Checking for finished SPI / DMA transfer - missing data?

[Miro Nohaj]

Posted on February 18, 2014 at 09:01

Hello all,

I've run through couple of SPI DMA threads here, but I didn't find what I was looking for... On my STM32F103 I'm sending data to host over SPI: - my STM32 acts as SPI slave - SPI transfer is done using DMA - data is transferred in blocks of ~520 bytes (260 words (word = 16 bits)), each block is a separate DMA transfer Everything works mostly fine, but from time to time I loose the first word of the block, so I guess I'm not waiting correctly for SPI DMA transfer to finish before starting a new one. I set a flag at the end of DMA transfer in interrupt, so I have DMA1_Channel2_IRQHandler and DMA1_Channel3_IRQHandler. When both transfer interrupts arrive, I assume that the SPI DMA finished and I start a new SPI DMA transfer. I guess that in the problematic case the DMA has finished, but the SPI is still transferring the last WORD, so I added a check / loop like this after the received interrupts:

void waitForSPIidle(void)
{
while((SPI1->SR & 2) == 0); // wait while TXE flag is 0 (TX is not empty)
while((SPI1->SR & (1 << 7)) != 0); // wait while BSY flag is 1 (SPI is busy)
}

The lost WORD is missing (or maybe arrived instead of the last word in the previous block) on the host part, so checking the TXE and then BSY flag should be enough to see if nothing more is transmitted. The thing is that this doesn't help. What am I missing? I don't want to add mostly unnecessary delay before starting each DMA SPI transfer, I would rather check some flags... Jookie #spi-dma #spi-dma

[Miro Nohaj]

Posted on February 25, 2014 at 10:14

''Okay, and if you try to avoid the underruns by allocating big enough buffer for both rx and tx, will your problem persist?''

I would have to test it (I might report on that later), but got it currently working with that workaround... But it maybe could resolve the problem at the cost of extra empty but used RAM... 

[grant]

Posted on June 16, 2015 at 11:38

Hi there

I have been running an STM32F207 with:

• SPI running in slave mode
• using DMA 

and also had the problem where the first word (I am using

16 bit transfers

) was corrupted, or simply not there.I believe I may have found the solution.  The problem is that nothing seems to trigger the FIRST load from the DMA into the SPI DR, and it is not until after the first 16 bits have been clocked out that the DMA controller does the first transfer into the DR of the SPI Channel.  So it behaves more like as in ''Figure 262: Reception using DMA'' in the STM32F207 Reference Manual.I got around this by :

• manually loading the first word from into the DR of the SPI channel
• configuring the DMA channel to start at the next word in my buffer (obviously reducing the length by 1 word)

It seems to work fine in my application.  Hope this helps.RegardsGrant