Are Pull-ups needed for SDIO

federicoj · ‎2011-09-22

Posted on September 23, 2011 at 03:30

Hello,

I noticed that the schematic for the STM3220G-EVAL which uses an STM32F207 has a 47K pull-up on each of the non-power pins associated with the micro-sd card. I am currently designing a very space constrained board so I would like to avoid any extra components. Are these resistors needed for SDIO communication with an SD card to work? Can internal pull-up resistors be used?

I wasn't able to find much information on this, so any insight would be helpful.

#stm32-sdio

aqueisser · ‎2011-09-23

Posted on September 23, 2011 at 18:41

You can use the internal pull-ups, usually the reason people add extra resistors is to make sure that there's always a pull-up regardless of what state the processor is in. Also, the internal pull-ups may not be the value you want. The data sheet for the 207 lists 30-50 so that should be a reasonable value.

It also depends on the peripheral you're connecting to. If you have socket that can take any card you have to be more defensive, in our case we have an eMMC that actually allows switching the pull-ups off during data transfers.

Also, note that the CLK line doesn't have a pull-up.

Andrew