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ETH_DMACSR_RBU error occurs and stalls the Ethernet receive on STM32H7. Is there a way around this issue with the DMA?

Associate II


I've been using the STM32H7 example which uses the new reworked Ethernet driver (

Changed the application layer of the example (LwIP_HTTP_Server_Netconn_RTOS) and during larger transfers, the DMA error (ETH_DMACSR_RBU) occurs and simultaneously Wireshark shows that the communication has issues (retransmissions, tcp window full) and the communication stops for a while.

Code below is located in the ethernetif.c file.

  * @brief  Ethernet DMA transfer error callback
  * @param  heth: ETH handle
  * @retval None
void HAL_ETH_ErrorCallback(ETH_HandleTypeDef *heth)
  SEGGER_RTT_printf(0, "DMA error occurred! %d\n",HAL_ETH_GetDMAError(heth) );
     SEGGER_RTT_printf(0, "DMA error occurred - releasing semaphore \n");

Here is the Wireshark picture which shows the issue:


How can this be fixed?

It causes the low throughput which is a big problem for our use case.

If this is an issue with the driver, what version to use then?

The important project files are in the attachment.

Can you please give a suggestion on what might be the issue.


Senior III

I got stuck in the same situation. Under stress-test I get ETH_DMACSR_RBU (Rx DMA error) after a few minutes of operation. HAL driver code is a newest from STM32 page. An ethernetif.c is also taken from recent H7 example. Just recreating DMX descriptors table doesn't help.

My error callback:

void HAL_ETH_ErrorCallback(ETH_HandleTypeDef *heth)
	printf( "ETH DMA Rx Error\n" );
	// Clear RBUS ETHERNET DMA flag
	heth->Instance->DMACSR = ETH_DMACSR_RBU;
	// Resume DMA reception
	heth->Instance->DMACRDTPR = 0;
	printf( "ETH DMA Tx Error\n" );
	//Clear TBU flag to resume processing
	heth->Instance->DMACSR = ETH_DMACSR_TBU;
	//Instruct the DMA to poll the transmit descriptor list
	heth->Instance->DMACTDTPR = 0;

According to all sources I found reset of DMACSR and DMACRDTPR/ETH_DMACSR_TBU should resume DMA controller, but it is not.

Associate II

Hi ktrofimo,

I managed to remove the issue with the stalling by modifying the lwipopts.h file, more


Modified the TCP_WND which is the tcp receive window. If set to high the download will be quicker but it can stall.

Maybe there is a disbalance of the send and receive.

Also, I increased the TCP_SND_BUF.

Here are my settings:

#define TCP_MSS                 (1500 - 40)	  /* TCP_MSS = (Ethernet MTU - IP header size - TCP header size) */
/* TCP sender buffer space (bytes). */
#define TCP_SND_BUF             (12*TCP_MSS)
/*  TCP_SND_QUEUELEN: TCP sender buffer space (pbufs). This must be at least
  as much as (2 * TCP_SND_BUF/TCP_MSS) for things to work. */
/* TCP receive window. */
#define TCP_WND                 (6*TCP_MSS)

Regarding to​ DMA error I also found that code:


heth->Instance->DMACSR = ETH_DMACSR_TBU;

heth->Instance->DMACTDTPR = 0;


shouldn't be called from within interrupt causing DMA failures as soon you stop at any breakpoint. ​(see discussion at

Also ​ethernet transmit could hang on osSemaphoreAcquire( TxPktSemaphore ) inside low_level_output()

Looks like ​this semaphore should be cleared at DMA error same way as RxPktSemaphore is cleared there already:


void HAL_ETH_ErrorCallback(ETH_HandleTypeDef *heth)




// ETH DMA Rx Error




+ {

+ // ETH DMA Tx Error\n" );

+ osSemaphoreRelease(TxPktSemaphore);

+ }



See ​

​I will check your lwIP settings and will see how it goes. Thanks!

Alexander KVASHIN

Can You try to increase buffers in the ethernetif.c on line 47:

#define ETH_RX_BUFFER_SIZE      1000

change it to 1536

Have it by default from CubeIDE set as 1524. Will set it to 1536 and try again.​

1524 does not have the correct alignment. 1536 is multiple of 512 bytes, and multiple of the cache line size.

Rx buffer size 1524 is actually aligned by 32bits and size of buffer is increased to 1536 bytes in ethernetif.c:

uint8_t buff[(ETH_RX_BUFFER_SIZE + 31) & ~31] __ALIGNED(32);


if I try 1524 or 1536 ethernet ping and http stop working.

Has anyone tried with different values of ETH_RX_BUFFER_SIZE?


>How can this be fixed?

RM0433 rev 7 page 2809:

"In the Suspend state, the DMA tries to acquire the descriptor again (and thereby return

to step 3). A poll demand command is triggered by writing any value to the Channel Tx

descriptor tail pointer register (ETH_DMACTXDTPR) when it receives a Transmit Poll

demand and the Underflow Interrupt Status bit is cleared. If the application stopped the

DMA by clearing Bit 0 of Transmit control register of corresponding DMA channel, the

DMA enters the Stop state."

RM0433 rev 7 page 2872:

"If the descriptors are not owned by the DMA (or no descriptor is available), the DMA

goes into Suspend state. The transmission or reception can be resumed by freeing the

descriptors and writing the ETH_DMACTXDTPR (see Channel Tx descriptor tail

pointer register (ETH_DMACTXDTPR) and ETH_DMACRXDTPR (see Channel Rx

descriptor tail pointer register (ETH_DMACRXDTPR))."

At I'd recommended using RBU. But if receive buffers can often be exhausted (and presumably no memory can be spared for more) the additional RBU handling will only waste cycles. Instead you might

  • clear RBUE in ETH_DMACIER to remove operation of RBU,
  • always write ETH_DMACTXDTPR after enabling DMA descriptors, and
  • following buffer exhaustion, notify your Ethernet-receive-thread to repopulate descriptors and update ETH_DMACTXDTPR to exit suspend state when the next buffer becomes available.