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SPI DMA vs Normal SPI, delay CS after transmission

Question asked by MarcZ on Jun 2, 2014
Latest reply on Jun 2, 2014 by waclawek.jan
Hi,

We are working on STM32F417.
We are getting a "problem" (if it is a problem) with the SPI through DMA.

The function which send data through SPI is selecting the device at the beggining and unselecting it after the transfer.

With a normal SPI transmission we get the "unselect device" right after the transmission is completed, for any baud rate prescaler. (see printscreen with a baud rate prescaler of 8)

SPItransfer.png

Now, if I am using the same baud rate prescaler and the same data are send. We are getting a delay after the transfer is complete. The fact is that the duration of the chip select is the same as for Normal SPI transfer. (see printscreen with also a baudrate prescaler of 8)

DMAtransfer.png

For a baud rate prescaler of 4, the DMA transfer is faster (sure it is) and so the delay is longer but almost the same as a baudrateprescaler of 8.

Also, with less data to send, the chip select keep the same duration in DMA mode.

Do you have any idea where this can come from?

Thank you,
Marc

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