SD CARDS

# SD Cards

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## 1. Overview

NAND flash is used as the main memory device inside of SD cards. There is also a memory controller IC (typically a microcontroller) which acts as an intermediary between the flash and the connected device reading/writing to/from the memory.

## 2. The TRIM command

The TRIM command is an ATA command which can be sent to an SSD controller by the OS. It is sent from the OS when a file is deleted from the filesystem, and the OS tells the SSD controller which NAND pages have been deleted. If the controller supports TRIM commands, then it will flag the pages for deletion so that the SSD controller can erase/free the pages. This increases the pool of available memory the wear-levelling algorithm can work with, allowing it to work more effectively and increasing the life time of the storage device.

# m     h       dom     mon     dow     command
0       1       *       *       *       ionice -c 3 fstrim -v /

## 3. Wear-levelling

Wear leveling is an intrinsic part of the erase pooling functionality of cards in the SanDisk microSD Card Product Family using NAND memory.[1]

There are two types of wear-levelling, static and global.

## 4. Speed Classes

There are three different systems for rating the access speed of an SD card[6]:

• Speed Class

• UHS Speed Class

• Video Speed Class

You will commonly see an SD card rated against more than one of these standards at the same time, e.g. "Class 10, UHS Class 3".

## 5. Multi-level Cells (MLCs)

In the context of NAND memory, a cell is an individual storage element (essentially a MOSFET with a floating gate). The first NAND cells were used to store one 1-bit of information and are called single-level cells. A multi-level cell (MLC) is a cell which can hold two bits of information. Most cheaper, high-density SD cards on the market in the 2010’s/2020’s utilize multi-level cell NAND flash to offer memory spaces of 64GB, 128GB and beyond. However, multi-level cells come at a cost --- they are less reliable than their single-layer cell (SLC) counterparts. This technology has been extended further into Triple-Level Cell (TLC, 3 bits per cell) and Quad-Level Cell (QLC, 4 bits per cell).

## Authors

### Geoffrey Hunter

Dude making stuff.