外设频率是一个比较难把握的知识，请按照我的理解  不可相信其他教程

晶振位12MHZ

配置system_LPC17xx.c 参数值如下：

#define CLOCK_SETUP 1
#define SCS_Val 0x00000020 //系统时钟源 ->12M晶振
#define CLKSRCSEL_Val 0x00000001
#define PLL0_SETUP 1
#define PLL0CFG_Val 0x00050063  //M  N位锁相环倍频参数分别为6和100
#define PLL1_SETUP 1
#define PLL1CFG_Val 0x00000023
#define CCLKCFG_Val 0x00000003 //锁相环输出分频位CPU时钟 CCLK=FCCO/4;
#define USBCLKCFG_Val 0x00000000 //无USB
#define PCLKSEL0_Val 0x00000000//0x00000140//   外设在CPU时钟的基础上选择是否为主时钟的1/4
#define PCLKSEL1_Val 0x00000000//0x00050000//  是否选择UART2、3为主时钟的1/4
#define PCONP_Val 0x042887DE
#define CLKOUTCFG_Val 0x00000000

系统初始化函数：

void SystemInit (void)
{
#if (CLOCK_SETUP) /* Clock Setup */
LPC_SC->SCS = SCS_Val;
if (SCS_Val & (1 << 5)) { /* If Main Oscillator is enabled */
while ((LPC_SC->SCS & (1<<6)) == 0);/* Wait for Oscillator to be ready */
}

LPC_SC->CCLKCFG = CCLKCFG_Val; /* Setup Clock Divider */

#if (PLL0_SETUP)
LPC_SC->CLKSRCSEL = CLKSRCSEL_Val; /* Select Clock Source for PLL0 */

LPC_SC->PLL0CFG = PLL0CFG_Val; /* configure PLL0 set M and N Value */
LPC_SC->PLL0FEED = 0xAA;
LPC_SC->PLL0FEED = 0x55;

LPC_SC->PLL0CON = 0x01; /* PLL0 Enable Lock M and N Value */
LPC_SC->PLL0FEED = 0xAA;
LPC_SC->PLL0FEED = 0x55;
while (!(LPC_SC->PLL0STAT & (1<<26)));/* Wait for PLOCK0 STAT include M and N */

LPC_SC->PLL0CON = 0x03; /* PLL0 Enable & Connect */
LPC_SC->PLL0FEED = 0xAA;
LPC_SC->PLL0FEED = 0x55;
while (!(LPC_SC->PLL0STAT & ((1<<25) | (1<<24))));/* Wait for PLLC0_STAT & PLLE0_STAT */
#endif

#if (PLL1_SETUP)
LPC_SC->PLL1CFG = PLL1CFG_Val;
LPC_SC->PLL1FEED = 0xAA;
LPC_SC->PLL1FEED = 0x55;

LPC_SC->PLL1CON = 0x01; /* PLL1 Enable */
LPC_SC->PLL1FEED = 0xAA;
LPC_SC->PLL1FEED = 0x55;
while (!(LPC_SC->PLL1STAT & (1<<10)));/* Wait for PLOCK1 */

LPC_SC->PLL1CON = 0x03; /* PLL1 Enable & Connect */
LPC_SC->PLL1FEED = 0xAA;
LPC_SC->PLL1FEED = 0x55;
while (!(LPC_SC->PLL1STAT & ((1<< 9) | (1<< 8))));/* Wait for PLLC1_STAT & PLLE1_STAT */
#else
LPC_SC->USBCLKCFG = USBCLKCFG_Val; /* Setup USB Clock Divider */
#endif

LPC_SC->PCLKSEL0 = PCLKSEL0_Val; /* Peripheral Clock Selection CCLK/4 CPU时钟的1/4 */
LPC_SC->PCLKSEL1 = PCLKSEL1_Val;

LPC_SC->PCONP = PCONP_Val; /* Power Control for Peripherals 0x042887DE=1110 1111 1110 1111 1111 0111 1101 1110 */

LPC_SC->CLKOUTCFG = CLKOUTCFG_Val; /* Clock Output Configuration */
#endif

#if (FLASH_SETUP == 1) /* Flash Accelerator Setup */
LPC_SC->FLASHCFG = FLASHCFG_Val;
#endif
}

时钟配置结果：

FOSC=12000000=12M

FCCO=2*FOSC*100/6=400M

FCCLK=FCCO/4=100M

对应PCLKSEL0_Val相应位的数据 00-》1/4分频 外设波特率位25M

01-》表示不分频 则外设 （如串口）的频率为100M

串口初始化：

void UART0_Init (void)
{
uint16_t usFdiv;
/* UART0 */
LPC_PINCON->PINSEL0 |= (1 << 4); /* Pin P0.2 used as TXD0 (Com0) */
LPC_PINCON->PINSEL0 |= (1 << 6); /* Pin P0.3 used as RXD0 (Com0) */

LPC_UART0->LCR = 0x83; /* 允许设置波特率 */
/*
usFdiv = (FPCLK / 16) / UART0_BPS; // 设置波特率
LPC_UART0->DLM = usFdiv / 256;
LPC_UART0->DLL = usFdiv % 256;

LPC_UART0->LCR = 0x03;
LPC_UART0->FCR = 0x06;
*/

若频率为25MHZ  波特率115200

LPC_UART0->FDR = (2<<4) | 1;
LPC_UART0->DLM = 0;
LPC_UART0->DLL = 9;
LPC_UART0->LCR = 0x03; // 锁定波特率


LPC_UART0->FCR = 0xc7;
/*
LPC_UART0->FDR = (14<<4) | 9;
LPC_UART0->DLM = 0;
LPC_UART0->DLL = 33;
LPC_UART0->LCR = 0x03; // 锁定波特率


LPC_UART0->FCR = 0xc7;
*/
/*
NVIC_SetPriority(UART0_IRQn,1);//UART0_INT_PRIORITY);
NVIC_EnableIRQ(UART0_IRQn);
LPC_UART0->IER = 0x03;
*/
}

参数配置规律如下：

确定外设频率FPCLK  （通常为25M  或100M）

FDR寄存器配置如下：Bits 3:0 为DIVADDVAL  7:4为MULVAL （>=1）;

UARTn_baudrate=PCLK/(  16*(256*UnDLM+UnDLL)*( 1+(DivAddVal/MulVal) )  )

1.  1<=MULVAL<=15

2.  0<=DIVADDVAL<=14

3.  DIVADDVAL<MULVAL

例子如下：

PCLK=12MHZ，BR=115200

由公式得知DL_est=PCLK/(16*BR)=6.51  不为整数

则用FR_est=1.5来确定DL_est的值DL_est=int(PCLK/(16*BR*1.5))=4;

FR_{est=PCLK/(16*BR*DLest)}=1.625   (1.1<FR_est<1.9)

1.628与表格table中1.625相邻  取1.625的数据

实际串口率为115384  与115200有0.16%的误差  但是不影响串口的正常工作

LPC1768串口时钟配置全解析