Using two different SDcard with SDMMC

[JC Lebreton]

Posted on March 13, 2018 at 10:00

Hi,

I was wondering if SDMMC can drive more than 1 SD card at a time. 

I understand that the cube or the SDMMC functions are not involved here.

I also noticed that FATFS_LinkDriver() use a variable called VOLUMES which is the number of logical drives that can be used. 

So my questions are : 

Do i have to simply change the variable VOLUMES from 1 to 2 ?

Can i do such a thing? using two sd card on the same SDMMC wires ?

and before you ask, i have a working project using SDMMC 1 bit wide bus !

Thanks in advance for all your answers!

[Tesla DeLorean]

Posted on June 18, 2018 at 19:10

The STM32F769I-EVAL and F779I-EVAL has two microSD sockets, and support two independent SDMMC peripherals.

This kind of adapter permits the use of two different kinds of cards, but only one at a time.

>>Do i have to simply change the variable VOLUMES from 1 to 2 ?

For FATFS to hold state for two cards you'd need to have settings, buffers and structures set up to accommodate that.

>>Can i do such a thing? using two sd card on the same SDMMC wires ?

It's complicated, you'd likely need buffer/transceivers on the socket side, and mutual exclusion and serialization on the software side.

The F76x and F77x parts do support two independent SDMMC peripherals. The H7 parts support a transceiver.

ST has this part

http://www.st.com/en/interfaces-and-transceivers/st6g3244me.html

 

You might be able to use one of the common TI 8-way automatic transceivers. Something like this might be viable, I'd have concerns about bandwidth and skewing.

http://www.ti.com/lit/ds/symlink/txb0108.pdf