对于无操作系统的软件来说,STM32上电后的第一件事,就是系统初始化。
Libaries\CMSIS_MDK\CM3\DeviceSupport\ST\STM32F10x\system_stm32f10x.c 中,官方给出了系统初始化函数 SystemInit(),我添加了一些中文注释,便于学习、模仿。
/**
* @brief Setup the microcontroller system
* Initialize the Embedded Flash Interface, the PLL and update the
* SystemCoreClock variable.
* @note This function should be used only after reset.
* @param None
* @retval None
*/
void SystemInit (void)
{
/* Reset the RCC clock configuration to the default reset state(for debug purpose) */
/* Set HSION bit */
RCC->CR |= (uint32_t)0x00000001; // 打开内部的 8MHz RC 振荡器, 使能内部高速时钟(HSI)
/* Reset SW, HPRE, PPRE1, PPRE2, ADCPRE and MCO bits */
#ifndef STM32F10X_CL
RCC->CFGR &= (uint32_t)0xF8FF0000;
#else
// MCO = 000: MCO 引脚设置成无时钟输出
// ADCPRE = 00: ADC 的时钟分频系数为2, ADC 的时钟频率=PCLK2/2
// PPRE2 = 000: APB2 的时钟分频系数为1, PCLK2 = HCLK
// PPRE1 = 000: APB1 的时钟分频系数为1, PCLK1 = HCLK
// HPRE = 0000: AHB 的时钟分频系数为1, HCLK = SYSCLK = 8MHz
// SW = 00: 选择 HSI 作为系统时钟
RCC->CFGR &= (uint32_t)0xF0FF0000;
#endif /* STM32F10X_CL */
/* Reset HSEON, CSSON and PLLON bits */
// PLLON = 0: 禁用 PLL
// CSSON = 0: 禁用时钟安全系统(CSS)
// HSEON = 0: 关闭外部高速时钟(HSE)振荡器
RCC->CR &= (uint32_t)0xFEF6FFFF;
/* Reset HSEBYP bit */
// HSEBY = 0: 外部高速时钟(HSE)振荡器没有准备好
RCC->CR &= (uint32_t)0xFFFBFFFF;
/* Reset PLLSRC, PLLXTPRE, PLLMUL and USBPRE/OTGFSPRE bits */
// OTGFSPRE = 0: USB OTG FS 的时钟频率 = 2*PLLCLK/3
// PLLMUL = 0000: 保留
// PLLXTPRE = 0: 如果选择 PREDIV1 为 PLL 的输入时钟, 则不对 PREDIV1 分频
// PLLSRC = 0: 选择 HSI 时钟二分器为PLL 的输入时钟
RCC->CFGR &= (uint32_t)0xFF80FFFF;
#ifdef STM32F10X_CL
/* Reset PLL2ON and PLL3ON bits */
RCC->CR &= (uint32_t)0xEBFFFFFF;
/* Disable all interrupts and clear pending bits */
RCC->CIR = 0x00FF0000;
/* Reset CFGR2 register */
RCC->CFGR2 = 0x00000000;
#elif defined (STM32F10X_LD_VL) || defined (STM32F10X_MD_VL) || (defined STM32F10X_HD_VL)
/* Disable all interrupts and clear pending bits */
RCC->CIR = 0x009F0000;
/* Reset CFGR2 register */
RCC->CFGR2 = 0x00000000;
#else
/* Disable all interrupts and clear pending bits */
// CSSC = 1: 清除"时钟安全系统"中断标志(CSSF)
// PLL3RDYC = 0: 无效
// PLL2RDYC = 0: 无效
// PLLRDYC = 1: 清除"PLL 准备"中断标志(PLLRDYF)
// HSERDYC = 1: 清除"外部高速时钟准备"中断标志(HSERDYF)
// HSIRDYC = 1: 清除"内部高速时钟准备"中断标志(HSIRDYF)
// LSERDYC = 1: 清除"外部低速时钟准备"中断标志(LSERDYF)
// LSIRDYC = 1: 清除"内部低速时钟准备"中断标志(LSIRDYF)
// PLL3RDYIE= 0: 禁止"PLL3 锁定"中断
// PLL2RDYIE= 0: 禁止"PLL2 锁定"中断
// PLLRDYIE = 0: 禁止"PLL 锁定"中断
// HSERDYIE = 0: 禁止"外部高速时钟准备"中断
// HSIRDYIE = 0: 禁止"内部高速时钟准备"中断
// LSERDYIE = 0: 禁止"外部低速时钟准备"中断
// LSIRDYIE = 0: 禁止"内部低速时钟准备"中断
RCC->CIR = 0x009F0000;
#endif /* STM32F10X_CL */
#if defined (STM32F10X_HD) || (defined STM32F10X_XL) || (defined STM32F10X_HD_VL)
#ifdef DATA_IN_ExtSRAM
SystemInit_ExtMemCtl();
#endif /* DATA_IN_ExtSRAM */
#endif
/* Configure the System clock frequency, HCLK, PCLK2 and PCLK1 prescalers */
/* Configure the Flash Latency cycles and enable prefetch buffer */
SetSysClock();
#ifdef VECT_TAB_SRAM
SCB->VTOR = SRAM_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal SRAM. */
#else
// 如果从内置 Flash 中开始运行程序, 则起始地址为 0x08000000
SCB->VTOR = FLASH_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal FLASH. */
#endif
}
执行 SetSysClock() 之前,时钟配置图如下: