SD card

The SD card is a proprietary, non-volatile, flash memory card format developed by the SD Association (SDA). They come in three physical forms: the full-size SD, the smaller miniSD (now obsolete), and the smallest, microSD. Owing to their compact form factor, SD cards have been widely adopted in a variety of portable consumer electronics, including digital cameras, camcorders, video game consoles, mobile phones, action cameras, and camera drones.

The format was introduced in August 1999 as Secure Digital by SanDisk, Panasonic (then known as Matsushita), and Kioxia (then part of Toshiba). It was designed as a successor to the MultiMediaCard (MMC) format, introducing several enhancements including a digital rights management (DRM) feature, a more durable physical casing, and a mechanical write-protect switch. These improvements, combined with strong industry support, contributed to its widespread adoption.

To manage licensing and intellectual property rights, the founding companies established SD-3C, LLC. In January 2000, they also formed the SD Association, a non-profit organization responsible for developing the SD specifications and promoting the format. As of 2023, the SDA includes approximately 1000 member companies. The association uses trademarked logos owned by SD-3C to enforce compliance with official standards and to indicate product compatibility.

History

In 1994, SanDisk made one of the first types of flash memory cards called CompactFlash. But many different kinds of cards came out later, making it hard for devices to use them all.

To fix this, SanDisk worked with Siemens and Nokia to make a new kind of card called the MultiMediaCard in 1996. But people didn’t use it much.

In 1999, SanDisk joined with Panasonic and Kioxia (part of Toshiba) to make a better card called Secure Digital, or SD. They wanted a card that worked well in many devices and could keep information safe. The first SD cards came out in early 2000 with 8 MB of storage, and bigger cards followed soon after.

At the Consumer Electronics Show in January 2000, the three companies created the SD Association to help all devices work with SD cards. Many companies joined this group.

In 2003, SanDisk showed a smaller version called miniSD, but by 2008, an even smaller version called microSD became popular. These small cards could still fit into regular SD slots with a special adapter.

Over the years, SD cards got bigger and faster. In 2009, they introduced SDXC cards that could hold up to 2 TB. By 2022, some cards could hold 2 TB, and in 2024, a new kind called SDUC was announced, which could hold up to 128 TB.

Capacity standards

There are four types of SD card capacities: Standard Capacity (SDSC), High Capacity (SDHC), Extended Capacity (SDXC), and Ultra Capacity (SDUC). Each type has its own storage limits and recommended ways to format the card.

SDSC cards hold up to 2 GB and use older file systems. SDHC cards, introduced in 2006, can hold up to 32 GB and use a newer file system called FAT32. SDXC cards, introduced in 2009, can hold up to 2 TB and use the exFAT file system. SDUC cards, introduced in 2018, can hold up to 128 TB and also use exFAT by default.

Bus marks

Bus marks show how fast a device can send and receive data. They are important for large files like photos and videos. Over time, the SD card standard has gotten faster by using a faster clock to move data.

The first SD cards could send data at up to 12.5 MB/s. Later, they could go up to 25 MB/s. Even faster versions, called UHS, can send data much quicker. These faster cards have special marks to show their speed.

UHS cards can reach speeds of up to 50 MB/s or even faster, depending on the model. There are also UHS-II and UHS-III cards, which can go even quicker, but not many devices use them yet.

The newest standard, SD Express, can send data at up to 985 MB/s by using technology from computer cards. This makes it very fast, but only a few cards and devices support it so far.

Default Speed

The original SD bus interface supported a maximum transfer rate of 12.5 MB/s. This mode is called Default Speed.

High Speed

The SD Association introduced High-Speed mode, which increased the maximum transfer rate to 25 MB/s. This was made for devices like digital cameras.

UHS (Ultra High Speed)

The Ultra High Speed bus interface allows faster data transfer on SDHC, SDXC and SDUC cards.

UHS-compatible cards are marked with Roman numerals next to the SD logo. These cards offer faster read and write speeds than earlier SD card types.

To achieve higher transfer speeds, UHS cards and devices use specialized electrical signaling and hardware interfaces. UHS-I cards operate at 1.8 V instead of the standard 3.3 V and use a four-bit transfer mode. UHS-II and UHS-III add a second row of interface pins to increase speed.

The following UHS speed classes are defined:

UHS-I

Support for the UHS-I interface was introduced in SD specification version 3.01. This version added several new transfer modes: SDR50, DDR50, and SDR104.

In 2018, SanDisk developed a proprietary mode known as DDR200, which achieves read speeds of up to 170 MB/s. In 2022, SanDisk introduced DDR225, increasing performance to up to 200 MB/s read and 140 MB/s write. In 2024 SanDisk released a 2TB microSD card that achieves 250/150 MB/s (R/W) using the faster DDR275. In the same year Silicon Motion announced the SM2709 card controller that supports the even faster DDR300 signalling.

UHS-II

Support for the UHS-II interface was introduced in SD specification version 4.0. It added two new transfer modes: FD156 and HD312.

Each LVDS lane can transfer up to 156 MB/s. In full-duplex mode, one lane is used for sending data and the other for receiving. In half-duplex mode, both lanes operate in the same direction.

As of 2025, only about 100 cameras support UHS-II cards.

UHS-III

Support for the UHS-III interface was introduced in SD specification version 6.0. It added two new full-duplex transfer modes: FD312 and FD624. UHS-III retains the same physical interface and pin layout as UHS-II for backward compatibility. However, as of 2025, UHS-III has seen limited adoption.

SD Express

SD Express was introduced in SD specification version 7.0. By incorporating a single PCIe lane and supporting the NVMe storage protocol, SD Express enables full-duplex transfer speeds of up to 985 MB/s. SD Express cards support direct memory access (DMA).

SD specification version 8.0 expanded the interface to support PCIe 4.0 and introduced dual-lane configurations for full-size cards.

Adoption has been gradual. In February 2024, Samsung began sampling its first microSD Express cards. In April 2025, Nintendo announced that the Switch 2 would support only microSD Express cards.

As of June 2025, only single-lane PCIe 3.1 SD Express cards are commercially available.

Card speed class ratings

Speed Class ratings help us know how fast an SD card can save data. This is very useful when you are recording videos because it makes sure the card can keep up and not miss any moments.

There are different kinds of speed classes. The original ones—Class 2, 4, 6, and 10—tell us the card can write data at least 2, 4, 6, and 10 megabytes per second, respectively. UHS speed classes—U1 and U3—can write at least 10 and 30 megabytes per second, good for recording things like 4K video. Video speed classes—V6, V10, V30, V60, and V90—can write at 6, 10, 30, 60, and 90 megabytes per second, perfect for very high-quality videos like 8K. And the newest SD Express speed classes—E150, E300, E450, and E600—can write at 150, 300, 450, and 600 megabytes per second, great for very big and fast tasks.

Originally, some companies used a “×” rating based on how fast a CD-ROM drive works, but this was confusing. Now, we use the Speed Class systems that tell us the minimum speed for sure.

Speed Class ratings tell us the minimum speed, but the real-world speed can change based on things like how files are stored and how the card’s controller works. Sometimes, cards with the same rating can act very differently in real use.

Performance ratings

In 2016, new ratings called Application Performance Class were added to help choose SD cards that work well for apps and other tasks. Before this, SD cards were mostly rated for how fast they could move big files, like photos or videos.

But apps and operating systems need to quickly open many small files, which is different. To handle this better, the SD Association made new ratings. The first one, called A1, needs the card to handle at least 1,500 small tasks for reading and 500 for writing each second. The next level, A2, needs even more: 4,000 reading tasks and 2,000 writing tasks each second. Both need the device to support certain features to reach these speeds. They also need to write files at least 10 MB/s, which is the same as older speed ratings like C10, U1, and V10.

Features

Card security

Commands to disable writes

A device can tell an SD card to only read information and not allow any writing. There are ways to do this that can be reversed and ways that cannot be undone.

Write-protect notch

Most full-size SD cards have a small sliding tab on the side. Moving this tab up allows the card to read and write, while sliding it down makes the card read-only. However, not all devices pay attention to this tab. Smaller cards like miniSD and microSD do not have this tab but can use special adapters that include one. Cards without this tab can always be written to.

Card password

A device can lock an SD card with a password chosen by the user. When locked, the card works normally except it will not allow reading or writing data. To unlock it, you must enter the same password. If the password is forgotten, the device can erase all data on the card, but the existing data cannot be recovered.

smartSD cards

A smartSD card is a tiny SD card that has extra parts inside to help with special tasks. These cards can work with systems that use special codes for secure communications, like those used in mobile payments. They have been used for things like paying for goods with a phone and keeping voice calls private.

Vendor enhancements

Companies have added special features to SD cards to make them stand out:

• Built-in Wi-Fi – Some SD cards have wireless parts that let cameras send pictures over the internet.
• Pre-loaded content – Some cards come with music or other content already saved on them.
• Built-in USB connector – Some cards have a USB plug so you can use them on a computer without a special card reader.
• Built-in display – One type of SD card has a small screen that shows how much space is left.

SDIO cards

SDIO cards are special SD cards that can do more than just store data. They can connect to things like GPS, Wi-Fi, cameras, barcode readers, and modems. However, not many devices support these cards.

Compatibility

Devices that follow the newer rules for SD cards can also use older cards. But older devices might not work with newer cards, especially if the newer cards have more space or use a different way to organize files.

Markets

Because they are very small, SD cards are used in many electronic devices to store data. Devices like digital cameras, camcorders, and video game consoles often use the full-sized SD cards. Smaller devices like mobile phones, action cameras, and camera drones usually use microSD cards.

MicroSD cards were very common in Android smartphones up until around 2015. After that, many newer phones stopped using them, preferring built-in storage or cloud services instead. Apple has never used microSD cards in the iPhone, choosing built-in storage and cloud services.

As of 2023, SD cards are the most common way to store data in digital cameras. Some computers can also use SD cards with a special slot or a USB adapter, but they cannot replace the main hard disk. SD cards are also used in small computers like the Raspberry Pi and in some older computers.

Technical details

Physical size

The SD card comes in three sizes. The SD and SDHC types are available in all three sizes, but SDXC and SDUC cards do not come in the mini size, and SDIO cards do not come in the micro size. Smaller cards can fit into larger slots using a special adapter.

Standard

• SD (SDSC), SDHC, SDXC, SDIO, SDUC
• 32 mm × 24 mm × 2.1 mm
• 32 mm × 24 mm × 1.4 mm (as thin as MMC) for Thin SD (rare)

MiniSD

• miniSD, miniSDHC, miniSDIO
• 21.5 mm × 20 mm × 1.4 mm

microSD

The micro size is the smallest SD card.

• microSD, microSDHC, microSDXC, microSDUC
• 15 mm × 11 mm × 1 mm

Transfer modes

SD cards can work in different ways to send and receive information. The SPI way and the one-bit SD way are required for all SD cards. Once a device and an SD card agree on how to connect, they use the same pins for all card sizes.

• SPI bus mode: Serial Peripheral Interface Bus is mainly used by small computers. It only works at 3.3 volts and does not need a special license.
• One-bit SD bus mode: Uses separate paths for commands and data with a special format.
• Four-bit SD bus mode: Uses extra pins for faster data transfer. All SD cards can do this. UHS-I and UHS-II cards need this way.
• Two differential lines SD UHS-II mode: Uses two special paths to send commands and data quickly. UHS-II cards include this way along with the SD bus ways.

The connection has 9 pins, except miniSD cards have two extra pins in the middle and microSD cards miss one of the two ground pins.

Notes:

1. Direction is relative to card. I = Input, O = Output.
2. PP = Push-Pull logic, OD = Open-Drain logic.
3. S = Power Supply, NC = Not Connected (or logical high).

Interface

Command interface

SD cards and devices first talk using a simple one-bit way, where the device gives a clock signal to control when information is sent and received. The device sends special codes and gets answers. The device can ask the card to use a different voltage, speed, or special connection.

The device can learn about the card’s type, size, and what it can do. It can also get ready to save or read information from the card.

The way SD cards talk is based on the MultiMediaCard (MMC) way. SD cards dropped some old commands but added new ones for protecting copies. By using only the common commands at first, a device can work with both SD and MMC cards.

Electrical interface

All SD cards start by working at 3.3 V. SDHC and SDXC cards can also switch to 1.8 V when asked.

When the card is turned on or put into a device, the voltage on pin 1 chooses whether to use the SPI way or the SD way. The SD way starts in one-bit mode, but the device can ask to switch to four-bit mode if the card can do it. Some cards can also switch to a higher speed.

Until the device knows what the card can do, it should not use a speed faster than 400 kHz. Most SD cards (except SDIO) work at 25 MHz by default. The device does not have to use the fastest speed the card allows; it can use a slower speed to save power. The device can also stop the clock between commands.

MBR and FAT

Most SD cards come ready to use with a special setup that lets them work like a small hard drive. They use a system called MBR and a way to organize files.

• For SDSC cards:
  • Smaller than 16 MB: FAT12
  • Between 16 MB and 32 MB: FAT16
  • Larger than 32 MB: FAT16B
• For SDHC cards:
  • Smaller than 7.8 GB: FAT32
  • Larger than 7.8 GB: FAT32
• For SDXC cards: exFAT

Most devices that use SD cards expect them to be set up this way. This lets the cards work on many computers with SD readers.

On these cards, tools like Mac OS X's Disk Utility or Windows' CHKDSK can fix problems and sometimes recover deleted files. Tools to organize files on FAT systems may help a little, but they also use up the card’s lifespan. The card’s memory has a limit to how many times it can be written to.

When reformatting an SD card larger than 32 MB but not more than 2 GB, using FAT16B is suggested for consumer devices. FAT16B does not work for cards larger than 4 GB.

The SDXC standard requires Microsoft's exFAT file system, which may need special software on some computers.

Risks of reformatting

Changing the way an SD card is set up can make it slower or shorten its life. Some cards change where they save information to balance wear, and this works best with the way the card comes formatted. Changing the way files are grouped can also make saving less efficient. The SD Association offers free software called SD Formatter for Windows and Mac OS X to help with this.

SD/SDHC/SDXC cards have a special area for security that normal formatters cannot erase. The SD Association says that devices or software using SD security might format this area.

Power consumption

The power SD cards use changes with their speed, maker, and model. While sending information, it can be between 66–330 mW. Standby power is much lower, less than 0.2 mA for some microSD cards.

New UHS-II cards can use up to 2.88 W if the device supports it. The smallest power use for UHS-II is 720 mW.

Storage capacity and compatibilities

SD cards allow devices to know how much information they can store. Early SD cards could hold up to 1 GB, but newer cards can hold much more.

When SD cards were first made, they could only hold up to 2 GB. Later changes let cards hold up to 32 GB or even more. Older devices might not work properly with these bigger cards.

Data recovery

If an SD card stops working, it might be fixed with special tools, as long as the main part that stores information isn’t broken. Sometimes, this can be done without using the card’s normal controller. However, this can be very difficult or impossible for certain types of cards where the controller is built into the same piece as the storage.

Adapters

Adapters help smaller SD cards fit into bigger SD card slots. For example, a microSD card can be used in an SD slot with a special adapter. These adapters simply connect the smaller card to the larger slot without doing anything else. Some cameras even came with these adapters for different types of cards.

Openness of specification

The SD format started in August 1999. Like many memory cards, SD cards are protected by patents and trademarks. Companies must pay royalties to make and sell SD cards, except for SDIO devices.

At first, only certain people could see the full details of how SD cards work. This made it hard to create free software for them. However, clever developers found ways around this and made free drivers for SD cards. Later, in 2006, the group behind SD cards began sharing simpler details more freely, so more people could use and understand SD cards without special agreements or fees.

Revisions