SD cards are a type of non-volatile memory known as flash memory. Non-volatile memory means that it retains data even when power is turned off. SD cards are commonly used in many consumer electronics devices such as digital cameras, mobile phones, tablets, and handheld video game consoles.
How Does SD Card Work?
SD cards store data in memory cells made up of floating-gate transistors. Each memory cell can store one bit of data as either a 1 or 0. When data needs to be written to the card, a voltage is applied to the floating gate of the transistors to charge them up and change their threshold voltage. This charge remains in place even when power is removed, allowing the data to be stored.
To read the data, a voltage is applied to the cell and based on the threshold voltage of the transistor, the presence or absence of charge can be detected. This allows the 1s and 0s to be read back out. SD cards use NAND flash memory, which means the cells are arranged in a grid with connections between each row and column.
Types of SD Cards
There are several types and classes of SD cards available:
- SD – The original SD card design with capacities up to 2GB.
- SDHC – SD High Capacity cards with capacities between 4GB and 32GB.
- SDXC – SD eXtended Capacity cards with capacities up to 2TB.
- SDSC – SD Standard Capacity cards from 1MB to 2GB.
In addition, SD cards have speed classes to indicate their minimum guaranteed speed:
| Speed Class | Minimum speed |
|---|---|
| Class 2 | 2 MB/s |
| Class 4 | 4 MB/s |
| Class 6 | 6 MB/s |
| Class 10 | 10 MB/s |
Higher speed classes allow for faster read/write performance.
SD Card Interface
SD cards use a serial interface protocol for communicating with devices. The interface consists of the following pins:
- Clock (SCK) – Provides the clock signal to synchronize data transfer.
- Command (CMD) – Carries commands and data between host and card.
- Power (VDD / VSS) – 3.3V power supply.
- Data (DAT0-3) – Bi-directional data bus for transferred data.
The host device controls the clock and uses the command pin to send commands like read, write, erase, etc. The SD card operates in either SPI mode or SD mode depending on the command sent during initialization.
SD Card File System
SD cards typically use the FAT32 file system to store files in a hierarchical directory structure. The file allocation table (FAT) keeps track of used and unused memory segments and locations of files on the disk. FAT32 supports individual files up to 4GB in size and partition sizes up to 2TB.
Some SD cards also support exFAT, a newer file system without the 4GB per file limit. SD cards are pre-formatted with the file system but can be reformatted if needed.
SD Card Capacities
SD card capacities have grown enormously over time as NAND flash memory technology has improved. Some common capacities include:
- 128MB
- 256MB
- 512MB
- 1GB
- 2GB
- 4GB
- 8GB
- 16GB
- 32GB
- 64GB
- 128GB
- 256GB
- 512GB
- 1TB
- 2TB
Higher density NAND flash chips and advanced manufacturing processes have enabled ever-greater storage capacities. SDXC supports capacities up to 2TB with future potential for even higher.
SD Card Speed Comparison
Faster SD cards provide quicker access to your data. The speeds are rated in megabytes per second (MB/s) or megabytes per second (MBps). Some typical sequential read/write speeds include:
| SD Card Class | Read Speed | Write Speed |
|---|---|---|
| Class 2 | 2 MB/s | 0.13 MB/s |
| Class 4 | 4 MB/s | 0.25 MB/s |
| Class 6 | 6 MB/s | 0.5 MB/s |
| Class 10 | 10 MB/s | 3 MB/s |
| UHS-I U1 | 10 MB/s | 5 MB/s |
| UHS-I U3 | 30 MB/s | 20 MB/s |
Faster cards allow you to transfer data quicker and improve performance, especially for apps, games, and 4K video capture.
SD Card Durability
SD cards are durable solid state memory with no moving parts. However, there are factors that can affect their lifespan:
- Rewrites – Flash memory has a limited number of write/erase cycles before failure.
- Physical damage – Drops, bends, water damage, etc can break the cards.
- High temperature – Heat can degrade the NAND flash memory over time.
- Magnetism – Magnetic fields can corrupt the data on the cards.
Typical SD card lifespans under normal usage range from 3-5 years. Good storage conditions and handling can extend the usable life further.
SD Card Security
SD cards have built-in security and privacy features:
- Write-protect switch – Slide switch to prevent accidental writes.
- Encryption – Optional AES encryption to secure data.
- Sanitize – Secure erase to completely delete data.
- Waterproof – Rugged durability and waterproofing.
Newer SD cards may also have advanced security like cryptographic erase for quickly deleting files beyond recovery. This helps keep sensitive data secure.
SD Card Use Cases
SD cards are a portable, reusable way to store and transfer data. Some common use cases are:
- Digital cameras – Store photos and videos.
- Smartphones – Expand storage for apps, media, etc.
- Tablets – Add space for music, movies, documents.
- Handheld games – Store game data, profiles, save files.
- Music players – Keep songs and audio files.
- Drones – Record high-quality aerial footage.
SD cards are ubiquitous across consumer electronics because they provide removable, interchangeable storage in a small physical footprint.
Conclusion
SD cards utilize NAND flash memory to provide secure, portable, high-capacity storage. Improvements in memory cell design and manufacturing allow SD cards to store more data in less physical space. Speed classes rating read/write performance enable optimal use for different devices and applications. With widespread usage across cameras, smartphones, handheld consoles, and more, SD cards have become the standard for removable storage in consumer electronics.