Hi all,

I've been trying to get DMA working on UART0. I've done memory to memory DMA, which seems to work.
However when I try to set up for UART0, I see one character appearing, and then it stops.
I have a callback set up for the interrupt on the DMA channel, and it does get called, and the END bit is set, however I only received the first character.
The DMA channel is set to 12 (UART_TX), I specify the UART0 DR register as the address (after converting the memory address space.
I tried setting the DMACR register on the UART to 0x07, but that also does nothing.
Any pointers?

I tried looking into the Circle code, but that only seems to implement DMA on SPI and I2S.

Regards,

Rene

did you setup dma pacing with dreq?

That was the right trigger, thanks. I also noticed you need to adapt for the fact that the data register is 32 bits but only holds a single character:
So here I use a stride of 1 to make sure we only advance one byte at a time.

Thanks for the help!

(mod edit for code tags)

renebarto wrote: ↑

Sun Sep 14, 2025 11:46 pm

That was the right trigger, thanks. I also noticed you need to adapt for the fact that the data register is 32 bits but only holds a single character:

Code: Select all

 m_controlBlock->TransferInformation = (static_cast<uint32>(dreq) << RPI_DMA_TI_PERMAP_SHIFT) |
 (RPI_DMA_DEFAULT_BURST_LENGTH << RPI_DMA_TI_BURST_LENGTH_SHIFT) |
 RPI_DMA_TI_SRC_WIDTH | RPI_DMA_TI_SRC_INC | RPI_DMA_TI_DST_DREQ | RPI_DMA_TI_WAIT_RESP;
 if (srcStride != 0)
 {
 // Writes are always a full 32 bit register, but we want to send a different width
 // This is a.o. used for UART writes
 m_controlBlock->TransferInformation |= RPI_DMA_TI_TDMODE;
 m_controlBlock->TransferLength = length << RPI_DMA_TXFR_LEN_YLENGTH_SHIFT | 1 << RPI_DMA_TXFR_LEN_XLENGTH_SHIFT;
 }
 else
 {
 m_controlBlock->TransferLength = length;
 }
 m_controlBlock->ModeStride2D = 0;
 m_controlBlock->SourceAddress = ARM_TO_GPU((uintptr)srcAddress);
 m_controlBlock->DestinationAddress = dstIOAddress;
 m_controlBlock->NextControlBlockAddress = 0;

So here I use a stride of 1 to make sure we only advance one byte at a time.

Thanks for the help!

Code (any language) works so much better on here between [code]... your code here ...[/code] tags.