diff --git a/stm/Makefile b/stm/Makefile
index 9c0fe2acf..f228e5fe2 100644
--- a/stm/Makefile
+++ b/stm/Makefile
@@ -18,6 +18,7 @@ FATFSSRC=fatfs
 CC3KSRC=cc3k
 DFU=../tools/dfu.py
 TARGET=PYBOARD
+OSC_VALUE=8000000
 
 AS = arm-none-eabi-as
 CC = arm-none-eabi-gcc
@@ -25,7 +26,7 @@ LD = arm-none-eabi-ld
 OBJCOPY = arm-none-eabi-objcopy
 SIZE = arm-none-eabi-size
 
-CFLAGS_CORTEX_M4 = -mthumb -mtune=cortex-m4 -mabi=aapcs-linux -mcpu=cortex-m4 -mfpu=fpv4-sp-d16 -mfloat-abi=hard -fsingle-precision-constant -Wdouble-promotion -DSTM32F40_41xxx -DUSE_STDPERIPH_DRIVER -DHSE_VALUE=8000000
+CFLAGS_CORTEX_M4 = -mthumb -mtune=cortex-m4 -mabi=aapcs-linux -mcpu=cortex-m4 -mfpu=fpv4-sp-d16 -mfloat-abi=hard -fsingle-precision-constant -Wdouble-promotion -DSTM32F40_41xxx -DUSE_STDPERIPH_DRIVER -DHSE_VALUE=$(OSC_VALUE)
 CFLAGS = -I. -I$(PY_SRC) -I$(FATFSSRC) -I$(CMSIS) -I$(STMSRC) -Wall -ansi -std=gnu99 $(CFLAGS_CORTEX_M4) -D$(TARGET)
 #CFLAGS += -I$(STMOTGSRC) -DUSE_HOST_MODE -DUSE_OTG_MODE
 
diff --git a/stm/system_stm32f4xx.c b/stm/system_stm32f4xx.c
index 8568a9b4f..12ac4d6b5 100644
--- a/stm/system_stm32f4xx.c
+++ b/stm/system_stm32f4xx.c
@@ -54,9 +54,9 @@
   *-----------------------------------------------------------------------------
   *        APB2 Prescaler                         | 2
   *-----------------------------------------------------------------------------
-  *        HSE Frequency(Hz)                      | 8000000     changed dpgeorge
+  *        HSE Frequency(Hz)                      | HSE_VALUE 
   *-----------------------------------------------------------------------------
-  *        PLL_M                                  | 8           changed dpgeorge
+  *        PLL_M                                  | (HSE_VALUE/1000000)
   *-----------------------------------------------------------------------------
   *        PLL_N                                  | 336
   *-----------------------------------------------------------------------------
@@ -251,7 +251,7 @@
 
 /************************* PLL Parameters *************************************/
 /* PLL_VCO = (HSE_VALUE or HSI_VALUE / PLL_M) * PLL_N */
-#define PLL_M      8
+#define PLL_M      (HSE_VALUE/1000000)
 /* USB OTG FS, SDIO and RNG Clock =  PLL_VCO / PLLQ */
 #define PLL_Q      7