XUartPs_SetFifoThreshold

/****************************************************************************/
/**
*
* This functions sets the receive FIFO trigger level. The receive trigger
* level specifies the number of bytes in the receive FIFO that cause a receive
* data event (interrupt) to be generated.
*
* @param    InstancePtr is a pointer to the XUartPs instance.
* @param    TriggerLevel contains the trigger level to set.
*
* @return    None
*
* @note        None.
*
*****************************************************************************/
void XUartPs_SetFifoThreshold(XUartPs *InstancePtr, u8 TriggerLevel)
{
    u32 RtrigRegister;

    /* Assert validates the input arguments */
    Xil_AssertVoid(InstancePtr != NULL);
    Xil_AssertVoid(TriggerLevel <= (u8)XUARTPS_RXWM_MASK);
    Xil_AssertVoid(InstancePtr->IsReady == XIL_COMPONENT_IS_READY);

    RtrigRegister = ((u32)TriggerLevel) & (u32)XUARTPS_RXWM_MASK;

    /*
     * Write the new value for the FIFO control register to it such that the
     * threshold is changed
     */
    XUartPs_WriteReg(InstancePtr->Config.BaseAddress,
               XUARTPS_RXWM_OFFSET, RtrigRegister);

}

这里 调用 XUartPs_SetFifoThreshold 函数 设置 寄存器 XUARTPS_RXWM_MASK ， 根据 UG585 手册 

 这里设置 接收 FIFO，接收到 多少字节，触发 中断 ，我 默认设置为 0和设置1 ，效果一样 。

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