diff --git a/mkspecs/features/stm32f4/syscalls.c b/mkspecs/features/stm32f4/syscalls.c
--- a/mkspecs/features/stm32f4/syscalls.c
+++ b/mkspecs/features/stm32f4/syscalls.c
@@ -11,6 +11,7 @@
 #include <uart.h>
 #include <stdint.h>
 #include <bsp.h>
+#include <core.h>
 
 #undef errno
 
diff --git a/src/stm32f4/I2C/i2c.c b/src/stm32f4/I2C/i2c.c
--- a/src/stm32f4/I2C/i2c.c
+++ b/src/stm32f4/I2C/i2c.c
@@ -40,7 +40,8 @@ i2c_t i2copen(int count)
 #define _INIT_DEV(_RCC_) \
     RCC_APB1PeriphClockCmd(_RCC_, ENABLE); \
     RCC_APB1PeriphResetCmd(_RCC_, ENABLE); \
-    RCC_APB1PeriphResetCmd(_RCC_, DISABLE);
+    RCC_APB1PeriphResetCmd(_RCC_, DISABLE); \
+    RCC_APB1PeriphClockCmd(_RCC_, ENABLE);
 
     switch(count)
     {