使用HSE振荡器作为PLL的时钟源，并配置PLL作为系统时钟源系统时钟设置为168MHZ，写出相关代码

使用HSE（高频外部时钟源）作为PLL（锁相环路频率合成器）的输入并将其配置为系统的主时钟源，通常涉及嵌入式微控制器如STM32系列，这里我会给出一个基于STM32F4的示例代码片段。请注意，具体的代码会因硬件库的不同而有所差异，这里假设我们已经在项目中包含了`stm32f4xx_hal.h`等头文件。

#include "stm32f4xx_hal.h"

// PLL Configuration Constants
#define HSE_VALUE        8000000 // Your HSE frequency (8 MHz in this example)
#define M_VALUE          8       // M value for the multiplication factor
#define N_VALUE          32768   // Prescaler value for system clock (HSI or HSI48)
#define PREDIV_VALUE     2       // PLL division factor

void SystemClock_Config(void) {
    RCC_OscInitTypeDef RCC_OscInitStruct;
    RCC_ClkInitTypeDef RCC_ClkInitStruct;

    // Enable Power Control clock
    __HAL_RCC_PWR_CLK_ENABLE();

    // HSE configuration
    RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSE;
    RCC_OscInitStruct.HSEState = RCC_HSE_ON; // Enable HSE Oscillator
    RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON; // Enable PLL
    RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSE; // Use HSE as PLL input
    RCC_OscInitStruct.PLL.PLLM = M_VALUE; // Set M (Multiplication Factor)
    RCC_OscInitStruct.PLL.PLLN = N_VALUE; // Set N (Prescaler value for HSI or HSI48)
    RCC_OscInitStruct.PLL.PLLP = PREDIV_VALUE; // Set PLLP (Division Factor)
    RCC_OscInitStruct.PLL.PLLQ = 5; // Set Q (Clock Division)

    if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK) {
        Error_Handler(); // Handle error
    }

    // Select PLL as system clock source and set HSI prescaler
    RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_SYSCLK | RCC_CLOCKTYPE_HCLK | RCC_CLOCKTYPE_PCLK1 | RCC_CLOCKTYPE_PCLK2;
    RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
    RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1; // HCLK at same speed as SYSCLK
    RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV4; // PCLK1 divider
    RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV2; // PCLK2 divider

    if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_5) != HAL_OK) {
        Error_Handler();
    }

    // Output clock frequency check
    uint32_t SysClockFreq = HAL_RCC_GetSysClockFreq();
    if (SysClockFreq != 168000000) {
        printf("Failed to set system clock to 16.8 MHz, current freq: %u MHz\n", SysClockFreq / 1000000);
    }
}

int main(void) {
    // ... (Your main function initialization here)
    SystemClock_Config();
    // ... (Continue with your main program)
}