一般我们在main()的开始部分都需要进行时钟的初始化，根据需要可以选择时钟源、是否开启PLL倍频、以及PLL配置达到所需要的输出时钟，然后再选择倍频后的时钟作为主时钟源。
如下图：需要注意几个时钟的概念，
#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--系统振荡器 */ 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，即系统PLL上电 */ while ( !(LPC_SYSCON->SYSPLLSTAT & 0x01) ); /* Wait until it's locked 确定PLL锁定以后向下执行 */ LPC_SYSCON->MAINCLKSEL = 0x03; /* Select PLL clock output--选择PLL输出作为主时钟 */ LPC_SYSCON->MAINCLKUEN = 0x01; /* Update MCLK clock source --允许更新主时钟*/ LPC_SYSCON->MAINCLKUEN = 0x00; /* Toggle update register once */ LPC_SYSCON->MAINCLKUEN = 0x01; while ( !(LPC_SYSCON->MAINCLKUEN & 0x01) ); /* Wait until updated 确定主时钟锁定以后向下执行 */ 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_SETUP SystemFrequency = ClockSource * (MAIN_PLL_M_Val+1);#else SystemFrequency = ClockSource;#endif SystemAHBFrequency = (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，即系统系统振荡器上电 */ /* Wait 200us for OSC to be stablized, no status indication, 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 or most of the I/O related peripherals won't work. */ LPC_SYSCON->SYSAHBCLKCTRL |= (1<<16); //使能IO模块的时钟 return;}
-电子元器件采购网（www.ruidan.com）是本土元器件目录分销商，采用“小批量、现货、样品”销售模式，致力于满足客户多型号、高质量、快速交付的采购需求。 自建高效智能仓储，拥有自营库存超过50,000种，提供一站式正品现货采购、个性化解决方案、选型替代等多元化服务。
