Having trouble receiving more data than my USART DMA buffer is set to

[JNoon.1]

Hi All,

I am trying to get my Nucleo-U575ZI-Q to receive low level USART data using DMA. I have set it up to be a circular buffer in the IOC file but everytime I receive 1024 bytes for example after that it won't receive anything.

When I call LL_DMA_GetBlkDataLength it gives me the space in the DMA buffer left and once it hits 0 it stays at 0. Instead of rolling around back to 1024. 

Attached is the settings for my USART1 Rx using DMA channel 0.

Any other info required please ask.

Thanks.

[FBL]

Hello @JNoon.1 

Would you please share your IOC file, and your implementation?

Please note, according to the reference manual, once the last data transfer is completed (BNDT[15:0] = 0):
- if GPDMA_CxLLR.UB1 = 1, this field is updated by the LLI in the memory.
- if GPDMA_CxLLR.UB1 = 0 and if there is at least one non null update bit, this field is internally restored to the programmed value.

[JNoon.1 · Accepted solution]

Hi @FBL,

Thank you for your response. I have managed to find a solution.

In the USART init function generated by CubeMX, it generates a shadow variable - LL_DMA_LinkNodeTypeDef Node_GPDMA1_Channel0 = {0};

But this has already been generated at the top of the file as a global (not static). I had to remove this line from the init function.

Also I used to call LL_DMA_SetDestAddress and LL_DMA_SetBlkDataLength in a separate file as my Rx buffer didn't live in main.c. Now this works on other MCs like the F722 and F446 that I have used but they don't for the U575. Instead I had to set these in the NodeConfig struct inside the USART init in main, meaning I had to get the address and size of my buffer dynamically. 

With these changes it works. Which means the CubeMX generation is wrong. I had also had to set the SrcAddress in the same NodeConfig manually too instead of it being generated by CubeMX. If I regenerate my IOC file then I have to add these changes in, which is a real pain.

Hopefully this makes some sense. If you want an example of my Init function before and after the changes I can add them here.

Thanks.