UART DMA not transmitting data properly

[fa31]

Hi,

I am working on a project and having the following problem:
My UART DMA is not working. I am transmitting a string, that should be visible in the terminal but it is not. The TX complete callback function is being triggered, so it looks like it works but there is nothing in the terminal.

Previously, I was using UART without DMA and things were working fine. It confirms that the hardware connections and the VCP connections are correctly set up.

What I have done so far:

- I enabled UART2, tested UART (without DMA). It worked well.

- I enabled DMA which gave me:

- Then I set up GPDMA1 for huart2_TX and huart2_RX. My TX configs are:

My main function:

int main(void)
{
  HAL_Init();
  SystemClock_Config();

  MX_GPIO_Init();
  MX_GPDMA1_Init();
  MX_ICACHE_Init();
  MX_USART2_UART_Init();


  /* Infinite loop */
  while (1)
  {
	  static uint32_t counter = 0;
	  sprintf((char *)pTxBuff, "Count: %lu\r\n", counter++);
	  if (huart2.gState == HAL_UART_STATE_READY) {
	      HAL_UART_Transmit_DMA(&huart2, pTxBuff, strlen((char *)pTxBuff));
	  }
	  HAL_Delay(1000);
  }
}

Does anyone have insights into why UART2 DMA transmission is not functioning correctly and why no output is appearing on the terminal, despite the transmission complete callback being triggered?

Thanks!

[fa31]

Thank you for the reply @Andrew Neil !
Yes, the variable is local to the function. And I have tried the following:

• Local to the function (without static)
• Local to the function (with static)
• Globally
• Static to the file

But nothing seems to be working. 

[Harryskin]

Hey! It sounds like you’ve done a solid job narrowing things down—especially confirming that UART works without DMA and that the TX complete callback fires. That rules out hardware and basic UART config, which is a strong starting point.

That said, one common issue is buffer accessibility. Make sure pTxBuff is in a memory region that's accessible by DMA. If you’re using cacheable memory or regions like DTCM on certain STM32 devices, the DMA controller might not be able to read from it reliably. Also, ensure your DMA stream is properly configured—check the direction (memory-to-peripheral), addresses, and length. Even if the TX complete callback is triggered, it doesn’t guarantee that data was successfully pushed out; it just means the DMA operation completed.

Another thing worth reviewing is whether the peripheral (USART2) itself is receiving the buffer content correctly. Try with a static string like "Hello\r\n" to rule out formatting issues from sprintf. If that works, then dynamic string handling or memory alignment might be at fault.

Also, take caching into account. If you're on STM32H7 or U5, and cache is enabled, you might need to clean or invalidate the cache before DMA can reliably access the memory. Adding HAL functions like SCBCleanDCachebyAddr() for the buffer before calling HALUARTTransmitDMA can sometimes resolve this.

Lastly, make sure you’re not missing DMA IRQ enabling or callback linking. Review MXGPDMA1Init to ensure nothing is misconfigured and confirm your HALUARTTxCpltCallback is implemented and doesn’t suppress errors silently.