diff --git a/libraries/mbed/targets/cmsis/TARGET_STM/TARGET_STM32F7/TARGET_NUCLEO_F746ZG/stm32f7xx_hal_conf.h b/libraries/mbed/targets/cmsis/TARGET_STM/TARGET_STM32F7/TARGET_NUCLEO_F746ZG/stm32f7xx_hal_conf.h
index 8c67585b9b..313fa4355e 100644
--- a/libraries/mbed/targets/cmsis/TARGET_STM/TARGET_STM32F7/TARGET_NUCLEO_F746ZG/stm32f7xx_hal_conf.h
+++ b/libraries/mbed/targets/cmsis/TARGET_STM/TARGET_STM32F7/TARGET_NUCLEO_F746ZG/stm32f7xx_hal_conf.h
@@ -102,7 +102,7 @@
   *        (when HSE is used as system clock source, directly or through the PLL).  
   */
 #if !defined  (HSE_VALUE) 
-  #define HSE_VALUE    ((uint32_t)25000000) /*!< Value of the External oscillator in Hz */
+  #define HSE_VALUE    ((uint32_t)8000000) /*!< Value of the External oscillator in Hz */
 #endif /* HSE_VALUE */
 
 #if !defined  (HSE_STARTUP_TIMEOUT)
diff --git a/libraries/mbed/targets/cmsis/TARGET_STM/TARGET_STM32F7/TARGET_NUCLEO_F746ZG/system_stm32f7xx.c b/libraries/mbed/targets/cmsis/TARGET_STM/TARGET_STM32F7/TARGET_NUCLEO_F746ZG/system_stm32f7xx.c
index 9c3676eb68..4ff3b6b2a4 100644
--- a/libraries/mbed/targets/cmsis/TARGET_STM/TARGET_STM32F7/TARGET_NUCLEO_F746ZG/system_stm32f7xx.c
+++ b/libraries/mbed/targets/cmsis/TARGET_STM/TARGET_STM32F7/TARGET_NUCLEO_F746ZG/system_stm32f7xx.c
@@ -133,7 +133,7 @@ HAL_StatusTypeDef HAL_Init(void);
   */
 
 /* Select the clock sources (other than HSI) to start with (0=OFF, 1=ON) */
-#define USE_PLL_HSE_EXTC (0) /* Use external clock --> NOT USED ON THIS BOARD */ 
+#define USE_PLL_HSE_EXTC (1) /* Use external clock --> NOT USED ON THIS BOARD */ 
 #define USE_PLL_HSE_XTAL (1) /* Use external xtal */
 
 /**
@@ -607,10 +607,10 @@ uint8_t SetSysClock_PLL_HSE(uint8_t bypass)
   {
     RCC_OscInitStruct.HSEState          = RCC_HSE_BYPASS; /* External clock on OSC_IN */
   }
-  // Warning: this configuration is for a 25 MHz xtal clock only
+  // Warning: this configuration is for a 8 MHz xtal clock only
   RCC_OscInitStruct.PLL.PLLState        = RCC_PLL_ON;
   RCC_OscInitStruct.PLL.PLLSource       = RCC_PLLSOURCE_HSE;
-  RCC_OscInitStruct.PLL.PLLM            = 25;            // VCO input clock = 1 MHz (25 MHz / 25)
+  RCC_OscInitStruct.PLL.PLLM            = 8;             // VCO input clock = 1 MHz (8 MHz / 8)
   RCC_OscInitStruct.PLL.PLLN            = 432;           // VCO output clock = 432 MHz (1 MHz * 432)
   RCC_OscInitStruct.PLL.PLLP            = RCC_PLLP_DIV2; // PLLCLK = 216 MHz (432 MHz / 2)
   RCC_OscInitStruct.PLL.PLLQ            = 9;             // USB clock = 48 MHz (432 MHz / 9) --> OK for USB