Inverts the run direction of the capture counter in the STM32 Dcc Dec…

…oder. (bakerstu#620)

* Inverts the run direction of the capture counter in the STM32 Dcc Decoder.
This is needed because certain timer resources in the STM32's can only
count upwards.

This is achieved by using upcounter mode, then inverting the capture value
before handing it back to the higher level implementation.

src/freertos_drivers/st/Stm32DCCDecoder.hxx

@@ -222,7 +222,7 @@ public:
         // This code handles underflow of the timer correctly. We cannot wait
         // longer than one full cycle though (65 msec -- typical RailCom waits
         // are 20-500 usec).
-        uint32_t new_match_v = usecTimerStart_ + TIMER_MAX_VALUE + 1 - usec;
+        uint32_t new_match_v = usecTimerStart_ + usec;
         new_match_v &= 0xffff;
         __HAL_TIM_SET_COMPARE(
             usec_timer_handle(), HW::USEC_CHANNEL, new_match_v);
@@ -245,7 +245,7 @@ public:
     static void set_cap_timer_time()
     {
         if (shared_timers()) {
-            usecTimerStart_ =  get_capture_counter();
+            usecTimerStart_ =  get_raw_capture_counter();
         } else {
             usecTimerStart_ =  __HAL_TIM_GET_COUNTER(usec_timer_handle());
         }
@@ -280,13 +280,18 @@ private:
         // 1 usec per tick
         handle->Init.Prescaler = configCPU_CLOCK_HZ / 1000000;
         handle->Init.ClockDivision = 0;
-        handle->Init.CounterMode = TIM_COUNTERMODE_DOWN;
+        handle->Init.CounterMode = TIM_COUNTERMODE_UP;
         handle->Init.RepetitionCounter = 0;
         handle->Init.AutoReloadPreload = TIM_AUTORELOAD_PRELOAD_DISABLE;
 
+        HASSERT(HAL_TIM_IC_DeInit(handle) == HAL_OK);
         HASSERT(HAL_TIM_IC_Init(handle) == HAL_OK);
     }
 
+    /// Helper function to read the raw capture register value.
+    /// @return the value of the CCx register matching the capture channel.
+    static inline uint32_t get_raw_capture_counter();
+
     /// @return the hardware pointer to the capture timer.
     static TIM_TypeDef *capture_timer()
     {
@@ -457,8 +462,34 @@ bool Stm32DccTimerModule<HW>::int_get_and_clear_capture_event()
 
 template <class HW> uint32_t Stm32DccTimerModule<HW>::get_capture_counter()
 {
-    return HAL_TIM_ReadCapturedValue(
-        capture_timer_handle(), HW::CAPTURE_CHANNEL);
+    // Simulates down-counting.
+    return TIMER_MAX_VALUE - get_raw_capture_counter();
+}
+
+template <class HW> uint32_t Stm32DccTimerModule<HW>::get_raw_capture_counter()
+{
+    // This if structure will be optimized away.
+    if (HW::CAPTURE_CHANNEL == TIM_CHANNEL_1)
+    {
+        return capture_timer()->CCR1;
+    }
+    else if (HW::CAPTURE_CHANNEL == TIM_CHANNEL_2)
+    {
+        return capture_timer()->CCR2;
+    }
+    else if (HW::CAPTURE_CHANNEL == TIM_CHANNEL_3)
+    {
+        return capture_timer()->CCR3;
+    }
+    else if (HW::CAPTURE_CHANNEL == TIM_CHANNEL_4)
+    {
+        return capture_timer()->CCR4;
+    }
+    else
+    {
+        DIE("Unknown capture channel");
+        return 0;
+    }
 }
 
 template <class HW>