APM_RC: Modified get_last_update() to be more interrupt friendly

libraries/APM_RC/APM_RC.cpp

@@ -5,5 +5,5 @@
  */
 #include "APM_RC.h"
 
-volatile uint32_t APM_RC_Class::_last_update;
+volatile uint8_t APM_RC_Class::_ppm_count;
 

libraries/APM_RC/APM_RC.h

@@ -50,20 +50,29 @@ public:
     virtual void            Force_Out2_Out3(void) = 0;
     virtual void            Force_Out6_Out7(void) = 0;
 
-    // get the time of the last radio update (_last_update modified by interrupt, so reading of variable must be interrupt safe)
+    // get the approximate time of the last radio ppm update
+	// - resolution of the return time in ms is defined by how often get_last_update() is called
+	// - _ppm_count is modified by interrupt, but type is volatile uint8_t making it atomic for reading and writting outside the interrupt
     virtual uint32_t        get_last_update() {
+        static uint32_t _last_update_time = millis(); // Initialize with current time to prevent false trigger
 
-        uint32_t _tmp = _last_update;
-        while( _tmp != _last_update ) _tmp = _last_update;
+		// PPM refresh?
+		if( _ppm_count > 0) {
+			// Reset _ppm_count
+			_ppm_count = 0;
 
-        return _tmp;
+			// Approximate time (very much so)  of last update
+			_last_update_time = millis();
+		}
+		
+        return _last_update_time;
     };
 
 protected:
     uint16_t                _map_speed(uint16_t speed_hz) {
         return 2000000UL / speed_hz;
     }
-    static volatile uint32_t         _last_update; // Modified by interrupt
+    static volatile uint8_t         _ppm_count; // Modified by interrupt
 
 };
 

libraries/APM_RC/APM_RC_APM1.cpp

@@ -58,7 +58,7 @@ void APM_RC_APM1::_timer4_capt_cb(void)
         // pass through values if at least a minimum number of channels received
         if( PPM_Counter >= MIN_CHANNELS ) {
             _radio_status = 1;
-            _last_update = millis();
+            _ppm_count++;
         }
         PPM_Counter=0;
     }

libraries/APM_RC/APM_RC_APM2.cpp

@@ -58,7 +58,7 @@ void APM_RC_APM2::_timer5_capt_cb(void)
         // pass through values if at least a minimum number of channels received
         if( frame_idx >= MIN_CHANNELS ) {
             _radio_status = 1;
-            _last_update = millis();
+            _ppm_count++;
         }
         frame_idx=0;
     } else {