SD Cards: Memory Types Explained
What is NAND Flash Memory?
NAND flash memory is one of the key components of an SD card. It stores information as a variation of its threshold voltage of the memory cell. This threshold voltage change is controlled by how many electrons are stored in the floating gate. The threshold voltage becomes higher when the electron count increases. Figure 1 shows a conceptual diagram of NAND flash memory storage mechanism.
Figure 1: Structure and threshold voltage of the memory cell
SLC, MLC, and TLC….Oh My!
A memory cell type of NAND flash memory is defined by how many individual threshold voltage distributions (memory states), exist in a memory cell. Each type varies in information capacity, as well as reliability. The three main types are:
SLC (Single-Level Cell): Stores 1-Bit binary information in each memory cell with two states per cell.
MLC (Multi-Level Cell): Stores 2-Bit binary information in each memory cell with four states per cell.
TLC (Triple-Level Cell): Stores 3-Bit binary information in each memory cell with eight states per cell.
The voltage gap between each state becomes smaller from SLC to MLC to TLC. This threshold voltage gap adversely affects reliability of the data stored in a memory cell as it becomes smaller. So, SLC NAND flash is more robust and reliable compared with other NAND types. However, it is more expensive than others, because of die size required for larger capacity chips.
On the other hand, the TLC NAND flash memory is cheaper, but has poor reliability due to a smaller gap margin between each threshold voltage distribution. See Figure 2 for a comparison between technologies.
Figure 2: Comparison of SLC, MLC, and TLC
The Benefits of SLC-Lite
Panasonic has engineered a special type of SD card to mimic the benefits of SLC with an MLC-like price. These SLC-Lite SD cards employ MLC NAND flash memory, but only utilize two of the four memory states (see figure 3). This way, the SD controller operates the MLC NAND flash memory as an SLC NAND. As a result, the reliability and robustness of the SD card becomes improved like a pure SLC NAND SD card.
Figure 3: SLC-Lite Differentiation
SLC-Lite NAND also provides a cost that is a third of pure SLC NAND, and offers equivalent features such as:
- Fast Write Speed
- Power-Fail Robustness
- Enhanced Endurance
- Lifetime 10x Better than MLC
If you're still not sure which technology is best for your application, we're here to help. Panasonic offers a free Use Case Analysis Service, which will help you choose the appropriate product from a more application-based perspective.
If you have any further questions about the Panasonic SD Card line, contact us today.
In the coming weeks, we’ll be doing a deeper dive into the Panasonic line, and the features that set us apart.