UART4自定义映射在芯片上发不出来

#define UART4_BASE            (APB1PERIPH_BASE + 0x4C00)
/******UART4寄存器地址映射**************************/
#define UART4_SR     (*(volatile unsigned long *) (UART4_BASE + 0x00))
#define UART4_DR     (*(volatile unsigned long *) (UART4_BASE + 0x04))
#define UART4_BRR     (*(volatile unsigned long *) (UART4_BASE + 0x08))
#define UART4_CR1     (*(volatile unsigned long *) (UART4_BASE + 0x0C))
#define UART4_CR2     (*(volatile unsigned long *) (UART4_BASE + 0x10))
#define UART4_CR3     (*(volatile unsigned long *) (UART4_BASE + 0x14))
#define UART4_GTPR     (*(volatile unsigned long *) (UART4_BASE + 0x18))
以上是自己定义的映射，下面是定义的初始化，可为什么芯片上就是发不出来呢？请大家多提意见，急死了啊！芯片是STM32F103RET6
RCC->APB2ENR |= 1 << 4;    //使能PORTC时钟  
RCC->APB1ENR |= 1 << 19;   //使能串口4UART时钟
    GPIOC->CRH |= 0x44444B22;

   
RCC->APB1RSTR |= 1 << 19;   //复位串口4
RCC->APB1RSTR &= ~(1<<19);//停止复位
      
//波特率设置
  UART4_BRR =  baud; // 波特率设置  
UART4_CR1 |= 0X200C;  //1位停止,无校验位.
//使能接收中断
    UART4_CR1 |= 1 << 8;    //PE中断使能
UART4_CR1 |= 1 << 5;    //接收缓冲区非空中断使能