ArduCopter: modified mainloop timing to be in sync with the arrival of new data from the IMU.

This reduces the maximum delay between when new sensor data arrives and when it is actually used by about 5ms.

ArduCopter/ArduCopter.pde

@@ -899,7 +899,7 @@ static byte gps_watchdog;
 // Time in microseconds of main control loop
 static uint32_t fast_loopTimer;
 // Time in microseconds of 50hz control loop
-static uint32_t fiftyhz_loopTimer;
+static uint32_t fiftyhz_loopTimer = 0;
 // Counters for branching from 10 hz control loop
 static byte medium_loopCounter;
 // Counters for branching from 3 1/3hz control loop
@@ -981,11 +981,12 @@ void setup() {
 void loop()
 {
     uint32_t timer                  = micros();
-    bool spare_time = true;
+    static bool run_50hz_loop = false;
 
     // We want this to execute fast
     // ----------------------------
-    if ((timer - fast_loopTimer) >= 10000 && imu.new_data_available()) {
+    if( imu.num_samples_available() >= NUM_IMU_SAMPLES_FOR_100HZ ) {
+
 		#if DEBUG_FAST_LOOP == ENABLED
         Log_Write_Data(50, (int32_t)(timer - fast_loopTimer));
         #endif
@@ -996,8 +997,60 @@ void loop()
 
         // Execute the fast loop
         // ---------------------
-        fast_loop();
+        fast_loop();////
-        spare_time = false;
+
+        // run the 50hz loop 1/2 the time
+        run_50hz_loop = !run_50hz_loop;
+
+        if( run_50hz_loop ) {
+
+            #if DEBUG_MED_LOOP == ENABLED
+            Log_Write_Data(51, (int32_t)(timer - fiftyhz_loopTimer));
+            #endif
+
+            // store the micros for the 50 hz timer
+            fiftyhz_loopTimer               = timer;
+
+            // port manipulation for external timing of main loops
+            //PORTK |= B01000000;
+
+            // reads all of the necessary trig functions for cameras, throttle, etc.
+            // --------------------------------------------------------------------
+            update_trig();
+
+            // Rotate the Nav_lon and nav_lat vectors based on Yaw
+            // ---------------------------------------------------
+            calc_loiter_pitch_roll();
+
+            // check for new GPS messages
+            // --------------------------
+            update_GPS();
+
+            // perform 10hz tasks
+            // ------------------
+            medium_loop();
+
+            // Stuff to run at full 50hz, but after the med loops
+            // --------------------------------------------------
+            fifty_hz_loop();
+
+            counter_one_herz++;
+
+            // trgger our 1 hz loop
+            if(counter_one_herz >= 50) {
+                super_slow_loop();
+                counter_one_herz = 0;
+            }
+            perf_mon_counter++;
+            if (perf_mon_counter > 600 ) {
+                if (g.log_bitmask & MASK_LOG_PM)
+                    Log_Write_Performance();
+
+                gps_fix_count           = 0;
+                perf_mon_counter        = 0;
+            }
+            //PORTK &= B10111111;
+        }
     } else {
 #ifdef DESKTOP_BUILD
         usleep(1000);
@@ -1007,60 +1060,6 @@ void loop()
     // port manipulation for external timing of main loops
     //PORTK &= B11101111;
 
-    if ((timer - fiftyhz_loopTimer) >= 20000) {
-
-		#if DEBUG_MED_LOOP == ENABLED
-    	Log_Write_Data(51, (int32_t)(timer - fiftyhz_loopTimer));
-        #endif
-
-        // store the micros for the 50 hz timer
-        fiftyhz_loopTimer               = timer;
-
-        // port manipulation for external timing of main loops
-        //PORTK |= B01000000;
-
-        // reads all of the necessary trig functions for cameras, throttle, etc.
-        // --------------------------------------------------------------------
-        update_trig();
-
-        // Rotate the Nav_lon and nav_lat vectors based on Yaw
-        // ---------------------------------------------------
-        calc_loiter_pitch_roll();
-
-        // check for new GPS messages
-        // --------------------------
-        update_GPS();
-
-        // perform 10hz tasks
-        // ------------------
-        medium_loop();
-
-        // Stuff to run at full 50hz, but after the med loops
-        // --------------------------------------------------
-        fifty_hz_loop();
-
-        counter_one_herz++;
-
-        // trgger our 1 hz loop
-        if(counter_one_herz >= 50) {
-            super_slow_loop();
-            counter_one_herz = 0;
-        }
-        perf_mon_counter++;
-        if (perf_mon_counter > 600 ) {
-            if (g.log_bitmask & MASK_LOG_PM)
-                Log_Write_Performance();
-
-            gps_fix_count           = 0;
-            perf_mon_counter        = 0;
-        }
-        //PORTK &= B10111111;
-        spare_time = false;
-    }
-
-    if (spare_time && g.compass_enabled) {
-        compass.accumulate();
-    }
 }
 //  PORTK |= B01000000;
 //	PORTK &= B10111111;