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;
}