stm8l051f3p6串口发送失败的原因有哪些？

想通过串口tx打印调试信息，但是始终调不通，没用中断，没用rx。示波器观察PA2始终高电平。
代码如下：
void SysClkInit(void)
{
    CLK_CKDIVR_bit.CKM = 3;
    CLK_ICKCR_bit.HSION = 1;
    CLK_SWR = 1;

    CLK_PCKENR1_bit.PCKEN15 = 1;       //USART1
    CLK_PCKENR1_bit.PCKEN10 = 1;       //tiM2

    CLK_PCKENR2_bit.PCKEN27 = 1;        //Boot ROM
    CLK_PCKENR2_bit.PCKEN20 = 1;        //ADC1
}


void SysUSART1Init(void)
{
    SYSCFG_RMPCR1_bit.USART1TR_REMAP = 1;       //USART1_TX on PA2 and USART1_RX on PA3
    PA_DDR_DDR2 = 1;            //output
    PA_ODR_ODR2 = 1;            //data
    PA_CR1_C12 = 1;             //push pull
    PA_CR2_C22 = 1;             //10M

    USART1_CR1_bit.M = 0;       //1 start bit, 8 data bits, n stop bit
    USART1_CR3_bit.STOP0 = 0;
    USART1_CR3_bit.STOP1 = 0;   //1 stop bit
    USART1_CR1_bit.PCEN = 0;    //parity disable
    USART1_BRR2 = 0x00;         //9600
    USART1_BRR1 = 0x0D;

    USART1_CR2_bit.TEN = 1;     //transmitter enable
    USART1_CR1_bit.USARTD = 0;  //usart enable
}


void UsartTxAByte(unsigned char ucData)
{
    USART1_DR = ucData;
    while (0 == USART1_SR_bit.TXE);
}