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SPI slave using DMA on STM32F405RG

Question asked by vmbg on Nov 15, 2013
I am working on the SPI communication between PC and STM32F405RG MCU. FTDI chip FT232H serves as a bridge between PC and MCU (USB to SPI converter). FTDI is set to be SPI master and MCU is slave. I am using DMA transfer on STM32 side to read/write over SPI, and the question is:
If byte is written in SPI->DR register on the MCU, and then DMA Tx transfer is initialized (number of bytes and buffer is set with data to be sent over SPI), will the byte previously written in DR be also received on master side, or only the data which are set to transfered via DMA.
During development, both cases occurred - sometimes byte in DR is received, and sometimes only the data set to DMA transfer.

Thanks in advance.

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