STM32: add lpuart_clock_source config

Keep same clock configuration as done before this PR (LSE and PCLK1).
Use a JSON file to change it.

Author: bcostm

targets/TARGET_STM/serial_api.c

@@ -32,6 +32,11 @@
 
 #include "serial_api_hal.h"
 
+// Possible choices of the LPUART_CLOCK_SOURCE configuration set in json file
+#define USE_LPUART_CLK_LSE    0x01
+#define USE_LPUART_CLK_PCLK1  0x02
+#define USE_LPUART_CLK_HSI    0x04
+
 int stdio_uart_inited = 0; // used in platform/mbed_board.c and platform/mbed_retarget.cpp
 serial_t stdio_uart;
 
@@ -367,29 +372,43 @@ void serial_baud(serial_t *obj, int baudrate)
     struct serial_s *obj_s = SERIAL_S(obj);
 
     obj_s->baudrate = baudrate;
+
 #if defined(LPUART1_BASE)
     /* Note that LPUART clock source must be in the range [3 x baud rate, 4096 x baud rate], check Ref Manual */
     if (obj_s->uart == LPUART_1) {
-        /* If baudrate is lower than 9600 try to change to LSE */
         RCC_PeriphCLKInitTypeDef PeriphClkInitStruct = {0};
+        PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_LPUART1;
+#if ((MBED_CONF_TARGET_LPUART_CLOCK_SOURCE) & USE_LPUART_CLK_LSE)
         if (baudrate <= 9600 && __HAL_RCC_GET_FLAG(RCC_FLAG_LSERDY)) {
-            PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_LPUART1;
             PeriphClkInitStruct.Lpuart1ClockSelection = RCC_LPUART1CLKSOURCE_LSE;
             HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct);
-        } else {
-            PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_LPUART1;
-            PeriphClkInitStruct.Lpuart1ClockSelection = RCC_LPUART1CLKSOURCE_PCLK1;
-            HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct);
+            if (init_uart(obj) == HAL_OK) {
+                return;
+            }
         }
+#endif
+#if ((MBED_CONF_TARGET_LPUART_CLOCK_SOURCE) & USE_LPUART_CLK_PCLK1)
+        PeriphClkInitStruct.Lpuart1ClockSelection = RCC_LPUART1CLKSOURCE_PCLK1;
+        HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct);
         if (init_uart(obj) == HAL_OK) {
             return;
         }
-        /* Change LPUART clock source and try again */
-        PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_LPUART1;
+#endif
+#if ((MBED_CONF_TARGET_LPUART_CLOCK_SOURCE) & USE_LPUART_CLK_HSI)
+        if (__HAL_RCC_GET_FLAG(RCC_FLAG_HSIRDY)) {
+            PeriphClkInitStruct.Lpuart1ClockSelection = RCC_LPUART1CLKSOURCE_HSI;
+            HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct);
+            if (init_uart(obj) == HAL_OK) {
+            return;
+            }
+        }
+#endif
+        // Last chance using SYSCLK
         PeriphClkInitStruct.Lpuart1ClockSelection = RCC_LPUART1CLKSOURCE_SYSCLK;
         HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct);
     }
 #endif /* LPUART1_BASE */
+
     if (init_uart(obj) != HAL_OK) {
         debug("Cannot initialize UART with baud rate %u\n", baudrate);
     }

targets/targets.json

@@ -730,6 +730,10 @@
                 "help": "Define if a Low Speed External xtal (LSE) is available on the board (0 = No, 1 = Yes). If Yes, the LSE will be used to clock the RTC, LPUART, ... otherwise the Low Speed Internal clock (LSI) will be used",
                 "value": "1"
             },
+            "lpuart_clock_source": {
+                "help": "Define the LPUART clock source. Mask values: USE_LPUART_CLK_LSE, USE_LPUART_CLK_PCLK1, USE_LPUART_CLK_HSI",
+                "value": "USE_LPUART_CLK_LSE|USE_LPUART_CLK_PCLK1"
+            },
             "stdio_uart_tx": {
                 "help": "default TX STDIO pins is defined in PinNames.h file, but it can be overridden"
             },