源程序为USART1的配置，更改USART3/4相应寄存器测试，测试一直显示有规律乱码，收发不符。
void uart_init(u32 pclk,u32 bound)
{
float temp;
u16 mantissa;
u16 fraction;
temp=(float)(pclk*1000000)/(bound*16);//得到USARTDIV
mantissa=temp; //得到整数部分
fraction=(temp-mantissa)*16; //得到小数部分
mantissa<<=4;
mantissa+=fraction;
//重映射USART到PC10、11
RCC->APB2ENR|=1<<0; //启动AFIO时钟
AFIO->MAPR&=~(0x1<<4);
AFIO->MAPR|=0x1<<4;
RCC->APB1ENR|=1<<18; //使能串口3时钟
RCC->APB2ENR|=1<<4; //使能PORTC口时钟
GPIOC->CRH&=0XFFFF00FF;//IO状态设置
GPIOC->CRH|=0X00008B00;//IO状态设置
RCC->APB1RSTR|=1<<18; //复位串口3
RCC->APB1RSTR&=~(1<<18);//停止复位
//波特率设置
USART3->BRR=mantissa; // 波特率设置
USART3->CR1|=0X200C; //1位停止,无校验位.
}发送：
printf("nihao啊");串口助手接收：
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USART1与其余的USART使用的时钟不一样，PCLK1最高为36Mhz
初始化时PCLK=36后串口助手接收正常
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