👩🏽 👩🏻‍✈️ 👧 Choosing Flash Cards: A Detailed Guide to Secure Digital Varieties 🍬 💸 🈂️

When choosing a hard drive, an "advanced" buyer usually does not have any special problems. Take the same 6 TB WD6003FZBX: the spindle speed is 7200 rpm, a capacious cache of 256 megabytes and the presence of a high-speed SATA III interface with a throughput of 6 Gb / s eloquently indicate that we have a high-performance solution that can satisfy the needs of both avid gamers and enthusiasts alike professionals. The only nuance that needs to be taken into account here is that the 6 Gb / s indicator is the bandwidth of the bus itself: although the speed of information transfer from the cache to the system is 550 MB / s, the actual data exchange rate between the system and the drive for the model in question reaches 227 MB / s,which is still an excellent result for classic hard drives. However, almost any technically savvy PC user knows this.

When it comes to flash cards, even an experienced admin can go into a dead end. You don’t have to go far for examples: just open the page of the SanDisk Extreme PRO UHS-I memory card . In addition to the usual lines “read speed: up to 170 MB / s” and “write speed: up to 90 MB / s”, the mysterious inscription “video speed: C10, U3, A2, V30” flaunts in the specifications. But let me, we already know the speed characteristics of the product. Why do we need some other obscure speed classes, and even in the amount of four pieces? And what is the fundamental difference between these indicators from each other? It is in these issues that we have to figure out in today's material.

The first version of Secure Digital Memory Card was developed in 1999 with the participation of SanDisk (currently a subsidiary of Western Digital), Panasonic and Toshiba. Already in 2000, the non-profit organization SD Card Association was announced as part of the International Consumer Electronics Show (CES), the founders of which were the above-mentioned corporations. To date, more than 1000 companies are participating in its work, whose engineers help in the design and development of a whole range of specifications that describe almost all the parameters of removable memory cards - from the form factor to the features of the data exchange interface with a client device. The document is called “SD Specification”, and its contents can be found on the official website https://www.sdcard.org. Let's carefully examine the resource and see what the various abbreviations and markings mean.

Secure Digital Memory Card Form Factor

Let's start with the simplest - form factor. SD Specification provides three sizes of memory cards:

SD is a classic flash card with overall dimensions of 24 × 32 × 2.1 mm. Of the interesting features, it is worth highlighting the presence of a mechanical overwrite protection switch. When set to the Lock position, it blocks the ability to write new files to the card, delete existing ones, or format it.

miniSD is a more compact version of flash cards with dimensions of 20 × 21.5 × 1.4 mm and focused on use in portable devices. Currently, cards of this format are practically not issued.

microSD — SD-, 11×15×1 . miniSD. , , MP3-, , . , microSD , — TransFlash (T-Flash TF).

In addition to the above, there is another, unique form factor - nanoSD, introduced in October 2018 by Huawei. In size, the data on the memory card is completely identical to nanoSIM and has dimensions of 12.3 × 8.8 × 0.67 mm, that is, they are almost 45% smaller than conventional microSD. They were released simultaneously with smartphones Huawei Mate 20 and Mate 20 Pro, supporting work exclusively with this format of flash cards, as well as with proprietary dual-port (USB + Type-C) card readers that allow working with other models of mobile devices, desktop computers and laptops .

Unfortunately, the miniature size turned out to be the only trump card of the new cards, while in terms of price and technical characteristics they are significantly inferior to the classic microSD. For example, Huawei Nano SD with a capacity of 128 GB and a sequential read speed of 90 MB / s, costs almost 6 thousand rubles in the official store. For comparison, Sandisk Extreme of a similar capacity, which has much more impressive characteristics (read / write speeds up to 160/90 MB / s, respectively, with protection from exposure to water, high and low temperatures, shock and x-ray radiation) will cost you 2200–2500 rubles according to Yandex.Market (the information given is relevant at the time of writing).

Since the new format has not yet been agreed with the SD Card Association and has not received official status, it is too early to talk about its further distribution. And in light of US sanctions against Huawei and the trade war with China, the prospects for nanoSD are becoming even more vague.

Secure Digital Card Standards

If with the form factor everything is quite obvious, then with the standards of flash cards the situation is somewhat more complicated. To date, SD Specification describes 5 generations of Secure Digital cards, differing in volume and other technical characteristics. Let's consider each of them separately.

SD

This standard is presented in two versions: 1.0 and 1.1. SD 1.0 memory cards range from 8 MB to 2 GB, while devices that meet the 1.1 specification reach 4 GB. This type of SD card uses byte addressing and 32-bit addresses (which explains the maximum capacity limit of 4 gigabytes), supporting the FAT16 and FAT32 file systems.

Sdhc

Memory cards that comply with SD Specification version 2.0. Their main difference from the previous ones is the support for sector-by-sector addressing, which has increased the maximum amount of flash cards to 32 gigabytes. The negative side of this advantage is the lack of backward compatibility with devices oriented to work with regular SD cards. The file system used is FAT32.

Sdxc

A standard introduced by the SD Association in 2009 as part of the International Consumer Electronics Show (CES). The abbreviation stands for Secure Digital eXtended Capacity ("SD-cards high capacity"). SDXC memory cards can have a capacity of up to 2 TB and use the exFAT file system, and also received a UHS bus (we will talk about it below), capable of working in four-bit mode and providing data transfer speeds up to 312 MB / s. Another feature of the new standard is direct and backward compatibility with previous standards: SDXC-enabled devices can work with SD and SDHC memory cards, and SDXC memory cards can work in SDHC-enabled devices, provided that they have been pre-formatted in FAT32.

SDUC

This standard is on the SD Specification Version 7.0. Like SDXC, memory card data uses the exFAT file system, but its maximum capacity can reach as high as 128 TB.

SD Express

Standard presented by the SD Association to the general public on June 27, 2018. It immediately included three types of flash cards that differ from each other in maximum capacity: SDHC Express (up to 32 GB), SDXC Express (up to 2 TB) and SDUC Express up to 128 TB. The new generation of memory cards is fundamentally different from its predecessors, as it uses the PCI Express 3.0 interface and the NVMe 1.3 protocol (on the second row of contacts), which allows them to develop data transfer speeds up to 0.9 GB / s. At the same time, flash cards of this type are backward compatible with devices that support operation with the UHS bus.

As for SDXC and SDUC, it is necessary to understand that the SD Card Association is ahead of the way in developing standards, since it takes quite a long time to create the appropriate technologies and practical implementation of the adopted specifications. To date, the most capacious and fastest memory card is 1 TB SanDisk Extreme: in addition to the record volume, this card demonstrates an impressive sequential read speed (up to 160 MB / s), while the SDXC standard itself, as mentioned above, provides the maximum card capacity is up to 2 TB and the data transfer speed is up to 312 MB / s, that is, a potential SDXC flash card can be 2 times more capacious and twice as fast.

The creation of this card was made possible through the use of the latest 64-layer 3D NAND BiCS 3D flash memory , made using the 28-nanometer process technology, which we described in detail in previous materials on solid-state drives. It was the increase in cell packing density by almost 1.4 times compared with the previous generation of three-dimensional memory that helped us create a flash card with a record capacity by the standards of the modern market. Further development of promising technology will allow in the future to increase the number of layers in a three-dimensional crystal to 128 and create even more advanced products that meet the SDUC specification.

Speed characteristics of SD cards

We've figured out the standards, it's time to study the speed classes. However, in order to avoid further confusion, it is necessary to deal with a concept such as UHS. This abbreviation stands for Ultra High Speed - "ultra-fast data transfer." The term has two meanings. First of all, this is the name of the bus, the specification of which first appeared in the third version of the Secure Digital standard. The main difference between UHS and its predecessors was support for the 4-bit data transfer mode, which allowed us to bring the performance of flash cards to a whole new level. So, UHS-I, the specifications of which were defined in the Secure Digital 3.01 standard, supports a data exchange rate of 50 or 104 MB / s, and UHS-II (included in the updated version of the Secure Digital 4.0 standard) - already 156 MB / s or 312 MB /with,whereas for SD cards with High Speed interface the limit of dreams was 25 MB / s.

By the way, writing “50 or 104 MB / s” is not an error. This is not a range of speeds, but two possible modes of operation. UHS-I cards are capable of operating in SDR50 or SDR104 mode. In SDR (Single Data Rate) mode, one data word is transmitted per cycle and one control command is received. Thus, at a frequency of 100 MHz, the bus is capable of transmitting 50 megabytes per second, and at a frequency of 208 MHz - already 104 MB / s.

With the UHS-II interface, things are a little more complicated. Such memory cards have two lines of contacts.

The upper one provides backward compatibility with the High Speed and UHS-I interfaces, while the lower one allows the card to function in two additional modes: FD156 and HF312. Using a pair of low-voltage (0.4 V) bands in duplex mode (and FD means nothing more than Full Duplex) allows you to achieve honest 156 MB / s at a frequency of 52 MHz, and half-duplex mode (HD, i.e. Half Duplex) - already 312 MB / s at the same frequency, however, data can only be transmitted in one direction at any given time.

The abbreviation UHS is also used to indicate the speed class of flash cards equipped with this interface (full name - UHS Speed Class). But, before proceeding to its discussion, it makes sense to clearly determine the terminology used. To do this, we turn to the technical specifications of SanDisk Extreme PLUS SDHC / SDXC UHS-I memory cards.

Here, the numbers 90 and 60 MB / s are the nominal speed achievable in certain scenarios (sequential reading and writing, respectively) under ideal conditions in which the client device fully reveals the potential of a particular card. In practice, such situations are extremely rare: most often the card works in mixed mode, and the device itself can be a “bottleneck”. In this regard, the concept of “speed class” appeared - the minimum stable performance indicator in the worst testing conditions on compatible equipment. Today there are 4 speed classes:

Speed Class
UHS Speed Class,
Video Speed Class
Application Performance Class.

The first three classifications determine the suitability of flash cards for recording video of a certain resolution, while the Application Performance Class declares high-speed performance of cards when working with applications and, in addition to bandwidth, also takes into account the minimum stable number of I / O operations per second.

The presence of three classifications at once, which determine the suitability of a particular card for working with streaming video, is explained both by the constant development of flash cards themselves and the spread of new video formats. The first classification was Speed Class, which includes 4 performance classes (from 2 to 10 MB / s). UHS Speed Class was introduced specifically for devices with a UHS bus and includes only two classes: 1 (10 MB / s) and 3 (30 MB / s). The Video Speed Class was first introduced by the SD Card Association during the annual CP + show held in Yokohama in 2016, and was included in the SD 5.0 specification. This classification takes into account support for recording ultra-high-resolution streaming video (4K, 8K and 3D) and covers a speed range from 6 to 90 MB / s.

The Application Performance Class is included in the enhanced version of SD Specification 5.1. The need for its development was largely due to the emergence of the possibility of using flash cards as internal storage in Android-based mobile devices (the possibility of fully combining the smartphone’s own memory with flash card space appeared since the 6th version of the Marshmallow operating system). Although memory cards showed good results when recording streaming video, their performance often turned out to be insufficient for working with applications, and the emergence of a new standard was designed to correct this drawback.

All these classifications use the same format of abbreviations: XY, where X is a letter indicating the type of classification used, and Y is a number denoting the class itself. The following Latin letters correspond to flash card speed classes:

Speed Class - C,
UHS Speed Class - U,
Video Speed Class - V,
Application Performance Class - A.

Since the above classifications overlap in many respects, we have prepared for you a comparative table in which the speed characteristics of flash cards are compared and correlated with videos of different resolutions.

Speed class	UHS Speed Class	Video Speed Class	Minimum sustained write speed	Application area
C2	-	-	2 MB / s	Recording standard definition video (SD, 720 by 576 pixels)
C4	-	-	4 MB / s	High Definition (HD) video recording, including Full HD (720p to 1080p / 1080i)
C6	-	V6	6 MB / s
C10	U1	V10	10 MB / s	Full HD (1080p) video recording and burst shooting in HD (High Speed bus), HD video streaming and large file transfer (UHS bus)
-	U3	V30	30 MB / s	4K video recording with a frame rate of 60/120 frames per second (UHS bus)
-	-	V60	60 MB / s	Recording video files with a resolution of 8K and a frame rate of 60/120 frames per second (UHS bus)
-	-	V90	90 MB / s

Separate is the classification Performance Application Class. In terms of data transfer rates, classes A1 and A2 correspond to U1 (10 MB / s). The differences lie in the smallest sustainable number of I / O operations per second.

Application Performance Class	Minimum steady-state recording speed	Random reading, minimum	Random record, minimum
A1	10 MB / s	1500 IOPS	500 IOPS
A2	10 MB / s	4000 IOPS	2000 IOPS

Workshop: decode mysterious letters

Now, armed with the acquired knowledge, you can easily decipher the marking of any flash card. Let's go back to the SanDisk Extreme PRO UHS-I and carefully read its specifications again.

Let's start in order:

microSD - the memory card has overall dimensions of 11 × 15 × 1 mm (width × height × thickness).
SDXC - before us is a Secure Digital eXtended Capacity card equipped with a high-speed UHS bus.
C10 - the card demonstrates a minimum stable write speed of 10 MB / s in backward compatibility mode (High Speed bus).
U3 - the card demonstrates a minimum steady write speed of 30 MB / s via the UHS bus.
V30 - the card can be used to record a video stream in resolution up to 4K with a frequency of 60/120 frames per second.
A2 - the card shows a minimum stable performance of 4,000 IOPS in random read operations and 2,000 IOPS in random write operations at a data transfer rate of 10 MB / s and is suitable for expanding the internal memory of smartphones and tablets based on the Android operating system.

As you can see, it was worth a little understanding of the issue, and the marking of the card provided us with comprehensive information about its performance, purpose and compatible devices. Now you know enough to navigate the abundance of specifications and choose a flash card that fully meets your needs.

Choosing Flash Cards: A Detailed Guide to Secure Digital Varieties