一般咱们在main()的开端部分都需求进行时钟的初始化，根据需求能够挑选时钟源、是否敞开PLL倍频、以及PLL装备到达所需求的输出时钟，然后再挑选倍频后的时钟作为主时钟源。
如下图：需求留意几个时钟的概念，
#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--体系振荡器 */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第七位写0，即体系PLL上电 */while ( !(LPC_SYSCON->SYSPLLSTAT & 0x01) ); /* Wait until its locked 确认PLL确认今后向下履行 */LPC_SYSCON->MAINCLKSEL = 0x03; /* Select PLL clock output--挑选PLL输出作为主时钟 */LPC_SYSCON->MAINCLKUEN = 0x01; /* Update MCLK clock source --答应更新主时钟*/LPC_SYSCON->MAINCLKUEN = 0x00; /* Toggle update register once */LPC_SYSCON->MAINCLKUEN = 0x01;while ( !(LPC_SYSCON->MAINCLKUEN & 0x01) ); /* Wait until updated 确认主时钟确认今后向下履行 */LPC_SYSCON->SYSAHBCLKDIV = SYS_AHB_DIV_Val; /* SYS AHB clock, typical is 1 or 2 or 4 --SYSAHBCLKDIV的值为1，即不分频 使AHB时钟设置为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的第五位写0，即体系体系振荡器上电 *//* 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模块的时钟return;}
