一般我們在main()的開始部分都需要進行時鐘的初始化，根據需要可以選擇時鐘源、是否開啟PLL倍頻、以及PLL配置達到所需要的輸出時鐘，然後再選擇倍頻後的時鐘作為主時鐘源。
如下圖：需要注意幾個時鐘的概念，
本文引用地址：http://www.eepw.com.cn/article/201611/323526.htm#define CLOCK_SETUP 1#define MAIN_PLL_SETUP 1#define MAIN_CLKSRCSEL_Val 0x00000001#define MAIN_PLL_M_Val 0x00000003#define MAIN_PLL_P_Val 0x00000001#define SYS_AHB_DIV_Val 1 /* 1 through 255, typical is 1 or 2 or 4 */#define XTAL (12000000UL) /* Oscillator frequency */#define OSC_CLK ( XTAL) /* Main oscillator frequency */#define IRC_OSC (12000000UL) /* Internal RC oscillator frequency */#define WDT_OSC ( 250000UL) /* WDT oscillator frequency */uint32_t ClockSource = IRC_OSC;uint32_t SystemFrequency = IRC_OSC; /*!< System Clock Frequency (Core Clock) */uint32_t SystemAHBFrequency = IRC_OSC;void Main_PLL_Setup ( void ){uint32_t regVal;ClockSource = OSC_CLK;LPC_SYSCON->SYSPLLCLKSEL = MAIN_CLKSRCSEL_Val; /* Select system OSC--系統振蕩器 */LPC_SYSCON->SYSPLLCLKUEN = 0x01; /* Update clock source */LPC_SYSCON->SYSPLLCLKUEN = 0x00; /* toggle Update register once */LPC_SYSCON->SYSPLLCLKUEN = 0x01;while ( !(LPC_SYSCON->SYSPLLCLKUEN & 0x01) ); /* Wait until updated */regVal = LPC_SYSCON->SYSPLLCTRL;regVal &= ~0x1FF;LPC_SYSCON->SYSPLLCTRL = (regVal | (MAIN_PLL_P_Val<<5) | MAIN_PLL_M_Val);/* P=1, M=4 FCLKOUT=12*4=48Mhz *//* Enable main system PLL, main system PLL bit 7 in PDRUNCFG. */LPC_SYSCON->PDRUNCFG &= ~(0x1<<7); /* PDRUNCFG第七位寫0，即系統PLL上電 */while ( !(LPC_SYSCON->SYSPLLSTAT & 0x01) ); /* Wait until its locked 確定PLL鎖定以後向下執行 */LPC_SYSCON->MAINCLKSEL = 0x03; /* Select PLL clock output--選擇PLL輸出作為主時鐘 */LPC_SYSCON->MAINCLKUEN = 0x01; /* Update MCLK clock source --允許更新主時鐘*/LPC_SYSCON->MAINCLKUEN = 0x00; /* Toggle update register once */LPC_SYSCON->MAINCLKUEN = 0x01;while ( !(LPC_SYSCON->MAINCLKUEN & 0x01) ); /* Wait until updated 確定主時鐘鎖定以後向下執行 */LPC_SYSCON->SYSAHBCLKDIV = SYS_AHB_DIV_Val; /* SYS AHB clock, typical is 1 or 2 or 4 --SYSAHBCLKDIV的值為1，即不分頻 使AHB時鐘設置為48Mhz */#if MAIN_PLL_SETUPSystemFrequency = ClockSource * (MAIN_PLL_M_Val+1);#elseSystemFrequency = ClockSource;#endifSystemAHBFrequency = (uint32_t)(SystemFrequency/SYS_AHB_DIV_Val);return;}/*** Initialize the system** @param none* @return none** @brief Setup the microcontroller system.* Initialize the System and update the SystemFrequency variable.*/void SystemInit (void){uint32_t i;#ifdef __DEBUG_RAM LPC_SYSCON->SYSMEMREMAP = 0x1; /* remap to internal RAM */#else#ifdef __DEBUG_FLASH LPC_SYSCON->SYSMEMREMAP = 0x2; /* remap to internal flash */#endif#endif#if (CLOCK_SETUP) /* Clock Setup *//* bit 0 default is crystal bypass,bit1 0=0~20Mhz crystal input, 1=15~50Mhz crystal input. */LPC_SYSCON->SYSOSCCTRL = 0x00;/* main system OSC run is cleared, bit 5 in PDRUNCFG register */LPC_SYSCON->PDRUNCFG &= ~(0x1<<5); /* PDRUNCFG的第五位寫0，即系統系統振蕩器上電 *//* Wait 200us for OSC to be stablized, no statusindication, dummy wait. */for ( i = 0; i < 0x100; i++ );#if (MAIN_PLL_SETUP)Main_PLL_Setup(); #endif#endif /* endif CLOCK_SETUP *//* System clock to the IOCON needs to be enabled ormost of the I/O related peripherals wont work. */LPC_SYSCON->SYSAHBCLKCTRL |= (1<<16); //使能IO模塊的時鐘return;}