These days memory cards, especially SD cards, can be found everywhere, in devices such as mobile phones, audio players, digital cameras, personal computers, printers, e-books, and car navigation systems, as well as many electronic devices for both consumers and business solutions
These days memory cards, especially SD cards, can be found everywhere, in devices such as mobile phones, audio players, digital cameras, personal computers, printers, e-books, and car navigation systems, as well as many electronic devices for both consumers and business solutions. SD, or Secure Digital cards have become the industry gold standard for memory storage. They became extremely popular in a short time due to their high storage capacities and superior speeds. SD Memory Cards have been classified as “removeable non-volatile memory” by the SD Association. There are three types of SD cards, categorised by their size: the SD Card, the Mini SD Card and the Micro SD Card. MicroSD cards are miniaturised version of the standard SD cards, and have the same attributes.
The SD Association has provided a useful classification system, outlined below, which explains the features and usages for the SD standard type, including the size of host devices.
As mentioned, SD cards are classified as removable non-volatile memory, and they are available in four classifications: Secure Digital (SD), Secure Digital High Capacity (SDHC), Secure Digital eXtended Capacity (SDXC) and Secure Digital Ultra Capacity (SDUC). Each type has a memory capacity range. These are:
The SD Association introduced faster speed bus interfaces in response to the need for higher performance to support newer and faster devices. These are UHS-I, UHS-II, UHS-III and SD Express, and they are all available for SDHC, SDXC and SDUC memory cards.
UHS-I use one row of pins to provide improved bus speeds. UHS-II, UHS-III and SD Express utilise 2 rows of pins to provide even faster speeds than UHS-I. The second row of pins enables Low Voltage Differential Signalling (LVDS) technology.
SD Express offers the fastest data transfer rates, with speeds up to 985MB/s using PCIe Gen.3 interface and NVMe application protocol.
|Bus Interface||Card Type||Bus Mark||Bus Speed||Spec Version|
|Default Speed||SD, SDHC, SDXC and SDUC||---||12.5MB/s||1.01|
|High Speed||SD, SDHC, SDXC and SDUC||---||25MB/s||1.10|
|UHS- I||SDHC, SDXC and SDUC||50MB/s (SDR50, DDR50) 104MB/s (SDR104)||3.01|
|UHS- II||SDHC, SDXC and SDUC||156MB/s Full Duplex 312MB/s Half Duplex||4.00|
|UHS- III||SDHC, SDXC and SDUC||312MB/s Full Duplex 624MB/s Full Duplex||6.00|
|SD Express||SDHC, SDXC and SDUC||985MB/s PCIe Gen.3||7.00|
Maximum speed differs from the bus I/F speed, depending on the card performance. The average write speed of a device to an SD card depends on the device, and the operation being formed. The amount of existing data already stored on the SD card can also affect the write speed.
SD Standard for Video Recording
There is a significant difference in memory access speeds on SD memory cards depending on the manufacturer and brand. The speed discrepancy makes it challenging to pinpoint which card can record streaming contents. Constant minimum write speed is required when recording to avoid ‘frame drop’ during recording, allowing a seamless playback. In response to demands for high quality video recording, the SD association has identified various Speed Class standards. These are indicated with symbols on both hosts and cards to help users choose the best combination to avoid frame drop. The three kinds of speed indications are shown .
Speed Class*, UHS Speed Class** and Video Speed Class*** symbols with a number indicate minimum writing speed. This is used primarily for camcorders, video recorders and other video recording devices. It is necessary to use a bus mode fast enough, as that does not influence memory write speed. C10 is used in High Speed mode or faster, U1 and U3 are used in SDR50/DDR50 or faster, and V60 and V90 are used in UHS-II mode or faster.
Video Speed Class is defined in response to a demand for high resolution and high quality 4K8K video recording. It also has the capacity to support next generation flash memory such as 3D NAND. Additionally, it meets the need of HD(2K) video, as such it is integrated into Video Speed Class.
* The Speed Classes defined by the SD Association are Class 2, 4, 6 and 10. Class 10 can be applied to High Speed Bus IF product family.
** The UHS Speed Classes defined by the SD Association are UHS Speed Class 1 (U1) and UHS Speed Class 3 (U3). U1 and U3 can be applied to UHS Bus IF product family (UHS-I, UHS-II & UHS-III).
*** The Video Speed Classes defined by the SD Association are V6, 10,30,60 and 90. V6 and V10 can be applied to High Speed and UHS Bus IF product family. V30 can be applied to UHS Bus IF product family. V60 and V90 can be applied to UHS-II / UHS-III product family.
Application Performance Class
It is now common practice to install applications and related data on SD memory cards. This can be done in addition to storage, and is commonly used as a workaround when a smartphone or other portable device does not have enough internal memory. The demand for application memory expansion has increased since the introduction of Android’s Adoptable Storage Device capability.
In response to the increasing need for a combination of random and sequential performance level improvement, the Application Performance Class was introduced. This simplified application manipulations, such as the compilation of stored data.
The Application Performance Class 1 (A1) was defined by SD Physical 5.1 specification. In addition to storing files such as maps, pictures, videos, music and other documents, A1 also allows users to edit and update data without sluggish processing.
The Application Performance Class 2 (A2) is defined by SD Physical 6.0 specification. A2 SD memory cards perform to a much higher level than A1 by utilising Command Queuing and Cache functions. Maintenance functions can further support efficient flash memory management. UHS SDHC/SDXC Memory Card products can also belong to the Application Performance Class.