Add force failsafe flag

src/modules/px4iofirmware/mixer.cpp

@@ -111,7 +111,7 @@ mixer_tick(void)
 		r_status_flags |= PX4IO_P_STATUS_FLAGS_FMU_OK;
 	}
 
-	/* default to failsafe mixing */
+	/* default to failsafe mixing - it will be forced below if flag is set */
 	source = MIX_FAILSAFE;
 
 	/*
@@ -154,6 +154,13 @@ mixer_tick(void)
 		}
 	}
 
+	/*
+	 * Check if we should force failsafe - and do it if we have to
+	 */
+	if (r_setup_arming & PX4IO_P_SETUP_ARMING_FORCE_FAILSAFE) {
+		source = MIX_FAILSAFE;
+	}
+
 	/*
 	 * Set failsafe status flag depending on mixing source
 	 */

src/modules/px4iofirmware/protocol.h

@@ -179,6 +179,7 @@
 #define PX4IO_P_SETUP_ARMING_ALWAYS_PWM_ENABLE	(1 << 5) /* Output of PWM right after startup enabled to help ESCs initialize and prevent them from beeping */
 #define PX4IO_P_SETUP_ARMING_RC_HANDLING_DISABLED	(1 << 6) /* Disable the IO-internal evaluation of the RC */
 #define PX4IO_P_SETUP_ARMING_LOCKDOWN		(1 << 7) /* If set, the system operates normally, but won't actuate any servos */
+#define PX4IO_P_SETUP_ARMING_FORCE_FAILSAFE	(1 << 8) /* If set, the system will always output the failsafe values */
 
 #define PX4IO_P_SETUP_PWM_RATES			2	/* bitmask, 0 = low rate, 1 = high rate */
 #define PX4IO_P_SETUP_PWM_DEFAULTRATE		3	/* 'low' PWM frame output rate in Hz */
@@ -253,6 +254,10 @@ enum {							/* DSM bind states */
 /* PWM failsafe values - zero disables the output */
 #define PX4IO_PAGE_FAILSAFE_PWM			55		/**< 0..CONFIG_ACTUATOR_COUNT-1 */
 
+/* PWM failsafe values - zero disables the output */
+#define PX4IO_PAGE_SENSORS			56		/**< Sensors connected to PX4IO */
+#define PX4IO_P_SENSORS_ALTITUDE		0		/**< Altitude of an external sensor (HoTT or S.BUS2) */
+
 /* Debug and test page - not used in normal operation */
 #define PX4IO_PAGE_TEST				127
 #define PX4IO_P_TEST_LED			0		/**< set the amber LED on/off */