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
@@ -996,18 +997,12 @@ void loop()
 
         // Execute the fast loop
         // ---------------------
-        fast_loop();
-        spare_time = false;
-    } else {
-#ifdef DESKTOP_BUILD
-        usleep(1000);
-#endif
-    }
+        fast_loop();////
 
-    // port manipulation for external timing of main loops
-    //PORTK &= B11101111;
+        // run the 50hz loop 1/2 the time
+        run_50hz_loop = !run_50hz_loop;
 
-    if ((timer - fiftyhz_loopTimer) >= 20000) {
+        if( run_50hz_loop ) {
 
             #if DEBUG_MED_LOOP == ENABLED
             Log_Write_Data(51, (int32_t)(timer - fiftyhz_loopTimer));
@@ -1055,12 +1050,16 @@ void loop()
                 perf_mon_counter        = 0;
             }
             //PORTK &= B10111111;
-        spare_time = false;
         }
-
-    if (spare_time && g.compass_enabled) {
-        compass.accumulate();
+    } else {
+#ifdef DESKTOP_BUILD
+        usleep(1000);
+#endif
     }
+
+    // port manipulation for external timing of main loops
+    //PORTK &= B11101111;
+
 }
 //  PORTK |= B01000000;
 //	PORTK &= B10111111;