L4_HAL/uart: Adapt UART HAL to avoid 64-bit integer division.

64-bit integer division brings a dependency on library functions. It is
avoided here by dividing fck and baud by a common divisior.  The error
is the better (1/(2*0x300)) as with 64 bit division (1/(0x300)).

This patch is originally from the MicroPython repository and due to
Tobias Badertscher <python@baerospace.ch>.

STM32L4xx_HAL_Driver/Inc/stm32l4xx_hal_uart.h

@@ -1007,7 +1007,8 @@ typedef struct
   * @param  __BAUD__: Baud rate set by the user.
   * @retval Division result
   */
-#define UART_DIV_LPUART(__PCLK__, __BAUD__)      ((((uint64_t)(__PCLK__)*256) + ((__BAUD__)/2)) / (__BAUD__))
+/* FIXME tobbad Adapted to avoid 64 bit division. */
+#define UART_DIV_LPUART(__PCLK__, __BAUD__)                HAL_UART_CalcBrr((__PCLK__), (__BAUD__))
 
 /** @brief  BRR division operation to set BRR register in 8-bit oversampling mode.
   * @param  __PCLK__: UART clock.
@@ -1425,6 +1426,8 @@ void UART_AdvFeatureConfig(UART_HandleTypeDef *huart);
 /**
   * @}
   */
+/* Functions added by MicroPython */
+uint32_t HAL_UART_CalcBrr(uint32_t fck, uint32_t baud);
 
 #ifdef __cplusplus
 }

STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_uart.c

@@ -2767,6 +2767,44 @@ static HAL_StatusTypeDef UART_Receive_IT(UART_HandleTypeDef *huart)
   }
 }
 
+
+/**
+  * @brief Calculate register BRR value without using uint64.
+  * @note   This function is added by the MicroPython project.
+  * @param  fck: Input clock frequency to the uart block in Hz.
+  * @param  baud: baud rate should be one of {300, 600, 1200, 2400, 4800, 9600, 19200, 57600, 115200}.
+  * @retval BRR value
+  */
+uint32_t HAL_UART_CalcBrr(uint32_t fck, uint32_t baud)
+{
+    const struct
+    {
+        uint32_t limit;
+        uint32_t div;
+    } comDiv[]= {
+        {1<<31, 300 }, /* must be >= 256 */
+        {1<<30, 150 }, /* must be >= 128 */
+        {1<<29,  75 }, /* must be >= 64 */
+        {1<<28,  50 }, /* must be >= 32 */
+        {1<<27,  20 }, /* must be >= 16 */
+        {1<<26,  10 }, /* must be >= 8 */
+        {1<<25,   5 }, /* must be >= 4 */
+        {1<<24,   2 }  /* must be >= 2 */
+    };
+    const uint32_t comDivCnt = sizeof(comDiv)/sizeof(comDiv[0]);
+    uint8_t i;
+    for (i=0; i<comDivCnt ;i++)
+    {
+        if (fck >= comDiv[i].limit)
+        {
+            fck  /= comDiv[i].div;
+            baud /= comDiv[i].div;
+            break;
+        }
+    }
+    return (fck<<8)/baud;
+}
+
 /**
   * @}
   */