diff --git a/boards.txt b/boards.txt
index adf880f..9c7fd64 100644
--- a/boards.txt
+++ b/boards.txt
@@ -2,6 +2,8 @@
 
 menu.pnum=Board Select
 
+menu.clock=Clock Select
+
 menu.xserial=U(S)ART support
 menu.usb=USB support (if available)
 menu.xusb=USB speed (if available)
@@ -49,6 +51,20 @@ CH32V00x_EVT.menu.upload_method.swdMethod.upload.options=
 CH32V00x_EVT.menu.upload_method.swdMethod.upload.tool=WCH_linkE
 
 
+# Clock Select
+CH32V00x_EVT.menu.clock.48MHz_HSI=48MHz Internal
+CH32V00x_EVT.menu.clock.48MHz_HSI.build.flags.clock=-DSYSCLK_FREQ_48MHz_HSI=48000000
+CH32V00x_EVT.menu.clock.24MHz_HSI=24MHz Internal
+CH32V00x_EVT.menu.clock.24MHz_HSI.build.flags.clock=-DSYSCLK_FREQ_24MHz_HSI=24000000
+CH32V00x_EVT.menu.clock.8MHz_HSI=8MHz Internal
+CH32V00x_EVT.menu.clock.8MHz_HSI.build.flags.clock=-DSYSCLK_FREQ_8MHz_HSI=8000000
+CH32V00x_EVT.menu.clock.48MHz_HSE=48MHz External
+CH32V00x_EVT.menu.clock.48MHz_HSE.build.flags.clock=-DSYSCLK_FREQ_48MHz_HSE=48000000
+CH32V00x_EVT.menu.clock.24MHz_HSE=24MHz External
+CH32V00x_EVT.menu.clock.24MHz_HSE.build.flags.clock=-DSYSCLK_FREQ_24MHz_HSE=24000000
+CH32V00x_EVT.menu.clock.8MHz_HSE=8MHz External
+CH32V00x_EVT.menu.clock.8MHz_HSE.build.flags.clock=-DSYSCLK_FREQ_8MHz_HSE=8000000
+
 
 # Optimizations
 CH32V00x_EVT.menu.opt.osstd=Smallest (-Os default)
diff --git a/platform.txt b/platform.txt
index be673c6..8aeed9e 100644
--- a/platform.txt
+++ b/platform.txt
@@ -32,15 +32,15 @@ compiler.objdump.cmd=riscv-none-embed-objdump
 
 compiler.libraries.ldflags=
 
-#"@{build.opt.path}" {build.flags.optimize}  {compiler.warning_flags} {build.flags.debug} -MP -MF
+#"@{build.opt.path}" {build.flags.clock} {build.flags.optimize}  {compiler.warning_flags} {build.flags.debug} -MP -MF
 
 compiler.extra_flags=-march={build.march} -mabi={build.mabi} -msmall-data-limit=8 -msave-restore -fmessage-length=0 -fsigned-char -ffunction-sections -fdata-sections -fno-common 
 
 compiler.S.flags={compiler.extra_flags} -x assembler-with-cpp "-I{build.system.path}/{build.series}/SRC/Startup/" "-I{build.core.path}/ch32/"
 
-compiler.c.flags={compiler.extra_flags} -c {build.flags.optimize} {build.flags.debug} {compiler.warning_flags} -std=gnu99 -MMD {compiler.ch.extra_include}
+compiler.c.flags={compiler.extra_flags} -c {build.flags.clock} {build.flags.optimize} {build.flags.debug} {compiler.warning_flags} -std=gnu99 -MMD {compiler.ch.extra_include}
 
-compiler.cpp.flags={compiler.extra_flags} -c {build.flags.optimize} {build.flags.debug} {compiler.warning_flags} -std={compiler.cpp.std}  -fno-threadsafe-statics  -fno-rtti -fno-exceptions -fno-use-cxa-atexit -MMD {compiler.ch.extra_include} -fpermissive
+compiler.cpp.flags={compiler.extra_flags} -c {build.flags.clock} {build.flags.optimize} {build.flags.debug} {compiler.warning_flags} -std={compiler.cpp.std}  -fno-threadsafe-statics  -fno-rtti -fno-exceptions -fno-use-cxa-atexit -MMD {compiler.ch.extra_include} -fpermissive
 
 compiler.ar.flags=rcs
 
@@ -99,6 +99,7 @@ build.enable_virtio=
 build.peripheral_pins=
 build.startup_file=
 build.flags.fp=
+build.flags.clock=
 build.flags.optimize=
 build.flags.debug=
 build.flags.ldspecs=
diff --git a/system/CH32V00x/USER/system_ch32v00x.c b/system/CH32V00x/USER/system_ch32v00x.c
index b735036..9adadee 100644
--- a/system/CH32V00x/USER/system_ch32v00x.c
+++ b/system/CH32V00x/USER/system_ch32v00x.c
@@ -17,20 +17,21 @@
 * If none of the define below is enabled, the HSI is used as System clock source. 
 */
 
+//Do it in menu selection in Arduino
 //#define SYSCLK_FREQ_8MHz_HSI    8000000
-//#define SYSCLK_FREQ_24MHZ_HSI   HSI_VALUE
-//#define SYSCLK_FREQ_48MHZ_HSI   48000000
+//#define SYSCLK_FREQ_24MHz_HSI   HSI_VALUE
+//#define SYSCLK_FREQ_48MHz_HSI   48000000
 //#define SYSCLK_FREQ_8MHz_HSE    8000000
 //#define SYSCLK_FREQ_24MHz_HSE   HSE_VALUE
-#define SYSCLK_FREQ_48MHz_HSE   48000000
+//#define SYSCLK_FREQ_48MHz_HSE   48000000
 
 /* Clock Definitions */
 #ifdef SYSCLK_FREQ_8MHz_HSI
   uint32_t SystemCoreClock         = SYSCLK_FREQ_8MHz_HSI;          /* System Clock Frequency (Core Clock) */
-#elif defined SYSCLK_FREQ_24MHZ_HSI
-  uint32_t SystemCoreClock         = SYSCLK_FREQ_24MHZ_HSI;        /* System Clock Frequency (Core Clock) */
-#elif defined SYSCLK_FREQ_48MHZ_HSI
-  uint32_t SystemCoreClock         = SYSCLK_FREQ_48MHZ_HSI;        /* System Clock Frequency (Core Clock) */
+#elif defined SYSCLK_FREQ_24MHz_HSI
+  uint32_t SystemCoreClock         = SYSCLK_FREQ_24MHz_HSI;        /* System Clock Frequency (Core Clock) */
+#elif defined SYSCLK_FREQ_48MHz_HSI
+  uint32_t SystemCoreClock         = SYSCLK_FREQ_48MHz_HSI;        /* System Clock Frequency (Core Clock) */
 #elif defined SYSCLK_FREQ_8MHz_HSE
   uint32_t SystemCoreClock         = SYSCLK_FREQ_8MHz_HSE;         /* System Clock Frequency (Core Clock) */
 #elif defined SYSCLK_FREQ_24MHz_HSE
@@ -49,10 +50,10 @@ static void SetSysClock(void);
 
 #ifdef SYSCLK_FREQ_8MHz_HSI
   static void SetSysClockTo_8MHz_HSI(void);
-#elif defined SYSCLK_FREQ_24MHZ_HSI
-  static void SetSysClockTo_24MHZ_HSI(void);
-#elif defined SYSCLK_FREQ_48MHZ_HSI
-  static void SetSysClockTo_48MHZ_HSI(void);
+#elif defined SYSCLK_FREQ_24MHz_HSI
+  static void SetSysClockTo_24MHz_HSI(void);
+#elif defined SYSCLK_FREQ_48MHz_HSI
+  static void SetSysClockTo_48MHz_HSI(void);
 #elif defined SYSCLK_FREQ_8MHz_HSE
   static void SetSysClockTo_8MHz_HSE(void);
 #elif defined SYSCLK_FREQ_24MHz_HSE
@@ -144,10 +145,10 @@ static void SetSysClock(void)
 {
 #ifdef SYSCLK_FREQ_8MHz_HSI
     SetSysClockTo_8MHz_HSI();
-#elif defined SYSCLK_FREQ_24MHZ_HSI
-    SetSysClockTo_24MHZ_HSI();
-#elif defined SYSCLK_FREQ_48MHZ_HSI
-    SetSysClockTo_48MHZ_HSI();
+#elif defined SYSCLK_FREQ_24MHz_HSI
+    SetSysClockTo_24MHz_HSI();
+#elif defined SYSCLK_FREQ_48MHz_HSI
+    SetSysClockTo_48MHz_HSI();
 #elif defined SYSCLK_FREQ_8MHz_HSE
     SetSysClockTo_8MHz_HSE();
 #elif defined SYSCLK_FREQ_24MHz_HSE
@@ -181,16 +182,16 @@ static void SetSysClockTo_8MHz_HSI(void)
     RCC->CFGR0 |= (uint32_t)RCC_HPRE_DIV3;
 }
 
-#elif defined SYSCLK_FREQ_24MHZ_HSI
+#elif defined SYSCLK_FREQ_24MHz_HSI
 
 /*********************************************************************
- * @fn      SetSysClockTo_24MHZ_HSI
+ * @fn      SetSysClockTo_24MHz_HSI
  *
  * @brief   Sets System clock frequency to 24MHz and configure HCLK, PCLK2 and PCLK1 prescalers.
  *
  * @return  none
  */
-static void SetSysClockTo_24MHZ_HSI(void)
+static void SetSysClockTo_24MHz_HSI(void)
 {
     /* Flash 0 wait state */
     FLASH->ACTLR &= (uint32_t)((uint32_t)~FLASH_ACTLR_LATENCY);
@@ -201,16 +202,16 @@ static void SetSysClockTo_24MHZ_HSI(void)
 }
 
 
-#elif defined SYSCLK_FREQ_48MHZ_HSI
+#elif defined SYSCLK_FREQ_48MHz_HSI
 
 /*********************************************************************
- * @fn      SetSysClockTo_48MHZ_HSI
+ * @fn      SetSysClockTo_48MHz_HSI
  *
  * @brief   Sets System clock frequency to 48MHz and configure HCLK, PCLK2 and PCLK1 prescalers.
  *
  * @return  none
  */
-static void SetSysClockTo_48MHZ_HSI(void)
+static void SetSysClockTo_48MHz_HSI(void)
 {
     /* Flash 0 wait state */
     FLASH->ACTLR &= (uint32_t)((uint32_t)~FLASH_ACTLR_LATENCY);