Plane: trigger OVERRIDE_CHAN in px4io

setup the MODESWITCH channel as our OVERRIDE_CHAN, allowing for
instant manual passthru in px4io when OVERRIDE_CHAN goes above 1750

This makes for faster switching, and avoids bugs where a still enabled
FMU channel disables override

ArduPlane/ArduPlane.cpp

@@ -273,9 +273,7 @@ void Plane::obc_fs_check(void)
  */
 void Plane::update_aux(void)
 {
-    if (!px4io_override_enabled) {
-        RC_Channel_aux::enable_aux_servos();
-    }
+    RC_Channel_aux::enable_aux_servos();
 }
 
 void Plane::one_second_loop()

ArduPlane/control_modes.cpp

@@ -81,10 +81,6 @@ void Plane::read_control_switch()
             }
         } 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"));
         }
@@ -95,17 +91,6 @@ void Plane::read_control_switch()
             // re-arm and take manual control
             hal.rcout->force_safety_off();
         }
-        if (px4io_override_enabled) {
-            /*
-              ensure that all channels are disabled from the FMU. If
-              PX4IO receives any channel input from the FMU then it
-              will think the FMU is still active and won't enable the
-              internal mixer.
-             */
-            for (uint8_t i=0; i<16; i++) {
-                hal.rcout->disable_ch(i);
-            }
-        }
     }
 #endif // CONFIG_HAL_BOARD
 }

ArduPlane/px4_mixer.cpp

@@ -20,6 +20,7 @@
 #include <stdio.h>
 #include <drivers/drv_pwm_output.h>
 #include <systemlib/mixer/mixer.h>
+#include <modules/px4iofirmware/protocol.h>
 
 #define PX4_LIM_RC_MIN 900
 #define PX4_LIM_RC_MAX 2100
@@ -307,7 +308,7 @@ bool Plane::setup_failsafe_mixing(void)
             config.rc_trim = constrain_int16(ch->radio_trim, config.rc_min+1, config.rc_max-1);
         }
         config.rc_dz = 0; // zero for the purposes of manual takeover
-        config.rc_assignment = i;
+
         // we set reverse as false, as users of ArduPilot will have
         // input reversed on transmitter, so from the point of view of
         // the mixer the input is never reversed. The one exception is
@@ -318,6 +319,25 @@ bool Plane::setup_failsafe_mixing(void)
         } else {
             config.rc_reverse = false;
         }
+
+        if (i+1 == g.override_channel.get()) {
+            /*
+              This is an OVERRIDE_CHAN channel. We want IO to trigger
+              override with a channel input of over 1750. The px4io
+              code is setup for triggering below 10% of full range. To
+              map this to values above 1750 we need to reverse the
+              direction and set the rc range for this channel to 1000
+              to 1833 (1833 = 1000 + 750/0.9)
+             */
+            config.rc_assignment = PX4IO_P_RC_CONFIG_ASSIGNMENT_MODESWITCH;
+            config.rc_reverse = true;
+            config.rc_max = 1833;
+            config.rc_min = 1000;
+            config.rc_trim = 1500;
+        } else {
+            config.rc_assignment = i;
+        }
+
         if (ioctl(px4io_fd, PWM_SERVO_SET_RC_CONFIG, (unsigned long)&config) != 0) {
             hal.console->printf("SET_RC_CONFIG failed\n");
             goto failed;

ArduPlane/system.cpp

@@ -751,10 +751,9 @@ bool Plane::arm_motors(AP_Arming::ArmingMethod method)
         return false;
     }
 
-    //only log if arming was successful
-    if (!px4io_override_enabled) {
-        channel_throttle->enable_out();
-    }
+    // only log if arming was successful
+    channel_throttle->enable_out();
+
     change_arm_state();
     return true;
 }