Demystifying Micro SD Cards

27th April 2024

Introduction to Micro SD Cards

In the ever-expanding universe of digital storage, few innovations have revolutionized portable memory quite like the Secure Digital (SD) card. From its humble beginnings to its modern incarnations, the SD card has undergone a remarkable evolution, reshaping the landscape of data storage in the process.

Initially introduced as a collaborative effort between Panasonic, SanDisk, and Toshiba in 1999, the SD card quickly gained traction as a compact, reliable solution for storing digital content. Its acronym, SD, stands for "Secure Digital," reflecting its emphasis on data security and integrity.

Over the years, the SD card has evolved to accommodate the escalating demands of modern technology, with advancements in capacity, speed, and form factor. Among these iterations, the Micro SD card has emerged as a pivotal player, offering unprecedented levels of portability and versatility.

In this article, we delve into the world of Micro SD cards, exploring their history, features, and applications. From their inception to their current state-of-the-art capabilities, we unravel the intricacies of Micro SD cards, shedding light on their significance in today's digital landscape.

Understanding Micro SD Form Factor

The most prevalent form factor for SD cards is the Micro SD, fitting snugly into the majority of smartphones worldwide. Measuring a mere 15mm x 11mm, Micro SD cards are remarkably compact, yet they pack a punch in terms of storage capacity, with some models offering up to 2TB of space, considering these are for smartphones. Most Micro SD cards come bundled with a standard-sized adapter for easy connectivity with computers and other devices.

Differentiating Micro SD Types

Micro SD Memory Card: Functionally identical to standard SD cards, Micro SD cards are distinguished primarily by their diminutive size, making them ideal for smartphones and other compact devices. They typically offer a maximum capacity of 16GB and are universally supported across smartphones.

Micro SDHC Memory Card: Introduced as an upgrade to Micro SD cards, Micro SDHC cards boast increased storage capacity of up to 32GB and faster transfer speeds of 10MB/s. However, compatibility issues may arise with older Micro SD devices.

Micro SDXC Memory Card: Representing the pinnacle of Micro SD technology, Micro SDXC cards offer expansive storage capacities ranging from 32GB to a staggering 2TB, coupled with lightning-fast transfer speeds of 90MB/s. These cards are ideal for power users requiring vast storage for gaming and multimedia content.

Micro SDUC Memory Card: The latest addition to the Micro SD family, Micro SDUC cards push the boundaries of storage capacity, supporting up to 128TB of memory. However, compatibility is restricted to Micro SDUC-compatible devices, ensuring seamless performance.

Navigating Speed Class and Bus Speed

The SD Association has devised a classification system to indicate the speed and performance capabilities of SD cards, encompassing various speed classes such as Speed Class, UHS Speed Class, Video Speed Class, and SD Express Speed Class. These classifications ensure optimal performance for tasks such as video recording and application execution.

Speed Class

One marking you may see on an SD card is the speed class marking. This class is at the lower end of the speed spectrum. This marking usually comes in the form of a number in a circle (as shown below) and represents the minimum read and write speed of the card in MB/s (Megabytes per second). This class can come as a 2, 4, 6, or 10.

A class 6 card can generally be used to film in HD and shoot basic JPEGs. A class 10 card can be used for burst shooting, full HD, some 4K filming, and raw photography.

UHS (Ultra High) Speed Class

The next speed class up is called UHS. Again, this symbol represents the minimum read and write speed of a card. It is printed with a number inside of a U and has two variants:

Class 1: 10 MB/s (equivalent to speed class 10)
Class 3: 30 MB/s

Many 4K cameras won’t accept SDs without a UHS class 3 rating. The UHS class 3 rating is generally a minimum requirement for ultra-high-resolution filmmaking.

Video Speed Class

The highest-reaching speed class is called the video speed class.

This class has the following variants:

V6: 6 MB/s
V10: 10 MB/s
V30: 30 MB/s
V60: 60 MB/s
V90: 90 MB/s

V60 and V90 speeds are often needed for filming in 8K.

SD Express Standard

The SD Express spec was last updated back in October 2023, which bumped it up from PCIe 3.0 to 4.0; it also defines four speed classes with read/write speeds of between 150MB and 600MB per second.

One issue blocking SD Express adoption is that the card and the device have to support SD Express to get the promised speeds; an SD Express card inserted into a regular run-of-the-mill UHS-I SD card slot will be limited to UHS-I speeds. And because both the slots and the cards are visually identical, it's not always easy to tell which slots support specific speeds.

Heat may also be a major limiting factor when using these SD Express cards to move around hundreds of gigabytes' worth of data or when using the SD card as the primary storage device in a computer (as you might in a Raspberry Pi or other single-board computers). There's no room for this kind of thing within the confines of a microSD card slot.

Bus Speed Class

Another speed class that is helpful to know (but not always necessary to know) is the Bus Speed Class. Each bus speed category is generally determined by the peak bus throughput. This is the maximum amount of bytes per second that can be exchanged between the host device and the SD card. Categories include:

Default speed (DS): 12.5 MB/s
High Speed (HS): 25 MB/s
Ultra-high speeds:
- Ultra-High Speed I (UHS-I): 50-104 MB/s
- Ultra-High Speed II (UHS-II): 156 - 312 MB/s
- Ultra-High Speed III (UHS-III): 213 - 624 MB/s
SD Express:
- 985 - 1969 MB/s
- 1969 - 3964 MB/s

Deciphering Nomenclature: Making Sense of Micro SD Labels

Understanding the various classifications and labels associated with SD cards is crucial for making informed purchasing decisions. With a multitude of specifications and features, deciphering the nomenclature can seem daunting at first glance. Let's delve deeper into the key elements of SD card labelling to shed light on their significance.

Speed Class:

One of the fundamental aspects of SD card classification is the speed class, denoted by symbols such as C2, C4, C6, and C10. These symbols represent the minimum write speeds of 2MB/s, 4MB/s, 6MB/s, and 10MB/s respectively. Additionally, the introduction of UHS Speed Classes—U1 and U3—further refined speed classification by indicating minimum write speeds of 10MB/s and 30MB/s. While these speed classes ensure reliable recording of streaming content, the latest Video Speed Class (V6, V10, V30, V60, V90) caters specifically to high-resolution video recording, such as 4K or 8K footage.

UHS Bus Interface:

The UHS bus interface, represented by Roman numerals (I or II), signifies the type of UHS bus interface supported by the SD card. Commonly denoted as "UHS-I" or "UHS-II," this classification dictates the maximum data transfer rate between the card and the host device. While UHS-I offers enhanced performance over traditional bus interfaces, UHS-II provides even faster speeds, ideal for demanding applications and high-resolution content capture.

App Performance Class:

With the advent of Android's Adopted Storage Device feature, the App Performance Class has gained prominence in SD card labelling. The A1 and A2 ratings guarantee minimum random and sequential performance speeds essential for running applications seamlessly. A1 cards ensure a minimum random read of 1500 IOPS [input/output operations per second] and a minimum random write of 500 IOPS, while A2 cards elevate performance further with a minimum random read of 4000 IOPS and a minimum random write of 2000 IOPS. Both classifications boast a minimum sustained write speed of 10MB/s, making them ideal for smartphones and mobile gaming devices.

Unveiling Additional Features: Boot Function, TCG Storage, and RPMB

Beyond speed and storage capacity, Micro SD cards offer a myriad of advanced features catering to diverse user requirements. Features such as Boot Function facilitate fast and secure boot code uploading, while TCG Storage enhances data security through encryption and access control. RPMB safeguards against data tampering and replay attacks, ensuring the integrity of stored content.

Fast Boot and Secure Boot features give cards the ability to serve as a device’s boot code memory by using a simple and easy fast boot code uploading process along with secured methods of providing boot code updates. Boot is an optional feature supported since SD Specification Part 1 v9.0.

The Trusted Computing Group (TCG) provides open standards for secure computing, including enterprise storage and mobile devices, to name a few. Thousands of vendors offer a variety of TCG-based products, including hardware, applications and services. TCG was adopted in the market mainly for self-encrypted drive (SED) applications. The TCG Storage protocol was already defined to be used over NVMe [Non-Volatile Memory Express] and other various command layer protocols. TCG storage performs two major functions:

1. Encrypt/Decrypt (AES) automatically any user data written/read to/from the user area of the card

2. May Lock/Unlock access to the user area.

It serves as access protection if the user area access is locked or as data protection, if the storage media is removed from the host.

TCG function may be supported by any SD, SD UHS-I or SD Express cards. TCG is not defined for UHS-II mode and cannot be implemented in UHS-II cards.

RPMB [Replay Protected Memory Block] is introduced to store data in an authenticated memory area for the purpose of protecting data from a replay attack or avoiding unexpected data updates.

SD Specification Part 1 v9.0 defines how RPMB may be used on SD memory cards, either through the SD protocol over the SD interface or through the NVMe protocol over the PCIe interface on SD Express cards.

RPMB feature allows cards to have:

1. Secured Write Protect capability. RPMB restricts the use of the Write Protect features, Permanent Write Protect and Write Protect Until Power Cycle, to occur after performing RPMB authentication.

2. Secured access to the boot partitions for updating or erasing the boot code saved in either of the two boot partitions.

Two popular usages for RPMB are:

· Software Version Authentication to Prevent a Downgrade Attack Software using RPMB

Secure Boot Prevention of undesired, or hacked, code from running on a device

Embracing Future Innovations

In the dynamic realm of technology, Micro SD cards stand at the forefront of innovation, constantly adapting to meet the evolving demands of the digital landscape. As we peer into the horizon of technological progress, Micro SD cards are poised to embrace a new era of advancements that promise to redefine the boundaries of storage capabilities.

One such innovation on the horizon is Low Voltage Signaling (LVS), a groundbreaking development that holds the key to enhanced power efficiency and performance. By leveraging LVS technology, Micro SD cards can operate seamlessly within devices that demand lower power consumption, extending battery life and enhancing overall efficiency. This shift towards energy-efficient storage solutions aligns perfectly with the growing emphasis on sustainability and eco-consciousness in the tech industry.

Additionally, the advent of Application Performance Class (APC) standards, denoted by classifications such as A1 and A2, represents a paradigm shift in the realm of storage performance. These standards are designed to deliver unparalleled levels of speed and reliability, catering to the ever-expanding needs of modern applications and devices. Whether it's seamless app installations, rapid data access, or smooth multimedia playback, Micro SD cards equipped with APC technology are primed to deliver optimal performance across a diverse range of scenarios.

By embracing these future innovations, Micro SD cards are not only staying ahead of the curve but also empowering users with a new level of efficiency, compatibility, and performance. Whether it's powering next-generation smartphones, IoT devices, or gaming consoles, Micro SD cards are poised to play a pivotal role in shaping the future of digital storage. As technology continues to advance at a rapid pace, the versatility and adaptability of Micro SD cards ensure that they remain indispensable companions in our journey toward a more connected and efficient world.

Conclusion: Choosing the Right Micro SD Card

Selecting the right Micro SD card involves considering factors such as form factor, storage capacity, speed class, and compatibility. By leveraging information in this article, users can navigate the intricate landscape of Micro SD cards with confidence, ensuring optimal performance and seamless integration with their devices. Remember, the perfect Micro SD card is not merely about size—it's about finding the perfect fit for your digital needs.

Let us know your thoughts and comments in the section below.

Sources:

https://www.sdcard.org/

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