White Paper

SD Express:
The Next Chapter ofIndustrial
Memory Cards?

© Swissbit AG 2023 – All rights reserved.

1. Introduction
A memory card, sometimes also called flash card, isa compact, rewritable storage medium on which any
data, such as logging data, images, audio and video,
can be stored. The data is stored using flash memory
technology. Memory cards are used for devices withspace constraints, such as dashcams or medical
equipment, and are available in several formats.
Today, SD and microSD cards are the formats mostwidely used in the industrial market.
The world’s reliance on SD memory card technology
keeps growing. Yet, the read and write performance
powering data-hungry applications has held backmanufacturers. In June 2018, the SD Association,
which establishes SD memory card standards, published a specification for SD Express memory cards
for the first time. This allows the cards to transfer
data at a rate of nearly four gigabytes per second,
thanks, in part, to the use of PCIe 4.0 and NVMe
interfaces commonly found in solid state drives.
That faster transfer rate might be essential as emerging technologies, such as 5G, artificial intelligence(AI), and decentralized edge-centric models, become
widely deployed. This changes the momentum froma “cloud-first” to a “data-first” approach. This puts
the workload back on the Core and Edge and not
only leads to massively increased need of storage,
but also requires more bandwidth/performancefrom a memory device. Because it is a major challenge to predict the amount and type of data to be
stored, OEM manufacturers tend not to use fixedstorage to avoid downtime due to depleted storage
devices.
With the introduction of two new SD memory card
formats, SD Express and micro SD Express, it appears
that a format war is on the horizon. The goal of thiswhite paper is to give an objective comparison of
the two competing SD representatives and to serve
as a possible decision support.

Table of Contents
1. Introduction
2. A brief history of SD memory cards
3. PCIe eats the world
4. What is SD Express?
5. Use cases
6. SD Express technical specifications
7. Design-in considerations
8. Conclusion

2. A brief history of SD memory cards
An SD memory card, short for Secure Digital memory
card, is a type of removable memory card used toread and write large quantities of data in a wide
variety of PLCs, including Industrial PCs, HMIs, medical devices, and more. The SD memory card debuted
in 2001 and is the successor to the MultiMediaCard(MMC). It was one of a number of competing memory card formats in use by consumer electronics,
such as the CompactFlash card, which, while still inuse, is much less common than it was in decades
past. Meanwhile, SD memory cards have become thedefault standard of memory cards for most consumer electronics and have brought ideal conditions, resulting in it becoming widely deployed inindustrial electronics. A smaller version, the Micro SD
memory card, is commonly used in dashcams,
drones, and other devices where physical space ismore of a premium.
The great benefit of SD memory cards lies in theirsmall form factor; the fact that they represent
removable storage; and the great host system availability through the huge consumer market. The SD
Association (SDA) standard is clearly specified andhas an extensive feature set.

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3. PCIe eats the world

4. What is SD Express?

Conventional SD memory cards supporting UHS-I,
which is the widely deployed speed class supported
by host controllers, provide maximum sequential
read speeds up to ~90 MB/s (theoretical max is
104MB/s), which is enough to transfer 1,000
high-resolution photos and 30 minutes of 4K video
in less than 5.5 minutes. For the random write use
case, the limit is about 1k IOPS, which is alreadystarting to become a bottleneck for some PLC applications. Other bottlenecks include the increased
demand for low latency and the power consumption
in relation to performance.
To eliminate these bottlenecks and to realize thevision of a universal Industry 4.0 solution, all electronic devices will have to communicate on the basisof PCIe/NVMe. The process will probably be analogous to the consolidation of data communication
protocols towards Ethernet, which started in the
mid-2000s with mobile backhaul networks and is
now tackling building automation networks.
If the user wants to operate several storage devices
in an application, it will be much easier to realize
with a uniform protocol system. Host controllers nolonger need to be able to handle different protocols.
A consolidation of bus systems also introduces thepotential for more innovation. The vision is one single interface for peripheral computer components
with a reduced number of bridge chips required for“translation” to legacy interfaces (e.g., memory,graphic cards, networking cards, etc.) communicating via PCIe/NVMe.
When the SD Association (SDA) originally launched SD
Express in June 2018, it set a high standard for manufacturers to incorporate supercharged removablestorage into their designs. Section 8.0, which outlines an NVMe-compliant, PCIe interface option forboth SD and microSD, has been updated with thenewest release. With the implementation of NVMe
1.3, the PCIe interface provides a scalable approachfor enhanced performance in flash-based storage
devices while adhering to industry standards currently in use on other devices, such as SSDs.

The basic idea of SD Express is to leverage the existing, state-of-the-art interface and integrate it into
the most popular form factors for removable storage– SD memory cards. There is a well-established ecosystem consisting of hardware, OS, and drivers andcommunities (e.g., SDA, PCI-SIG) supporting PCIe/
NVMe implementation on SD Express.
SD Express is the next generation SD memory card
standardized by the SD Association and is an evolution of the existing standard.
The SD interface, along with the most advanced
memory interface protocols of PCIe and NVMe, brings
the favorite SD standard to new SSD levels of performance, enabling exceptional new user experiences
for multi-processing environments in real time.
This takes advantage of the benefits of both the highperformance and reliability of SSDs as well as thesmall size of SD and microSD memory cards.

Speed Class
Lanes
Performance
Backward Compatibility
Size

Figure 1: SD Express unites the well-established SD interface withthe most advanced memory interface protocols of PCIe and NVMe.

The SD Express standard utilizes a single PCIe lane forboth full size SD Express and microSD Express – setting the data transfer performance of both options
to 1GB/sec. A comparison table below highlights allof the significant differences between the Express
and conventional SD and microSD cards:*SDR (Single Data Rate) 104 and all traditional SD speed classes are optionalSD

microSDSD Express

microSD Express

UHS-I

UHS-I

PCIe Gen4

PCIe Gen3--

1 or 2 lanes

1 laneUp to 104 MB/s

Up to 104 MB/s

Up to 4GB/s 40 x fasterUp to 1 GB/s 11 x faster

NA

NA

SD UHS-I (SDR 104*)

microSD UHS-I (SDR 104*)

32.0 x 24.0 x 2.1 mm

11.0 x 15.0 x 1.0 mm

32.0 x 24.0 x 2.1 mm

11.0 x 15.0 x 1.0 mm

Table 1: Key differentiators between conventional SD and microSD cards and their respective SD Express variants.

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Benefits
As shown above, performance is the key differentiator and one major benefit of SD Express. The read
and write performance serves data-hungry applications, such as video drones, 3D medical imagery, or
industrial VR cameras, compared to standard SD
memory cards. The speed difference is illustrated in
Figure 2. It is obvious that recording a medical imaging data set the size of up to 10 GB results in a catastrophic impact on the writing performance in thestandard SD memory card case. By employing an SD
Express card, the same data set size can be writtenwith negligible time.

SD Memory Card

SD Express Memory Card

110sec2,5 sec

Elapsed time for recording data
As a result of adding PCIe to the SD (UHS-I) interface,
SD Express cards can be used interchangeably withboth new SD Express-capable hosts and billions ofolder SD hosts in use today. Despite being backwards
compatible, SD cards, however, may not reach theirfull performance potential due to varying hosts.

Figure 2: Schematic representation of the time required to record
a data set of 10 GB for SD and SD Express cards.

SD Express Benefits

SD Express Cons• Compatible with SD hosts• Compatible with PCIe hosts• More than 40 times faster than SD speed classes• Low latency• Extremely compact form factor• Must have two controller functions in cardand host• Dual interface host ASIC required• Plastic housing with typical PCIe heat emission• PCIe not necessarily hot-pluggable(depends on host)

SSD PCIe/NVMe

SD Memory Card

– Latest SSD grade performance– PCIe/NVMe – a continuously
innovated marketwide platform– Scalable SW stack widely supported– Bus mastering and reduction ram and cost
– Low power options for mobile
implementations– Most popular removable card in
consumer market

SD PCIe/NVMe card with backward
compatibility to existing billions
host devices in the market.

– Enhanced features added:
Command Queue, Cache– SD UHS-I operations mode supported– Leveraging existing investments for cardand products manufacturers

Mobile Phones

Mobile Computing

GamingImaging

Automotive

IoT…

Figure 3: Combining the worlds of PCIe/NVMe and SD memory cards – SD Express.

45. Use cases
SD Express will be ideal for the demanding high-performance content applications of today and thefuture. Therefore, the new standard provides advantages for three major areas. The first major area isapplications that demand massive capacity, such as
high-end imaging with increased resolution andframes that requires increased bandwidth. Thisapplies to applications such as super-slow motion
video, RAW continuous burst-mode photography,
and 8K/16K video capture and playback.
The second type of applications can be found in
evolving, speed-hungry IIoT infrastructures. For those,
performance and minimal latency is required foraccessing databases, booting the OS, accessing
external cache, etc. of cutting edge Industrial IoT
and networking applications.

Dash Cams• High video speed class• Quick media swap

Medical / Healthcare• High resolution imaging• High speed data transfer

Automation
The third aspect where SD Express comes into play iswith the harmonization of chipsets in order toreduce architectural complexity. SD Express functions as one single interface for peripheral computercomponents with a reduced number of bridge chips
for interfacing with legacy interfaces, such as memory, graphic cards, networking cards, etc., communicating via PCIe/NVMe.

• Easy installation transfer• Wide temperature

Data Loging• Multiple channel recording• Easy data transport

6. SD Express technical specifications
Before delving into its design, it is important tounderstand that the SDA has not launched a microSD
card with PCIe 4.0 + NVMe technology. For the time
being, it is limited to full-sized SD memory cards.

Edge Computing

The new SD Express card comes in three variations:• Latency• Wide temperature• A single-lane PCIe 4.0 interface
capable of up to 2 GB/s (4.0 x 1)
• A dual-lane PCIe 3.1 interface
capable of up to 2 GB/s (3.1 x 2)
• A dual-lane PCIe 4.0 interface
capable of up to 4 GB/s (4.0 x 2)

High End Camera• High image rate
• Quick media swap

Figure 4: Overview on potential use cases for SD Express.

5SD Memory Card

SD Express Memory Card*

4000 MB/sec PCI Gen 4 x 2

PCIe Bus Interface

2000 MB/sec PCI Gen 3 x 22000 MB/sec PCI Gen 4 x 1

2000 MB/sec PCI Gen 4 x 11000 MB/sec PCI Gen 3 x 11000 MB/sec PCI Gen 3 x 1

624 MB/sec
UHS-III
312 MB/sec
UHS-IISD Bus Interface

104 MB/sec
UHS-I
25 MB/sec
High Speed* microSD Express supports only PCIe Gen 3 x 1

Capacity(file system)

SD Memory Card

SD Express Memory Card

Ultra up to
128 TB (exFAT)

Extended up to
2 TB (exFAT)High up to
32 GB (FAT32)
Figure 5: SD cards: past, present and future pin layouts, bus interfaces, and capacities.

6Removable Memory Cards Sequential Performance
SD Express
PCI Gen 4 x 2

x 40

SD Express
PCI Gen 3 x 2, Gen 4 x 1

x 20

SD Express & micro SD Express
PCI Gen 3 x 1

x2

x 10

SD UHS-III
UFS 2.0 Gen 3
SD UHS-IISD UHS-I
Bandwidth (MB/s) 0

500

1000

1500 2000 2500 3000 3500

4000

Figure 6: A performance comparison of different SD and SD Express card specifications.

Figure 7 shows that SD Express uses the identical ports and connections as the earlier UHS-II interface (butnot backwards compatible with UHS-II). The differential interface, on the other hand, currently supports up
to PCIe 4.0 and has a single lane. As previously stated, it can reach 2 GB/s. As can be seen in Figure 7, there isa third row of pins to accommodate two PCIe lanes. It is capable of implementing PCIe 3.1 and 4.0 interfaces
with speeds up to 4GB/s. Aside from that, an additional pin 19 has been added to dual-lane cards to provide
greater power (3.3 V). Furthermore, pin 18 has been saved for future usage of an optional 1.2 V supply.

SD Memory Card

SD UHS-I

SD UHS-II

SD Express Memory Card

PCIe 1-lane

Figure 7: SD pin layout and SD Express with two PCIe lanes pin layout.

The first row of the new SD Express card architecture
is based on the UHS-I interface, which is a very beneficial feature. This implies that, while being a cutting-edge and incredibly fast memory card, SD
Express will be backward compatible with billions ofexisting SD host devices. Users will be able to use
the SD Express card on older devices and card readers without the need to purchase a suitable SD
Express card reader. However, it should be notedthat the specified Gigabit transmission speed is notavailable in this case.

PCIe 2-lane

Magnification:
SD Express PCIe 1-Lane

microSD Express
PCIe 1-Lane
SD Express PCIe 2-Lane

For the sake of completeness, it should be mentionedthat the SDA also announced the UHS-III Speed Classin February 2017. However, since the standard wasannounced, no UHS-III SD card have yet appeared on
the market or been announced. There are also no
devices equipped according to this standard.
In addition, the SDA has implemented the following
security features in the latest SD 9.0 standard. An
exact consideration of all details of the individualfeatures, as well as potential use cases, is not part ofthis white paper.
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Boot
Fast Boot and Secure Boot features give cards theability to serve as a device’s boot code memory byusing a simple and easy fast boot code uploading
process, along with secured methods of providing
boot code updates.

TCG Storage
A secured storage method defined by the Trusted
Computing Group adds a self-encrypted drive capability.

Replay Protected Memory Block (RPMB)RPMB offers a secured, hidden memory accessible
only through a secured authentication process andprovides a secured write-protect mechanism and asecured boot code update and replay protection
security mechanism.

7. Design-in considerations
There are three different host types that can use
SD Express:
1. The host supports both SD and PCIe interface. Thisis the most flexible solution. The performanceadvantage of SD Express cards can be used, but normal SD cards still work.
Host Controller

PCIe
SD

Connector

Card

Figure 8: PCIe / SD support – most flexible solution – any card/
interface can be used

2. The host supports only the SD interface. In thiscase, an SD Express card is operated like a normal SD
card, and there is no performance advantage.
Host Controller

SD

Connector

Card

Figure 9: SD-only host (not reasonable to use SD Express card here)

3. The host supports PCIe only. This is possible inprinciple, since SD Express cards can boot in PCIe
mode. However, this configuration is not recommended because it is not possible to operate normal
SD cards in the host.
Host Controller

PCIe

Connector

Card

Figure 10: Native connection – PCIe-only support – not recommended because there is a SD card slot but SD cards are not recognized

8. Conclusion
As one of the globally leading manufacturers ofindustrial SD cards, Swissbit is also a driving force
behind the research and development of industrial-grade SD Express and microSD Express products.
Swissbit’s work on SD Express technology is built on
the established SDA specification 9.0 and is using
the same footprint as existing SD and microSD memory products.
Whether the IIoT and medical market is ready andreceptive to this new technology remains to be seen.
As with any technology revolution, it takes time tounderstand which product will prevail. What is certain is that PCIe will eat up the world and eventually
all storage products will migrate their bus systems to
PCIe. SD Express competes with CF Express and, ofcourse, with embedded solutions such as PCIe BGA
or SSDs. It remains to be determined, however,which variant will prevail. In the meantime, Swissbit
is preparing itself and its customers for the migration to SD Express.

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Authors

About Swissbit

Dr. Thomas Wellinger
Head of Product Management, Swissbit

Swissbit AG is the leading European
manufacturer of storage, security
and embedded IoT solutions fordemanding applications. As trusted
partner, Swissbit empowers the digital and connected world by reliably
storing and protecting data in industrial, security and IoT applications.

Alexander Lenz
Product Manager Embedded Memory
and Flash Cards, Swissbit

www.swissbit.com
List of abbreviations
AI

Artificial Intelligence

BGA

Ball Grid Array

DDR

Double Data Rate

HMI

Human Machine Interface

IIoT

Industrial Internet of Things

IOPS

Input/output operations per second

IoT

Inter of Things

LLC

Limited Liability Company

ML

Machine Learning

NVMe Nonvolatile Memory Express
OEM

Original Equipment Manufacturer

OS

Operating System

PCIe

Peripheral Component Interconnect Express

PLC

Programmable Logic Controller

SDR

Single Data Rate

TCG

Trusted Computing Group

VR

Virtual Reality© Swissbit AG 2023 – All rights reserved.

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