << 0)) * 1000 + 998;
+		values[16] = (frame[SBUS_FLAGS_BYTE] & (1 << 0)) * 1000 + 998;
 		/* channel 18 (index 17) */
-		values[17] = (frame[23] & (1 << 1)) * 1000 + 998;
+		values[17] = (frame[SBUS_FLAGS_BYTE] & (1 << 1)) * 1000 + 998;
 	}
 
 	/* note the number of channels decoded */
 	*num_values = chancount;
 
+	/* decode and handle failsafe and frame-lost flags */
+	if (frame[SBUS_FLAGS_BYTE] & (1 << SBUS_FAILSAFE_BIT)) { /* failsafe */
+		/* emulate throttle failsafe for maximum compatibility and do not destroy any channel data */
+		values[2] = 900;
+	}
+	else if (frame[SBUS_FLAGS_BYTE] & (1 << SBUS_FRAMELOST_BIT)) { /* a frame was lost */
+		/* set a special warning flag or try to calculate some kind of RSSI information - to be implemented 
+		 * 
+		 * Attention! This flag indicates a skipped frame only, not a total link loss! Handling this 
+		 * condition as fail-safe greatly reduces the reliability and range of the radio link, 
+		 * e.g. by prematurely issueing return-to-launch!!! */
+
+		// values[2] = 888; // marker for debug purposes
+	}
+
 	return true;
 }