LPUART CLOCK SOURCE SELECTION LEFT TO SERIAL DRIVER.

The clock source selection of LPUART depends on System clocks but also on
the serial baudrate. There is a specific computation done in serial driver
targets/target_STM/serial_api.c

At first start-up the LPUART1 clock selected in SetSysClock was anyway
overridden by the serial driver, so this was of no effect. But in case
of deep sleep SetSysClock is called again, while the driver isn't, so
SetSyClock was corrupting the serial clock configuration.

So let's remove these few lines of code which are causing trouble.

For targets L496 and L5.

Taking into account device TRNG in L5 configuration

targets/TARGET_STM/TARGET_STM32L4/TARGET_STM32L496xG/system_clock.c

@@ -154,13 +154,6 @@ MBED_WEAK uint8_t SetSysClock_PLL_HSE(uint8_t bypass)
     }
 #endif /* DEVICE_USBDEVICE */
 
-    /* Select HSI as clock source for LPUART1 */
-    RCC_PeriphClkInit.PeriphClockSelection = RCC_PERIPHCLK_LPUART1;
-    RCC_PeriphClkInit.Lpuart1ClockSelection = RCC_LPUART1CLKSOURCE_HSI;
-    if (HAL_RCCEx_PeriphCLKConfig(&RCC_PeriphClkInit) != HAL_OK) {
-        return 0; // FAIL
-    }
-
     // Output clock on MCO1 pin(PA8) for debugging purpose
 #if DEBUG_MCO == 2
     if (bypass == 0) {
@@ -224,13 +217,6 @@ uint8_t SetSysClock_PLL_HSI(void)
     }
 #endif /* DEVICE_USBDEVICE */
 
-    /* Select HSI as clock source for LPUART1 */
-    RCC_PeriphClkInit.PeriphClockSelection = RCC_PERIPHCLK_LPUART1;
-    RCC_PeriphClkInit.Lpuart1ClockSelection = RCC_LPUART1CLKSOURCE_HSI;
-    if (HAL_RCCEx_PeriphCLKConfig(&RCC_PeriphClkInit) != HAL_OK) {
-        return 0; // FAIL
-    }
-
     // Output clock on MCO1 pin(PA8) for debugging purpose
 #if DEBUG_MCO == 3
     HAL_RCC_MCOConfig(RCC_MCO1, RCC_MCO1SOURCE_HSI, RCC_MCODIV_1); // 16 MHz
@@ -303,13 +289,6 @@ uint8_t SetSysClock_PLL_MSI(void)
         return 0; // FAIL
     }
 
-    /* Select LSE as clock source for LPUART1 */
-    PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_LPUART1;
-    PeriphClkInitStruct.Lpuart1ClockSelection = RCC_LPUART1CLKSOURCE_LSE;
-    if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct) != HAL_OK) {
-        return 0; // FAIL
-    }
-
     // Output clock on MCO1 pin(PA8) for debugging purpose
 #if DEBUG_MCO == 4
     HAL_RCC_MCOConfig(RCC_MCO1, RCC_MCO1SOURCE_MSI, RCC_MCODIV_2); // 2 MHz

targets/TARGET_STM/TARGET_STM32L5/system_clock.c

@@ -177,15 +177,13 @@ uint8_t SetSysClock_PLL_MSI(void)
         return 0; // FAIL
     }
 
-    // Default STDIO is LPUART1
     RCC_PeriphCLKInitTypeDef PeriphClkInitStruct = {0};
+
 #if DEVICE_TRNG
-    PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_LPUART1 | RCC_PERIPHCLK_RNG;
+    PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_RNG;
     PeriphClkInitStruct.RngClockSelection = RCC_RNGCLKSOURCE_HSI48;
-#else
-    PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_LPUART1;
 #endif
-    PeriphClkInitStruct.Lpuart1ClockSelection = RCC_LPUART1CLKSOURCE_LSE;
+
     HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct);
 
     return 1; // OK