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I was looking to port the code to a board with no external oscillator, I figured out the code changes and post them here if anyone else is interested:

--- a/targets/stm32/jshardware.c
+++ b/targets/stm32/jshardware.c
@@ -2487,13 +2487,14 @@ bool jshSleep(JsSysTime timeUntilWake) {
 [#endif](https://forum.espruino.com/search/?q=%23endif)
 [#endif](https://forum.espruino.com/search/?q=%23endif)
     // recover oscillator
-    RCC_HSEConfig(RCC_HSE_ON);
-    if( RCC_WaitForHSEStartUp() == SUCCESS) {
+      RCC_HSICmd(ENABLE);
+//    RCC_HSEConfig(RCC_HSE_ON);
+//    if( RCC_WaitForHSEStartUp() == SUCCESS) {
       RCC_PLLCmd(ENABLE);
       while(RCC_GetFlagStatus(RCC_FLAG_PLLRDY) == RESET);
       RCC_SYSCLKConfig(RCC_SYSCLKSource_PLLCLK);
       while(RCC_GetSYSCLKSource() != 0x08);
-    }
+//    }
     RTC_WaitForSynchro(); // make sure any RTC reads will be done
 [#ifdef](https://forum.espruino.com/search/?q=%23ifdef) USB
     jshSetUSBPower(true);
@@ -2916,14 +2917,14 @@ unsigned int jshSetSystemClock(JsVar *options) {
   if (pclk2<-1) return 0;
 
   // Run off external clock - 8Mhz - while we configure everything
-  RCC_SYSCLKConfig(RCC_SYSCLKSource_HSE);
+  RCC_SYSCLKConfig(RCC_SYSCLKSource_HSI);
   // set latency
   if (latency!=255) FLASH_SetLatency(latency);
   if (pclk1>=0) RCC_PCLK1Config(pclk1);
   if (pclk2>=0) RCC_PCLK2Config(pclk2);
   // update PLL
   RCC_PLLCmd(DISABLE);
-  RCC_PLLConfig(RCC_PLLSource_HSE, m, n, p, q);
+  RCC_PLLConfig(RCC_PLLSource_HSI, m, n, p, q);
   RCC_PLLCmd(ENABLE);
   while(RCC_GetFlagStatus(RCC_FLAG_PLLRDY) == RESET) {}
   RCC_SYSCLKConfig(RCC_SYSCLKSource_PLLCLK);

and

--- a/targetlibs/stm32f4/lib/system_stm32f4xx.c
+++ b/targetlibs/stm32f4/lib/system_stm32f4xx.c
@@ -256,7 +256,7 @@
 // SYSCLK = PLL_VCO / PLL_P 
 
 [#define](https://forum.espruino.com/search/?q=%23define) PLL_M      8
-#define PLL_Q      7
+#define PLL_Q      4
 
 [#if](https://forum.espruino.com/search/?q=%23if) defined (STM32F40_41xxx)
 [#define](https://forum.espruino.com/search/?q=%23define) PLL_N      336
@@ -269,8 +269,8 @@
 [#endif](https://forum.espruino.com/search/?q=%23endif) /* STM32F427_437x || STM32F429_439xx */
 
 [#if](https://forum.espruino.com/search/?q=%23if) defined (STM32F401xx) // 84Mhz
-#define PLL_N      336
-#define PLL_P      4
+#define PLL_N      84
+#define PLL_P      2
 [#endif](https://forum.espruino.com/search/?q=%23endif) /* STM32F401xx */
 
 [#if](https://forum.espruino.com/search/?q=%23if) defined (STM32F411xx) // 96Mhz 
@@ -499,29 +499,29 @@ static void SetSysClock(void)
 /******************************************************************************/
 /*            PLL (clocked by HSE) used as System clock source                */
 /******************************************************************************/
-  __IO uint32_t StartUpCounter = 0, HSEStatus = 0;
+//  __IO uint32_t StartUpCounter = 0, HSEStatus = 0;
 
   /* Enable HSE */
-  RCC->CR |= ((uint32_t)RCC_CR_HSEON);
+//  RCC->CR |= ((uint32_t)RCC_CR_HSEON);
 
   /* Wait till HSE is ready and if Time out is reached exit */
-  do
-  {
-    HSEStatus = RCC->CR & RCC_CR_HSERDY;
-    StartUpCounter++;
-  } while((HSEStatus == 0) && (StartUpCounter != HSE_STARTUP_TIMEOUT));
-
-  if ((RCC->CR & RCC_CR_HSERDY) != RESET)
-  {
-    HSEStatus = (uint32_t)0x01;
-  }
-  else
-  {
-    HSEStatus = (uint32_t)0x00;
-  }
-
-  if (HSEStatus == (uint32_t)0x01)
-  {
+//  do
+//  {
+//    HSEStatus = RCC->CR & RCC_CR_HSERDY;
+//    StartUpCounter++;
+//  } while((HSEStatus == 0) && (StartUpCounter != HSE_STARTUP_TIMEOUT));
+//
+//  if ((RCC->CR & RCC_CR_HSERDY) != RESET)
+//  {
+//    HSEStatus = (uint32_t)0x01;
+//  }
+//  else
+//  {
+//    HSEStatus = (uint32_t)0x00;
+//  }
+
+//  if (HSEStatus == (uint32_t)0x01)
+//  {
     /* Select regulator voltage output Scale 1 mode */
     RCC->APB1ENR |= RCC_APB1ENR_PWREN;
     PWR->CR |= PWR_CR_VOS;
@@ -547,7 +547,7 @@ static void SetSysClock(void)
 
     /* Configure the main PLL */
     RCC->PLLCFGR = PLL_M | (PLL_N << 6) | (((PLL_P >> 1) -1) << 16) |
-                   (RCC_PLLCFGR_PLLSRC_HSE) | (PLL_Q << 24);
+                   (RCC_PLLCFGR_PLLSRC_HSI) | (PLL_Q << 24);
 
     /* Enable the main PLL */
     RCC->CR |= RCC_CR_PLLON;

Is there an easy way to switch to the internal oscillator on the pico board?

Thanks Gordon, that worked great.

Is there a way to use the STM32 Serial Bootloader on the Pico board? I can't use dfu-util since I want to program it from another board that only has a UART available. Is there any easy way to access Boot1 pin? Do I have to tie it to ground?