SSD, SD, eMMC, Raw NAND what are the differences

sd cardssdterminology

so the underlying technology with SSD, eMMC, SD, USB Flash etc… is NAND flash correct? So is the difference between all of them just the way controllers are implemented? Or are the technologies different all together?

From what I know, I think SSDs are for desktops and eMMCs are for mobile devices, but is there some intricate difference between all of these storage technologies?

Best Answer

NAND stands for Negated AND. It often refers to the way the a logic gate is build from silicon.

Flash memory is also built from silicon chips and uses NAND gates. This leads to the term NAND flash. I suspect that this is the NAND you refer to, but for completeness sake I wanted to mention the background.

You can build storage with NAND flash, but you will need some way to access it.

E.g. you can put a NAND flash chip on a PCB, add a controller chip and some USB logic and you get a USB pen drive. Or you can add a SD controller and put it in this format and you get a SD card.

enter image description here

Both of these are relative simple devices and when you store information on them you will write to a fixed location. This is a bad thing, because the number of writes to NAND flash is limited.

You can add a controller to the device which makes sure that all writes are spread evenly across the NAND, while providing a consistent image to the computer. This requires a lot more intelligence on the device part and is done in SSDs. (SSDs are supposed to replace mechanical harddisks and thus are expected to get a lot of writes).

Is the difference between all of them just the way controllers are implemented?

For SD/USB pendive: mostly the same, just with a different interface.
For SSDs: completely different controllers.

Or are the technologies different all together?

There are several ways to implement nand storage. The main implemented differences seem to come down to:

  • Single cells in which you can store a high voltage or a low one (SLC, or Single level). Basically either 'on' or 'off', or '1' or '0'.
  • chips which allow multiple levels of power. (off, slightly charged, mostly charged, fully charged. Compare it to signaling with a stereo. SLC would be music on or music off. MLC would be 'off, soft music, loud music, and extra-deaf-mode).

That leaves eMMC.

I never heard of it before, but according to Wikipedia it is a a flash memory memory card standard.