MMC vs SD Interface

When we look at card readers, we often see a slot that says SD/MMC. So it leads one to question what the differences between MMC and SD interfaces are. Actually, SD emerged from the MMC standard. Thus, the electrical interface of SD was derived from and is a subset of MMC. The two are electrically compatible except that they no longer have identical sets of commands; while SD retained some of the commands, it removed some while added others.

Even though they do not use the same command sets, all SD hosts start off by using the MMC command sets. They then negotiate what signaling commands to use in order to achieve optimum speed. So if you stick an MMC card into an SD slot, the host will still be able to read and write to that card.

But despite the compatibility of the interfaces of SD and MMC, you cannot fit an SD card into an MMC card slot. This is not because of the electrical interface but because of the physical differences between MMC and SD cards. MMC cards are about 1.4mm and SD cards are about 2.1mm thick. This makes it easy to stick an MMC card into an SD slot but not the other way around. There is a version of SD that has the same thickness as an MMC, but those are pretty rare.

Even if you sidestepped the size issues and are able to stick one card into the other host, it is still not recommended to do so. This is because of how the two standards have diverged over time. If you use an SD card in an MMC slot, you will not be able to achieve the optimum transfer speed as the MMC host will not be able to use the advanced commands in SD.

SD is now considered to be the standard when it comes to removable storage media. It has outgrown MMC fairly quickly and is used in virtually all portable devices with removable storage.

Summary:
1.The SD interface is a subset of the MMC interface.
2.SD hosts are able to read MMC cards.
3.SD cards will not fit into MMC card slots.
4.SD cards will not attain optimum speed in a MMC slot and vice versa.

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