int Usart_Printf(const char *fmt,...)  {      while(transmission_flag==0);   	      va_list ap;      int n;      char send_buf[256];      va_start(ap, fmt);      n=vsprintf(send_buf, fmt, ap);      va_end(ap);      Dma_Enable(send_buf,n);  	transmission_flag=0;      return n;  }

#include "dma.h"  extern uint8_t transmission_flag;    void DMA1_NVIC_Init(void)  {       NVIC_InitTypeDef NVIC_InitStructure;          //DMA1通道4 NVIC 配置      NVIC_InitStructure.NVIC_IRQChannel =  DMA1_Channel4_IRQn;          NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority=0 ;//抢占优先级3      NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;        //子优先级3      NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;            //IRQ通道使能         NVIC_Init(&NVIC_InitStructure);    //根据指定的参数初始化VIC寄存器  }  void Dma_Init()  {  	DMA_InitTypeDef  DMA_InitStructure;  	RCC_AHBPeriphClockCmd(RCC_AHBPeriph_DMA1, ENABLE);  	DMA1_NVIC_Init();  	DMA_DeInit(DMA1_Channel4);  	DMA_InitStructure.DMA_PeripheralBaseAddr = (uint32_t)& (USART1->DR);  	//DMA_InitStructure.DMA_MemoryBaseAddr = (uint32_t)NULL;  	DMA_InitStructure.DMA_DIR = DMA_DIR_PeripheralDST;  	DMA_InitStructure.DMA_BufferSize = 0;  	DMA_InitStructure.DMA_PeripheralInc = DMA_PeripheralInc_Disable;  	DMA_InitStructure.DMA_MemoryInc = DMA_MemoryInc_Enable;  	DMA_InitStructure.DMA_PeripheralDataSize = DMA_PeripheralDataSize_Byte;  	DMA_InitStructure.DMA_MemoryDataSize = DMA_MemoryDataSize_Byte;  	DMA_InitStructure.DMA_Mode = DMA_Mode_Normal;  	DMA_InitStructure.DMA_Priority =DMA_Priority_VeryHigh;  	DMA_InitStructure.DMA_M2M = DMA_M2M_Disable;  	DMA_Init(DMA1_Channel4, &DMA_InitStructure);  	DMA_ITConfig(DMA1_Channel4, DMA_IT_TC, ENABLE);  	  	  }    void Dma_Enable(uint8_t* buffer,uint32_t len)  {  	DMA_Cmd(DMA1_Channel4, DISABLE);  	DMA1_Channel4->CMAR =  (uint32_t)buffer;  	DMA1_Channel4->CNDTR =len ;  	DMA_Cmd(DMA1_Channel4, ENABLE);  }    void DMA1_Channel4_IRQHandler()  {  	if(DMA_GetFlagStatus(DMA1_FLAG_TC4)==SET)  	{	  		transmission_flag=1;  		DMA_ClearFlag(DMA1_FLAG_TC4);  	}  	  }