时钟

用哪个外设就要开他对应的时钟

例子:RCC->APB2ENR |=(0x01<<3);//时钟

需要在APB2上开启对应的时钟

拓展：系统时钟如何配置

static void SetSysClockTo72(void)

{

__IO uint32_t StartUpCounter = 0, HSEStatus = 0;

/* SYSCLK, HCLK, PCLK2 and PCLK1 configuration ---------------------------/

/ 使能外部高速晶振 */

RCC->CR |= ((uint32_t)RCC_CR_HSEON);

/* 等待高速晶振就绪 /

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)//时钟开启成功

{

//单片机内部flash配置

/ Enable Prefetch Buffer /

FLASH->ACR |= FLASH_ACR_PRFTBE;

/ Flash 2 wait state /

FLASH->ACR &= (uint32_t)((uint32_t)~FLASH_ACR_LATENCY);

FLASH->ACR |= (uint32_t)FLASH_ACR_LATENCY_2;

/ HCLK = SYSCLK /

RCC->CFGR |= (uint32_t)RCC_CFGR_HPRE_DIV1;

/ PCLK2 = HCLK /

RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE2_DIV1;

/ PCLK1 = HCLK /

RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE1_DIV2;

/ 配置PLL时钟的倍频 /

RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_PLLSRC | RCC_CFGR_PLLXTPRE |

RCC_CFGR_PLLMULL));

RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLSRC_HSE | RCC_CFGR_PLLMULL9);

/ 开启PLL时钟 /

RCC->CR |= RCC_CR_PLLON;

/ 等待PLL时钟开启 /

while((RCC->CR & RCC_CR_PLLRDY) == 0)

{

}

/ 选择PLL作为系统时钟 /

RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_SW));

RCC->CFGR |= (uint32_t)RCC_CFGR_SW_PLL;

/ 等待时钟就绪 /

while ((RCC->CFGR & (uint32_t)RCC_CFGR_SWS) != (uint32_t)0x08)

{

}

}

else

{ / If HSE fails to start-up, the application will have wrong clock

configuration. User can add here some code to deal with this error */

}

}

更改倍频

单片机如何实现延时？

NOP延时：空指令，在执行这个指令的时候，单片机啥也不干，等待一个时钟周期

单片机主频72Mhz

1个nop等待 1/72000000 秒 1/72000 ms 1/72us

define delay_1us() __nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();#define delay_1us() __nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();

__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();

__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();

__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();

__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();

__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();

__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();__nop();

__nop();__nop();

void delay_us(uint32_t time)

{

while(time--)

{

delay_1us();

}

}

void delay_ms(uint32_t time)

{

while(time--)

{

delay_us(1000);

}

}

ST公司，为了降低功耗，默认外设的时钟是关闭的，需要使用外设时，第一步就是要先开启这个时钟