c - 带 PDC 的 UART 写缓冲器

Question

我在使用 const char buffer 和 char arrray 写入 USARt 时遇到问题。

这是我的 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;    
}

以下使用 const char* 的函数如下:

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

如果我使用如下所示的数组缓冲区，它会显示乱码，想知道为什么吗？

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

Answer 1

Ricardo Crudo 是正确的。您遇到了以下问题:

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 */

解决方案:

• 复制到全局缓冲区或
• 在启用 tx 请求和检查 PDC 是否完成之间等待一些周期(可能是整个波特率 tick)或
• 读回 PERIPH_TCR 并检查它是否为零而不是检查标志

理想情况下，您将为字符串分配一些动态内存，并在 PDC 与您的实际代码异步完成后释放它。您可能想检查在 PDC/外围设备完成后是否可以获得某种中断，然后释放它读取的内存。

如果您没有动态内存分配，则使用全局环形缓冲区并将您的字符串/字符发送函数抽象化以改为使用此缓冲区。