Interfacing MCUs to external SDRAM

Question

Hi all,

I'm looking for a way to interface external SDRAM to MCUs (MCUs having external memory controller / MMU).

I don't know well how drive SDRAM memory (because usually I interface MCUs having RAM internally), but my main problem is external memory terminations.

Please, have anyone explanations, documentation or books titles to address me in solving this issue?

Thanks in advance.

Regards.

Marco.

lpetacchi · Accepted Answer

Hi Marco, modern MCUs with SDRAM controller are very easy to interface to SDRAM. Just connect the pins, with small series resistors (10Ohm is quite common) to control rise times and dump reflections if needed. Clock trace should follow specific rules depending on the path lenght (if the distance between MCU and SDRAM is less than 1" you don't need to). On my experience, problems arise when you need to program the controller with the correct set of timing parameters for the SDRAM. Be careful and use only well documented chips. regards

Former Member · Answer

Hello Marco, an external memory interface is always interesting and can involve a lot of issues. Usually the supplier of your controller chip will have Design Guides how to implement the memory interface. Of course you should take a look at the Spec of your DRAMs (JEDEC and from manufacturer). And I also might be able to answer your questions, but they need to be more specific and I need some more information (wchich MCU, how many DRAMs, which DRAM generation, which DRAM organization, Stackup of your board and minimum feature size you intend to use, .. ) Best regards Hermann

DAB · Answer

Hi Marco,

To access external memory, you need to configure a way to address the memory and read or write data over its buss width. Go look at the old Z80 or 8085 cookbooks, they will show you the basic architecture of how to set up a memory bank. If you really dig, you can find the software design used to set up the memory as a storage medium called a ram disk. We used to make these all the time to get over the old floppy disk or casette tape delay times.

The interface can be simple or complex, it depends upon the MCU you have, how many digital pins you have free, and how wide of a data bus you want. After that, the configuration of the logic circuit is fairly simple. One caveat, dynamic ram requires a refresh circuit or controller which can get pretty involved. So I would go to static ram if you have a choice.

Good luck,

DAB