Merge pull request #2381 from Guozhanxin/hw_us

[bsp][stm32] add rt_hw_us_delay

bsp/stm32/libraries/HAL_Drivers/drv_common.c

@@ -86,6 +86,41 @@ void _Error_Handler(char *s, int num)
     /* USER CODE END Error_Handler */
 }
 
+/**
+ * This function will delay for some us.
+ *
+ * @param us the delay time of us
+ */
+void rt_hw_us_delay(rt_uint32_t us)
+{
+    rt_uint32_t ticks;
+    rt_uint32_t told, tnow, tcnt = 0;
+    rt_uint32_t reload = SysTick->LOAD;
+
+    ticks = us * reload / (1000000 / RT_TICK_PER_SECOND);
+    told = SysTick->VAL;
+    while (1)
+    {
+        tnow = SysTick->VAL;
+        if (tnow != told)
+        {
+            if (tnow < told)
+            {
+                tcnt += told - tnow;
+            }
+            else
+            {
+                tcnt += reload - tnow + told;
+            }
+            told = tnow;
+            if (tcnt >= ticks)
+            {
+                break;
+            }
+        }
+    }
+}
+
 /**
  * This function will initial STM32 board.
  */