Also on STM32 using dma+ serial port to obtain variable length data

The latest one STM SCM development needs to connect a module , Serial communication .

STM32 SCM is a typical F103Cx, It is a simple operation in this paper , I didn't expect to do it 2 God . There are many posts on the Internet, including various existing routines , Use RXNE I won't say if I interrupt the reception , Single chip computers with heavy load are not recommended . about DMA Use , Most articles use DMA+ Serial port is free IDLE sign , Get variable length data , The usual way is ：

1. To configure DMA, Use single pass mode

2. Configure serial port , Interrupt flag IDLE,

3 Receive data in the serial port interrupt , Reset DMA buffer

Found in actual use ：

The serial port data does not spit smoothly ,IDLE Interrupts occur frequently , The buffer read by multiple interrupts needs to be spliced together to be the complete data , So I set DMA Buffers are basically wasteful . But also need to add splicing code . The use effect is not ideal .

Since the nature of the module itself cannot be changed , Can only solve their own software problems . In the observation of serial port data . The data interval of each frame exceeds 500ms, The amount of data per frame is 400-800byte Unequal . So we have the following improvements ：

The general principle is ： Give Way DMA Go to buffer Fill in the data , When one frame is finished , There was a time when there was no growth in data , Check this period of time , Judge the end of the frame , I also see people using timers to detect this period of time , The effect is the same , But I will definitely skip a timer , Just count directly .

Methods and steps ：

1. To configure DMA, The buffer is set to be greater than the number of frame bytes （1000）, Use continuous mode （ Data exceeding the buffer will overwrite the previous ）

2. Configure serial port without interruption （ Because the single-chip microcomputer has display timing requirements , Do not use interrupts at will , Will cause display disorder ）

3. stay while The process of querying the number of buffers in CheckDMA();

#define RXIDLE 500

#define USART1_MAX_RECV_LEN 1000

void CheckDMA(void){
    u16 temp;
    static u16 lastcnt,idletime;
    if(DMA1_Channel5->CNDTR<USART1_MAX_RECV_LEN){// There's data coming in
        if(DMA1_Channel5->CNDTR==lastcnt){// Have new data arrived
            if(++idletime>RXIDLE){// No new data has arrived over time
                temp = USART1_MAX_RECV_LEN -DMA1_Channel5->CNDTR;// Number of data read
                USART1_RX_BUF[temp]=0;// Terminator
                DATA_Analysis((u8*)USART1_RX_BUF);
                DMA_Restart();// Buffer reset , Although it is a continuous mode , But you want every frame to be written from the header .
            }
        }else idletime=0;
        lastcnt=DMA1_Channel5->CNDTR;
    }

}

void DMA_Restart(void)
{ 
    DMA_Cmd(DMA1_Channel5, DISABLE );  // close USART1 TX DMA1 Indicated channel      
     DMA_SetCurrDataCounter(DMA1_Channel5,USART1_MAX_RECV_LEN);//DMA The tunnel DMA Cache size
     DMA_Cmd(DMA1_Channel5, ENABLE);  // Can make USART1 TX DMA1 Indicated channel  
}     

After the above transformation , It realizes the reading of serial port data of irregular data sending module , By adjusting the idle time just in the right range （ No more than the frame interval , wide ）, The above procedure can work stably and reliably , And get buffer The data is continuous and accurate . because STM32 Of IDLE The time of the sign cannot be adjusted , Therefore, the above method is an effective method to solve this problem . At the same time, due to the adoption of DMA+ How to query , Avoiding other problems caused by interruptions won't be right CPU Bring extra burden