Compass: add last_update_usec per instance

libraries/AP_Compass/AP_Compass_Backend.cpp

@@ -34,7 +34,7 @@ void AP_Compass_Backend::publish_field(const Vector3f &mag, uint8_t instance)
     apply_corrections(state.field, instance);
 
     state.last_update_ms = hal.scheduler->millis();
-    _compass._last_update_usec = hal.scheduler->micros();
+    state.last_update_usec = hal.scheduler->micros();
 }
 
 /*

libraries/AP_Compass/Compass.cpp

@@ -275,7 +275,6 @@ const AP_Param::GroupInfo Compass::var_info[] PROGMEM = {
 // their values.
 //
 Compass::Compass(void) :
-    _last_update_usec(0),
     _backend_count(0),
     _compass_count(0),
     _board_orientation(ROTATION_NONE),
@@ -286,6 +285,7 @@ Compass::Compass(void) :
     AP_Param::setup_object_defaults(this, var_info);
     for (uint8_t i=0; i<COMPASS_MAX_BACKEND; i++) {
         _backends[i] = NULL;
+        _state[i].last_update_usec = 0;
     }    
 
 #if COMPASS_MAX_INSTANCES > 1
@@ -604,7 +604,7 @@ void Compass::setHIL(uint8_t instance, const Vector3f &mag)
 {
     _hil.field[instance] = mag;
     _hil.healthy[instance] = true;
-    _last_update_usec = hal.scheduler->micros();
+    _state[instance].last_update_usec = hal.scheduler->micros();
 }
 
 const Vector3f& Compass::getHIL(uint8_t instance) const 

libraries/AP_Compass/Compass.h

@@ -233,7 +233,7 @@ public:
     void        set_hil_mode(void) { _hil_mode = true; }
 
     // return last update time in microseconds
-    uint32_t last_update_usec(void) const { return _last_update_usec; }
+    uint32_t last_update_usec(void) const { return _state[get_primary()].last_update_usec; }
 
     static const struct AP_Param::GroupInfo var_info[];
 
@@ -255,9 +255,6 @@ private:
     void _add_backend(AP_Compass_Backend *(detect)(Compass &));
     void _detect_backends(void);
 
-    //< micros() time of last update
-    uint32_t _last_update_usec;
-
     // backend objects
     AP_Compass_Backend *_backends[COMPASS_MAX_BACKEND];
     uint8_t     _backend_count;
@@ -321,6 +318,7 @@ private:
 
         // when we last got data
         uint32_t    last_update_ms;
+        uint32_t    last_update_usec;
     } _state[COMPASS_MAX_INSTANCES];
 
     // if we want HIL only