Why Are Storage Cards Smaller Than Fingernails Comparable to SSD Speeds?

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A storage card is a universal storage peripheral suitable for digital devices such as mobile phones, tablets, cameras, drones, dashboard cameras, and surveillance cameras.However, the read and write speeds of storage cards have always been their biggest bottleneck, and the main reason high-end smartphones have discontinued support for them.So, what standards do storage cards have, and do they have a chance to achieve performance comparable to SSDs and be accepted back by high-end smartphones?

Continuously Expanding Storage Cards

Storage cards once included Secure Digital Memory Card (SD), Micro SD Card (also known as TF), Compact Flash (CF), Memory Stick (MS), etc., but today only SD and MicroSD remain popular.

As the capacity of SD cards continues to increase, these two types of storage cards have further derived the SDHC (SD High Capacity), SDXC (SD eXtended Capacity), and SDUC (SD Ultra Capacity) standards. Please refer to the table below (Table 1) for their differences.

In terms of capacity, SD cards have successfully fulfilled the role of external storage devices, providing just the right capacity to meet the expansion needs of digital products at the same time.

However, while the capacity is sufficient, what about the speed?

Continuously Increasing Speed of Storage Cards

Before discussing the speed of storage cards, let’s take a moment to review the development history of SSDs.

Like storage cards, SSDs also use NAND flash memory as the storage medium, achieving read speeds of about 550MB/s and write speeds of about 520MB/s (sequential speeds, same below) when using SATA 3.0 bus interfaces, far surpassing HDD mechanical hard drives.

However, as the main consumer force for SSDs, PCs have an endless demand for storage performance, and the SATA bus quickly became a bottleneck limiting overall performance.

Laptops Support SATA/PCIe and M.2 Slots That Only Support PCIe Bus

Thus, SSDs began to change their approach, attempting to use the PCI-E bus (including PCI-E2.0 and PCI-E3.0, as well as dual-channel and quad-channel, for example, PCI Express Gen3.0×4 is a quad-channel PCI-E3.0 bus) M.2 interfaces, coupled with the NVMe protocol (an advanced version of the AHCI protocol), allowing NVMe SSDs to achieve read speeds of up to 3500MB/s and write speeds of about 3000MB/s, which is 6 to 7 times that of the SATA bus!

Clearly, the evolution of bus interfaces is the core competitive advantage that allows SSDs to continually increase their speed.

Now, looking back at storage cards, these NAND-derived peripherals, which are only the size of a fingernail, also have their speed determined by the bus interface, and the relevant technical standards are set by the SD Association (SDA). As of today, storage card standards have been upgraded from SD1.0, SD2.0… all the way to SD7.1 standards (Table 2).

When storage cards were first introduced, the corresponding SD1.0 standard was interesting; it defined the performance of storage cards based on the speed of CD-ROM as 1X speed, with a maximum read speed of only 12.5MB/s. Later, the SD 1.1 standard introduced HS high-speed mode, further increasing the speed to 25MB/s.

During the era when digital cameras and smartphones became popular, users needed storage cards to have stable write speeds to meet the needs of continuous shooting. Therefore, during the SD2.x standard period, the SD Association introduced the Class x definition method, with the highest level Class 10 requiring storage cards to have a minimum write speed of 10MB/s, easily recording standard-definition videos.

Starting from SD3.0, storage cards support the new UHS-I ultra-high-speed bus interface, which can coexist with DS and HS modes, achieving theoretical read speeds of 50MB/s or 104MB/s without changing the storage card’s pin configuration.

At the same time, the SD3.0 standard also introduced the U1 and U3 definitions, further increasing the write speed of storage cards to 30MB/s, meeting the basic requirements for recording FHD full HD videos.

With the rise of 4K video recording demands, storage cards require higher speeds to meet the needs of enthusiasts. Thus, the SD Association successively introduced UHS-II and UHS-III bus interfaces in the SD4.0 and SD6.0 standards. The biggest difference between UHS-I and UHS-II is that it uses an additional row of pins, where the traditional pins on top are compatible with DS, HS, and UHS-I buses, while the newly added pins below are used to identify the new FD156 and HD312 working modes of UHS-II.

Standard UHS-II SD Card

Comparison of UHS-II Micro SD Card and Standard Micro SD Card

The two rows of pins allow UHS-II to have two data transmission lines, one line can be allocated to the host to the storage card, while the other line can be used from the storage card to the host, achieving a dual-channel half-duplex working mode.

In “full-duplex mode” (FD), the two lines are used for transmitting and receiving data, with a theoretical transmission rate of 156MB/s, i.e., FD156.In the dual-channel “half-duplex mode” (HD), data can be sent or received simultaneously in both directions, achieving a doubling of bandwidth and bus performance, increasing the theoretical transmission rate to 312MB/s, i.e., HD312.

The UHS-III bus interface introduced in the SD6.0 standard is even more impressive; although it operates only in full-duplex mode, it boosts speeds to FD312 (312MB/s) and FD624 (624MB/s), while also reducing the time required for the storage card to transition from a power-saving state to an active state.

Indeed, the UHS-III bus interface is more like a new solution specifically designed for professional cameras and smart devices, where installed apps not only run fast, enabling continuous shooting and 4K video recording without delay, but also have lower wake-up latency from standby and do not hinder the storage card from remaining in a power-saving state for extended periods.

It can be said that starting from the UHS-II bus interface, storage cards have the strength to challenge the eMMC5.1 flash memory used in smartphones, while the UHS-III bus interface can rival the speed of UFS2.0 flash memory and SATA bus interface SSDs.

Unfortunately, although the SD6.0 standard and UHS-III bus technology were already established and released in 2017, the highest-end storage cards on the market still remain at the UHS-II bus interface level.

With the launch of the SD7.0 standard in 2018, storage cards are likely to skip the UHS-III bus and directly embrace the latest PCI-E bus interface and NVMe protocol.

Further Reading:Samsung UFS Storage Cards

In addition to the UHS-III bus interface defined by the SD Association providing SSD-comparable performance, the UFS standard led by JEDEC (Solid State Technology Association) is also a surefire way to speed up storage cards. The UFS v1.1 standard launched in 2018 can achieve maximum storage speeds of 1.5GB/s (the sequential read and write speeds of the UFS1.0 standard are 530MB/s and 170MB/s, respectively).UFS storage cards are similar in size to Micro SD cards, but their appearance resembles a “shark fin,” and the gold fingers have also been redesigned.

SD7.x Declares War on SSDs

In June 2018, the SD Association officially announced the SD7.0 standard, which includes two new features:The first is that storage cards can transmit data through the PCIe Gen3.0×1 bus interface and NVMe protocol, increasing the theoretical transmission (read) speed from the maximum 624MB/s of the UHS-III bus to 985MB/s;The second defines the new “SDUC” standard, expanding the storage card capacity from 2TB to a maximum of 128TB.

Storage cards that meet the SD7.0 standard will be called “SD Express” (the surface of the storage card will add the Express or EX mark), and their appearance is the same as existing SD cards, except that like UHS-II/III, 10 new pins are added below the existing metal contacts.

In early 2019, the SD Association updated the SD7.1 standard, adding Micro SD Express to meet the future application needs of small devices such as smartphones, providing a theoretical bandwidth of 985MB/s through the PCI-E bus interface, while using the NVMe v1.3 communication protocol to enhance data throughput.

Tip:SDUC is merely a capacity standard; storage cards using the UHS bus interface that exceed 2TB can also be called SDUC storage cards.Currently, the capacity of SD Express storage cards is still at the GB level, so for a long time to come, SD Express will still belong to the category of SDXC storage cards.

As in the past, SD Express storage cards are also backward compatible with all SD card readers (or mobile phones, drones, digital cameras, etc., hereinafter referred to as terminals), but to achieve the theoretical value of 985MB/s requires a dedicated SD Express card reader; on readers (or devices) that support UHS-II/III bus interfaces, only a transmission speed of 104MB/s can be obtained.Similarly, UHS-II/III storage cards will also be limited to 104MB/s when encountering dedicated SD Express card readers (or devices).

Currently, Realtek and Phison have released controllers that support SD7.x, laying the foundation for the popularization of such ultra-fast storage cards.Among them, the Realtek controller model that meets the SD 7.0 standard is “RTS5261,” with a maximum read speed of 985MB/s.The Phison controller model for the SD 7.1 standard is “PS5017,” which fully supports SD 7.0 and SD 7.1, and is currently the first solution that simultaneously implements SD Express and Micro SD Express.PS5017 supports 3D TLC/QLC flash memory, with read/write speeds of up to 900MB/s and 500MB/s, comparable to entry-level PCIe NVMe SSDs in the PC field.

Recently, Western Digital also showcased Micro SD Express standard storage cards at ComputeX 2019, achieving a continuous read speed of 888.9MB/s and a write speed of 428.2MB/s during on-site DEMO testing, transferring a 12GB file in less than 30 seconds.At the same time, JMicro (Zhiwei Technology) also announced that it will launch corresponding SD7.x card readers in sync with Western Digital, enabling this new generation of high-speed storage cards to have a place to be utilized.

Long Road Ahead for Practical Applications

When PCs have adopted SSDs and smartphones have integrated UFS flash memory, using traditional storage cards as data intermediaries is undoubtedly a regression in experience.With the advent of the SD7.1 standard and Micro SD Express storage cards, the entire mobile industry has been provided with an eye-catching new choice—mobile devices such as smartphones can finally use “mobile SSDs” that can be installed or removed at any time, with transfer speeds close to 1000MB/s, sufficient to meet the interactive needs of applications with high-speed requirements on smartphones, evolving gaming systems, multi-channel IoT devices, various automotive applications, higher resolution mobile videos, action cameras, 360-degree videos, and VR applications.

Tip:The latest SD7.1 standard has a hidden advantage, which is that the PCI-E 3.1 bus used includes low-power sub-states (L1.1, L1.2), which can further reduce the power consumption of storage cards during standby.

However, just as UHS-II storage cards have yet to gain popularity in the smartphone and PC circles, (Micro) SD Express storage cards, despite their extreme speed performance, face an incomplete surrounding ecosystem and relatively high production costs. In the short term, their main application areas will still be high-end fields such as 4K/8K video cameras and drones. Only when our newly purchased laptops and smartphones come standard with SD Express card readers will they have a real opportunity for widespread adoption.

Moreover, storage devices face a severe test when running for extended periods, namely heat generation.PCIe NVMe SSDs on PCs often require metal heat sinks (or rely on the laptop’s bottom cover for cooling) to prevent speed drops, and flash drives running at speeds above 200MB/s can become very hot during full-speed operation.

As a storage card with speeds approaching 1000MB/s, a size only as small as a fingernail, made of plastic substrate with no heat dissipation measures, can you imagine its “enthusiasm”?

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