Question about SDMMC_DLEN register

[Yosuf Taraki]

Posted on January 23, 2018 at 19:57

We are developing a new product using STM32F767 processor, with a microSD card for storing data.  Page 

1472 of the Reference Manual (RM0410) contains a description of the SDMMC data length register (SDMMC_DLEN).  There is a cryptic note on this page that reads:

For an SDMMC multibyte transfer the value in the data length register must be between 1 and 512.

I am trying to understand the meaning of this sentence.  I have found that we can read up to 262144 bytes per transfer when reading from the SD card.  This corresponds to 512 sectors. I don't understand why we cannot read more than 262144 bytes per transfer, but am wondering if it is related to the note above.  I would appreciate any help on this question.

[Tesla DeLorean]

Posted on January 24, 2018 at 09:34

More likely to relate to maximal DMA transfer length. Reading 256KB at a time should be pretty efficient.