Plane: added OVERRIDE_CHAN parameter

this allows both for testing PX4IO override on the ground, and for
forced manual override while flying, including re-arming after in-air
reboot

ArduPlane/ArduPlane.pde

@@ -350,6 +350,11 @@ static uint8_t oldSwitchPosition = 254;
 // This is used to enable the inverted flight feature
 static bool inverted_flight     = false;
 
+#if CONFIG_HAL_BOARD == HAL_BOARD_PX4
+// This is used to enable the PX4IO override for testing
+static bool px4io_override_enabled     = false;
+#endif
+
 static struct {
     // These are trim values used for elevon control
     // For elevons radio_in[CH_ROLL] and radio_in[CH_PITCH] are
@@ -1006,7 +1011,9 @@ static void obc_fs_check(void)
  */
 static void update_aux(void)
 {
-    RC_Channel_aux::enable_aux_servos();
+    if (!px4io_override_enabled) {
+        RC_Channel_aux::enable_aux_servos();
+    }
 
 #if MOUNT == ENABLED
         camera_mount.update_mount_type();

ArduPlane/Parameters.h

@@ -128,6 +128,7 @@ public:
         k_param_takeoff_flap_percent,
         k_param_flap_slewrate,
         k_param_rtl_autoland,
+        k_param_override_channel,
 
         // 100: Arming parameters
         k_param_arming = 100,
@@ -469,6 +470,9 @@ public:
     AP_Int8 fbwa_tdrag_chan;
     AP_Int8 rangefinder_landing;
     AP_Int8 flap_slewrate;
+#if CONFIG_HAL_BOARD == HAL_BOARD_PX4
+    AP_Int8 override_channel;
+#endif
 
     // RC channels
     RC_Channel rc_1;

ArduPlane/Parameters.pde

@@ -874,6 +874,14 @@ const AP_Param::Info var_info[] PROGMEM = {
     // @User: Advanced
     GSCALAR(land_flap_percent,     "LAND_FLAP_PERCNT", 0),
 
+#if CONFIG_HAL_BOARD == HAL_BOARD_PX4
+    // @Param: OVERRIDE_CHAN
+    // @DisplayName: PX4IO override channel
+    // @Description: If set to a non-zero value then this is an RC input channel number to use for testing manual control in case the main FMU microcontroller on a PX4 or Pixhawk fails. When this RC input channel goes above 1750 the FMU will stop sending servo controls to the PX4IO board, which will trigger the PX4IO board to start using its failsafe override behaviour, which should give you manual control of the aircraft. That allows you to test for correct manual behaviour without actually crashing the FMU. This parameter is normally only set to a non-zero value for ground testing purposes. When the override channel is used it also forces the PX4 safety switch into an armed state. This allows it to be used as a way to re-arm a plane after an in-flight reboot. Use in that way is considered a developer option, for people testing unstable developer code.
+    // @User: Advanced
+    GSCALAR(override_channel,      "OVERRIDE_CHAN",  0),
+#endif
+
     // @Param: RSSI_PIN
     // @DisplayName: Receiver RSSI sensing pin
     // @Description: This selects an analog pin for the receiver RSSI voltage. It assumes the voltage is 5V for max rssi, 0V for minimum

ArduPlane/config.h

@@ -445,6 +445,10 @@
  # define INVERTED_FLIGHT_PWM 1750
 #endif
 
+#ifndef PX4IO_OVERRIDE_PWM
+ # define PX4IO_OVERRIDE_PWM 1750
+#endif
+
 //////////////////////////////////////////////////////////////////////////////
 // Developer Items
 //

ArduPlane/control_modes.pde

@@ -59,6 +59,43 @@ static void read_control_switch()
         // fly upside down when that channel goes above INVERTED_FLIGHT_PWM
         inverted_flight = (control_mode != MANUAL && hal.rcin->read(g.inverted_flight_ch-1) > INVERTED_FLIGHT_PWM);
     }
+
+#if CONFIG_HAL_BOARD == HAL_BOARD_PX4
+    if (g.override_channel > 0) {
+        // if the user has configured an override channel then check it
+        bool override = (hal.rcin->read(g.override_channel-1) > PX4IO_OVERRIDE_PWM);
+        if (override && !px4io_override_enabled) {
+            if (setup_failsafe_mixing()) {
+                px4io_override_enabled = true;
+                // disable output channels to force PX4IO override
+                for (uint8_t i=0; i<16; i++) {
+                    hal.rcout->disable_ch(i);
+                }
+                gcs_send_text_P(SEVERITY_LOW, PSTR("PX4IO Override enabled"));
+            } else {
+                // we'll try again next loop. The PX4IO code sometimes
+                // rejects a mixer, probably due to it being busy in
+                // some way?
+                gcs_send_text_P(SEVERITY_LOW, PSTR("PX4IO Override enable failed"));
+            }
+        } else if (!override && px4io_override_enabled) {
+            px4io_override_enabled = false;
+            // re-enable output channels
+            for (uint8_t i=0; i<8; i++) {
+                hal.rcout->enable_ch(i);
+            }
+            RC_Channel_aux::enable_aux_servos();
+            gcs_send_text_P(SEVERITY_LOW, PSTR("PX4IO Override disabled"));
+        }
+        if (px4io_override_enabled && 
+            hal.util->safety_switch_state() != AP_HAL::Util::SAFETY_ARMED) {
+            // we force safety off, so that if this override is used
+            // with a in-flight reboot it gives a way for the pilot to
+            // re-arm and take manual control
+            hal.rcout->force_safety_off();
+        }
+    }
+#endif // CONFIG_HAL_BOARD
 }
 
 static uint8_t readSwitch(void)