Write SPI dummy bytes using DMA circular mode

Question

I am trying to read a fixed number of bytes (27 bytes in total) from an SPI slave device using DMA. I'm running an STM32F4 chip.

"In order to read from the SPI bus, a clock needs to be generated. So you need to write in order to read." I set up my DMA controller to write a dummy byte (0xFF) in circular mode.

uint8_t tx_buffer[] = {0xFF};

DMA_InitStruct.DMA_Mode = DMA_Mode_Circular;
DMA_InitStruct.DMA_BufferSize = 1;
DMA_InitStruct.DMA_Memory0BaseAddr = (uint32_t)tx_buffer;

and my Rx dma stream is setup using double buffer mode:

uint8_t rx_buffer0[27];
uint8_t rx_buffer1[27];

DMA_InitStruct.DMA_Mode = DMA_Mode_Normal;
DMA_InitStruct.DMA_BufferSize = 27;
DMA_InitStruct.DMA_Memory0BaseAddr = (uint32_t)rx_buffer0;

DMA_DoubleBufferModeConfig(DMA2_Stream0, (uint32_t)rx_buffer1, DMA_Memory_0);
DMA_DoubleBufferModeCmd(DMA2_Stream0, ENABLE);

(I have omitted other irrelevant initialisations.)

After the 27 bytes are received a Transfer Complete interrupt is triggered.

void DMA2_Stream0_IRQHandler(void)
{
    if(DMA_GetITStatus(DMA2_Stream0, DMA_IT_TCIF0) == SET)
    {
        DMA_ClearITPendingBit(DMA2_Stream0, DMA_IT_TCIF0);
        DMA_Cmd(DMA2_Stream1, DISABLE); // disable Tx circular writes
    }
}

The problem is as follows: I see correct data in the first buffer. But while the interrupt triggers the Tx circular DMA continues, filling the first few bytes of the second buffer until it is disabled.

I want to avoid reserving 27 bytes for dummy data, so is there any way to stop a circular DMA stream after a fixed amount of cycles?

tilz0R tilz0R · Accepted Answer · 2017-08-16T09:58:18

Enable DMA in normal mode and set number of elements to 27.

Later, disable memory increase for memory and for peripheral and your approach will work.

Write SPI dummy bytes using DMA circular mode

1 Answers