AP_Scheduler: added get_last_loop_time_s()

this gives compatibility with previous G_Dt in copter

libraries/AP_Scheduler/AP_Scheduler.cpp

@@ -213,8 +213,11 @@ void AP_Scheduler::loop()
 
     const uint32_t sample_time_us = AP_HAL::micros();
     
-    if (loop_timer_start_us == 0) {
-        loop_timer_start_us = sample_time_us;
+    if (_loop_timer_start_us == 0) {
+        _loop_timer_start_us = sample_time_us;
+        _last_loop_time_s = get_loop_period_s();
+    } else {
+        _last_loop_time_s = (sample_time_us - _loop_timer_start_us) * 1.0e-6;
     }
 
     // Execute the fast loop
@@ -241,9 +244,9 @@ void AP_Scheduler::loop()
 #endif
 
     // check loop time
-    perf_info.check_loop_time(sample_time_us - loop_timer_start_us);
+    perf_info.check_loop_time(sample_time_us - _loop_timer_start_us);
         
-    loop_timer_start_us = sample_time_us;
+    _loop_timer_start_us = sample_time_us;
 }
 
 void AP_Scheduler::update_logging()

libraries/AP_Scheduler/AP_Scheduler.h

@@ -131,6 +131,11 @@ public:
     float get_filtered_loop_time(void) const {
         return perf_info.get_filtered_time();
     }
+
+    // get the time in seconds that the last loop took
+    float get_last_loop_time_s(void) const {
+        return _last_loop_time_s;
+    }
     
     static const struct AP_Param::GroupInfo var_info[];
 
@@ -185,8 +190,11 @@ private:
     uint8_t _spare_ticks;
 
     // start of loop timing
-    uint32_t loop_timer_start_us;
+    uint32_t _loop_timer_start_us;
 
+    // time of last loop in seconds
+    uint32_t _last_loop_time_s;
+    
     // performance counters
     AP_HAL::Util::perf_counter_t *_perf_counters;