带有PDC的UART写缓冲区

Question

我在使用const缓冲区和char arrray写入USARt时遇到了问题。

下面是我的UART写函数：

unsigned int USART_Send(  unsigned char *p_pucData, 
                              unsigned int p_unLen)
{


  AT91C_BASE_US2->US_TPR = (unsigned int)p_pucData;
  AT91C_BASE_US2->US_TCR =  p_unLen;
  AT91C_BASE_US2->US_PTCR = AT91C_PDC_TXTEN;


  while((AT91C_BASE_US2->US_CSR & ((0x1 << 11) | (0x1 << 4) ) ) == 0);

   AT91C_BASE_US2->US_PTCR = AT91C_PDC_TXTDIS;

    return p_unLen;    
}

下面的函数与const char*类似：

USART_Send("IsitDone?",9);   //Working

如果我使用像下面这样的数组缓冲区来显示垃圾字符，那么为什么呢？

 unsigned char arr[10];
  memcpy(arr, "HelloWorld", 10);
  USART_Send(arr, sizeof(arr));  //Not working properly displaying Garbage chars

Answer 1

里卡多·克鲁多是对的。您会遇到以下问题：

arr is created on the stack
arr is filled
call USART_Send
    fill transmit pointer, counter, enable tx requests
    /* peripheral state is TXBUFE = '0' and ENDTX = '0' because:           */
    /* TXBUFE = '0' when PERIPH_TCR != 0 and                               */
    /* ENDTX = '0' when PERIPH_TCR != 0                                    */
    /* but we just wrote to PERIPH_TCR, so it's != 0                       */
    /* both conditions are satisfied, b/c the transfer hasn't started yet! */
    wait until (TXBUFE = '0' and ENDTX = '0')
    /* your code thinks PDC is done here      */
    /* but in reality, PDC is getting started */
    disable tx requests
return from sub-function
overwrite stack (and arr) with unrelated data here
/* PDC might push out last word(s) here due to pipelining/        */
/* instruction cache/side effects/you-name-it                     */
/* even though its matrix requests were disabled a few cycles ago */

解决办法：

• 复制到全局缓冲区或
• 在启用tx请求和检查PDC是否已完成(可能是整个波特刻度)、或之间等待一些周期。
• 读取PERIPH_TCR并检查它是否为零，而不是检查标志

理想情况下，您应该为字符串分配一些动态内存，并在PDC异步执行到实际代码之后释放它。在完成PDC/外设之后，您可能需要检查是否可以得到某种中断，然后释放从它读取的内存。

如果您没有动态内存分配，那么使用全局循环缓冲区并抽象您的字符串/char发送函数来代替此缓冲区。