HardwareTimer: Enable preload for ARR/overflow register

This changes the default for preload (i.e. delay updating the overflow
value until an update event such as timer overflow) to enabled, since
this is will probably make the timer output behave as expected as much
as possible (e.g. you cannot make it "skip" an overflow and run it all
the way up to 0xffff by lowering the overflow value).

Author: matthijskooijman

cores/arduino/HardwareTimer.cpp

@@ -93,7 +93,7 @@ HardwareTimer::HardwareTimer(TIM_TypeDef *instance)
 #if defined(TIM_RCR_REP)
   _timerObj.handle.Init.RepetitionCounter = 0;
 #endif
-  _timerObj.handle.Init.AutoReloadPreload = TIM_AUTORELOAD_PRELOAD_DISABLE;
+  _timerObj.handle.Init.AutoReloadPreload = TIM_AUTORELOAD_PRELOAD_ENABLE;
   HAL_TIM_Base_Init(&(_timerObj.handle));
 }
 
@@ -442,6 +442,11 @@ uint32_t HardwareTimer::getOverflow(TimerFormat_t format)
 
 /**
   * @brief  Set overflow (rollover)
+  *
+  *         Note that by default, the new value will not be applied
+  *         immediately, but become effective at the next update event
+  *         (usually the next timer overflow). See setPreloadEnable()
+  *         for controlling this behaviour.
   * @param  overflow: depend on format parameter
   * @param  format of overflow parameter. If ommited default format is Tick
   *           TICK_FORMAT:     overflow is the number of tick for overflow
@@ -680,8 +685,9 @@ void HardwareTimer::setMode(uint32_t channel, TimerModes_t mode, PinName pin)
 /**
   * @brief  Enable or disable preloading for overflow value
   *         When disabled, changes to the overflow value take effect
-  *         immediately. When enabled, the value takes effect only at
-  *         the next update event (typically the next overflow).
+  *         immediately. When enabled (the default), the value takes
+  *         effect only at the next update event (typically the next
+  *         overflow).
   *
   *         Note that the capture/compare register has its own preload
   *         enable bit, which is independent and enabled in PWM modes