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，然后再選擇倍頻后的時鐘作為主時鐘源。

如下圖：需要注意幾個時鐘的概念，

#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 it's locked 確定PLL鎖定以后向下執行 */
  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 確定主時鐘鎖定以后向下執行 */
  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_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的第五位寫0，即系統系統振蕩器上電 */
  /* 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模塊的時鐘
  return;
}