GD32 system_clock詳解

時鍾就是整個系統正常運行的命脈，時鍾配置不對或者理解不全，后面定時器的配置就會出問題

系統運行時首先是從下圖這里開始，先加載SystemInit函數，然后加載main函數

 

 SystemInit()是廠家封裝好的，不用改動，只要理解就行了。

void SystemInit (void)
{
  /* FPU settings ------------------------------------------------------------*/
  #if (__FPU_PRESENT == 1) && (__FPU_USED == 1)
    SCB->CPACR |= ((3UL << 10*2)|(3UL << 11*2));  /* set CP10 and CP11 Full Access */
  #endif
  /* Reset the RCU clock configuration to the default reset state ------------*/
  /* Set IRC16MEN bit */
  RCU_CTL |= RCU_CTL_IRC16MEN;

  RCU_MODIFY;　　//源代碼沒‘；’這會導致代碼查看有問題（雖然執行上沒問題）
    
  /* Reset CFG0 register */
  RCU_CFG0 = 0x00000000U;

  /* Reset HXTALEN, CKMEN and PLLEN bits */
  RCU_CTL &= ~(RCU_CTL_PLLEN | RCU_CTL_CKMEN | RCU_CTL_HXTALEN);

  /* Reset PLLCFGR register */
  RCU_PLL = 0x24003010U;

  /* Reset HSEBYP bit */
  RCU_CTL &= ~(RCU_CTL_HXTALBPS);

  /* Disable all interrupts */
  RCU_INT = 0x00000000U;
         
  /* Configure the System clock source, PLL Multiplier and Divider factors, 
     AHB/APBx prescalers and Flash settings ----------------------------------*/
  system_clock_config();
}

前面都不用管，都是系統進行的初始化操作，重點看system_clock_config()里，它會根據用戶定義的時鍾源選擇相應 的時鍾初始化函數

時鍾源的選擇要看下用戶手冊，然后跟硬件工程師確認下。這里我就犯了一個錯誤，看到GD的demo里他們用的是外部25M的高速時鍾，我也理所當然的用了這個。結果調試定時器死活調不對，輸出的PWM也不對。問了硬件工程師才知道，原來我們外部的高速時鍾源是8M的。而內部的16M的雖然也可以用，但是它的精度不高，不推薦用。當初從STM換到GD，移植這塊的時候就糊里糊塗的沒細究，結果后面還是會被卡脖子。

 

 

 

我們這里選擇的是外部高速時鍾__SYSTEM_CLOCK_200M_PLL_8M_HXTAL

 

 

 其時鍾初始化函數如下：

static void system_clock_200m_25m_hxtal(void)
{
    uint32_t timeout = 0U;
    uint32_t stab_flag = 0U;
    
    /* enable HXTAL */
    RCU_CTL |= RCU_CTL_HXTALEN;

    /* wait until HXTAL is stable or the startup time is longer than HXTAL_STARTUP_TIMEOUT */
    do{
        timeout++;
        stab_flag = (RCU_CTL & RCU_CTL_HXTALSTB);
    }while((0U == stab_flag) && (HXTAL_STARTUP_TIMEOUT != timeout));

    /* if fail */
    if(0U == (RCU_CTL & RCU_CTL_HXTALSTB)){
        while(1){
        }
    }
         
    RCU_APB1EN |= RCU_APB1EN_PMUEN;
    PMU_CTL |= PMU_CTL_LDOVS;

    /* HXTAL is stable */
    /* AHB = SYSCLK = 200M */
    RCU_CFG0 |= RCU_AHB_CKSYS_DIV1;
    /* APB2 = AHB/2 = 100M */
    RCU_CFG0 |= RCU_APB2_CKAHB_DIV2;
    /* APB1 = AHB/4 = 50M */
    RCU_CFG0 |= RCU_APB1_CKAHB_DIV4;

    /* Configure the main PLL, PSC = 25, PLL_N = 400, PLL_P = 2, PLL_Q = 9 */ 
    RCU_PLL = (25U | (400U << 6U) | (((2U >> 1U) - 1U) << 16U) |
                   (RCU_PLLSRC_HXTAL) | (9U << 24U));

    /* enable PLL */
    RCU_CTL |= RCU_CTL_PLLEN;

    /* wait until PLL is stable */
    while(0U == (RCU_CTL & RCU_CTL_PLLSTB)){
    }
    
    /* Enable the high-drive to extend the clock frequency to 200 Mhz */
    PMU_CTL |= PMU_CTL_HDEN;
    while(0U == (PMU_CS & PMU_CS_HDRF)){
    }
    
    /* select the high-drive mode */
    PMU_CTL |= PMU_CTL_HDS;
    while(0U == (PMU_CS & PMU_CS_HDSRF)){
    } 
    
    /* select PLL as system clock */
    RCU_CFG0 &= ~RCU_CFG0_SCS;
    RCU_CFG0 |= RCU_CKSYSSRC_PLLP;

    /* wait until PLL is selected as system clock */
    while(0U == (RCU_CFG0 & RCU_SCSS_PLLP)){
    }
}

1. 開啟高速時鍾源

RCU_CTL |= RCU_CTL_HXTALEN;

2.配置 APB1 = AHB/4

RCU_CFG0 |= RCU_APB1_CKAHB_DIV4;

3.配置PLL為系統時鍾源

這樣系統時鍾就是200M的時鍾了

那么APB1是多少呢？

根據RCU_CFG1的寄存器說明：

 

 

 

AHB = SYSCLK；

APB1 = AHB/4= 200M/4 = 50MHz；
CK_TIMERx = 2 * CK_APB1 = 100MHz
若對時鍾配置寄存器 1 進行時鍾配置的話，
rcu_timer_clock_prescaler_config(RCU_TIMER_PSC_MUL4);
則：

CK_TIMERx = 4 * CK_APB1 = 200MHz