Mobile Device Innovations: Application
Performance Class Expansion and New Low
Voltage Signaling for SD Memory Cards
White Paper | February 2017www.sdcard.org | ©2017 SD Association. All rights reserved

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Mobile Device Innovations: Application Performance Class Expansion and New Low Voltage Signaling for SD Memory Cards

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Executive Summary
In February 2017, the SD Association (SDA) released the SD
6.0 specification with a variety of new functions that may
be implemented in SD memory cards. While
complementary, each of the new features may be used
independent of the others: Application Performance Class
2 (A2); Command Queuing, Cache and Self-Maintenance,
new protocol functions that allow performance
improvements; Ultra High Speed III (UHS-III), providing up
to 624 megabytes per second transfer rates; and Low
Voltage Signaling (LVS), allowing SD memory card
operation with hosts that support only 1.8V signaling.
This document provides the background and details on the
new App Performance Class A2 and related new features aswell as LVS. UHS-III is explained in a separate document
available on our website.
In November 2016, the SDA released the SD 5.1
specification to answer new market needs with Application
Performance Class, introducing Application Performance
Class 1 (A1) (see related white paper) as its first and basic
class. The new A2, introduced in SD 6.0 specification, is thenext class level and provides more flexibility in the market
for cost-performance optimization per product orapplication need and various market segments. The newly
introduced functions of Command Queuing, Cache, andSelf-Maintenance are SD protocol functions that enablehigher performance levels.

LVS introduces new 1.8V signaling to let an LVS host device
start in UHS-I/1.8V mode directly, without traditional 3.3V
initialization process after power up. The LVS card is usable byboth conventional hosts connected by 3.3V signaling and LVS
hosts connected by 1.8V signaling. An LVS host cannot use
conventional cards except when UHS- II mode is available
using UHS- II cards. This new feature lets productmanufacturers take advantage of new SOC designs enablingsmartphones to offer higher performance with less power
consumption.
As with all SD features, consumers are encouraged to match
their SD memory card’s performance level to their product’s
requirements.

This document provides the
background and details on the new
App Performance Class A2 and related
new features as well as LVS. UHS-III is
explained in a separate document
available on our website.

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Background on Market Situations
GeneralMost of today’s mobile operating systems (Win-Mobile andAndroid) allow application data to be saved on removablecards or run applications from cards. When Google released
Android Marshmallow in 2015, it expanded microSD
memory card support by offering Adoptable Storage
Devices, which allows users to run applications from theinstalled memory card. It’s helpful to know there are somebasic performance requirements based on a combinationof random and sequential memory access that optimize
user experiences when using memory cards as storage
and/or to run applications.
Product manufacturers may use the Application
Performance Class (App Performance) with its associatedsymbol to define a certain level of minimum performancefor customers, ensuring the SD memory card delivers theright performance with their products. This also helps
consumers choose the proper card for their product at the
point of sale.
The App Performance Class 1 (A1) was introduced in SD 5.1
specification to answer these evolving market needs byindicating a minimum basic level of performance. While
today’s A1 cards may meet most users’ and application
developers’ needs, there may be a need for cards in themarket with higher performance capabilities designed tomeet higher host/product performance needs to give users
and product developers more flexibility to optimize costperformance needs for their product and applications
requirements for their intended market segments.
Therefore, SD 6.0 specification introduces App Performance
Class 2 (A2) with higher minimum random write/read
performance levels and a new set of features that enables
memory cards to achieve these higher levels. In short, SD
memory cards are now used for running applications, andthis use requires a combination of higher random +sequential performance.

The SDA initiated new SD specification enhancements tomeet market requirements by:• Standardizing a common language for consumers andproduct manufacturers, allowing best utilization of
specific products with related best cost-effective card type• Introducing the new concept of Application Performancewith SD 5.1 specification, along with the first App
Performance Class named A1• Defining the next level of minimum performance named
A2 with SD 6.0 specification, allowing users to match withthe best cost-performance needs• Improving SD memory card protocol, enabling higher
random and sequential performance, with SD 6.0
specification

Application Performance Class 2 – A2
The Application Performance Class standard defines:• App Performance symbol for use on cards, packaging
and manuals• Assured combination of minimum random + sustained
sequential performance levels under certain conditions

App Performance Class SD memory cards
App Performance Class A1 and A2 SD memory cards mustmeet the performance measures as given in Table 1.

Table 1 Application Performance Classes and their related
minimum Input-Output access Per Second (IOPS)

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Note that the given performance levels are assured under theconditions defined in SD 5.1 specification for A1 and SD 6.0
specification for A2. Any SD memory card that indicates itself
as an A1 or A2 card by the logo and in the internal SD Card
Status register must meet the defined specification
requirements. Please refer to the SD 6.0 Specification forfurther details. Figure 1 shows the new App Performancesymbols in both short and long forms.

Product manufacturers supporting SD memory cards andrequiring certain minimum random and sequential
performance levels that are satisfied by either A1 or A2
may check for the desired Application Performance Classlevel as follows:
1. Check the Application Performance Class type of the SD
memory card in the SD Card Status and confirm
whether it matches its requirement level.
2. The host may also perform an internal benchmark test
and check for the absolute execution time to qualify thememory card. If it matches or exceeds the execution
time expected from the corresponding Application
Performance Class expected by the host, the cardshould be accepted for use.

Figure 1 App Performance Logo

What is expected from product manufacturers
and consumers?
Similar to the concept of Speed Classes, it is recommended
that consumers choose the SD memory card with theApplication Perf Class that is recommended by their productmanufacturers, as indicated on the product, package oruser manual.
There is an important new expectation for productmanufacturers seeking A2 designation. While there was noneed to support any new SD protocol functions for A1, A2
products must support the new functions of Command

An important note about A1 cards: The performance levelthat SD memory cards indicate during the Android’s Card
Adoption process is designed1 to allow A1 SD memory
cards to pass, but they cannot be guaranteed by Android.
The reason is that the Android benchmark test isdependent on the device environment (i.e. clock speed,background apps, etc.), not only the memory card.
Therefore, the SDA highly recommends that productmanufacturers first test their devices with A1 SD memory
cards before they would recommend their customers to
use A1 SD memory cards, assuring acceptance during thecard adoption process.
Note that in any case, an SD memory card indicates
Application Performance Class support, it shall meet the
SD standard specification for Application Performance
Class conditions as defined in SD 6.0 specification.

Queuing and Cache. Therefore, before referring customers to
A2 cards, manufacturers must make sure that their productsupports these functions.

1

Refer to Application Performance Class White Paper on the detailed
conditions for the Android Benchmark passing A1 cards

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www.sdcard.org | ©2017 SD Association. All rights reservedNew SD protocol functions for A2
SD memory cards have been progressing in two aspects:
greater storage capacity and faster bus speed. Figure 2
shows the progress of SD bus interface throughput. As SDA
propels higher storage capacity and faster bus speed
specifications, it enables applications to manage big datawith an SD memory card.
Following are the three new features:
Command Queue• Contributes mainly to random read performance• Multiple tasks can be handled at one time with
arbitrary order• New tasks can be assigned during data transmission• New commands (CMD43,44,45,46,47)
Cache function• Contributes mainly to random write performance• Card may use higher speed volatile memory to cache thehost data during memory card access operation• Data loss is allowed if it’s not moved to non-volatilestorage• Cache flushing is triggered by host followed by up to
1 second of card’s busy time.
Self-Maintenance• Contributes to better memory access performance• Allows internal background data management• Initiated by card when there are no pending commands• Initiated by host based on the maintenance event by
the card

What is Command Queuing?

transfers planned by the host is provided. By having suchinformation, the card may optimize its internal memory
management and even execute the commands in adifferent order than issued by the host. As such, a related
performance improvement can be mainly seen in small
random accesses.
Queue depth is the total number of commands that may
be queued at one time. SD supports a queue depth of 32,
which is quite common in similar standards (32independent RD or WR commands without requiring any
specific order of execution).
The Command Queuing (CQ) function defined in SD has
met the following concepts:• Minimum changes to the SD protocol (including
state machines)• No added pins to the I/O interface• Backward compatibility to the legacy SD standard• Functional similarity to existing, commonly usedhigher layers of CQ management, allowing re-useof widely implemented drivers in the industry forsimilar applications
The legacy SD protocol suffered from a command
response type of protocol. It means that every commandsequence looks like the following (see Figure 2). Hostissues commandcard response
data transfer (ifassociated) follows. The host is not allowed to issue thenext operational command (except status) until the current
command completes execution.

It is assumed that memory card performance may be
improved if early information about the intended data

Figure 2 Legacy SD command response concept

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Figure 3 Flow example for data transfer with Command Queue

Such method eliminates delivery of any advanced
information on the next command. SD bus interface is notefficiently utilized in this method.
The new CQ mechanism allows the SD memory card toaccept several commands in a series (without theirassociated data) and execute them (with the data)whenever the memory card is ready. During the datatransfer, additional commands may be sent to the card aslong as the maximum number of queued commands does
not exceed 32. Figure 3 shows an example of queuing two
commands in a series, wherein the SD memory card
executes the command that the card is ready and thentransfers the third command during the data transfer of theongoing executed command.
The advantages of CQ are:• Advanced information on intended commands isprovided to the card
• The card may manage and optimize its internal
operations to prepare for the various commands inadvance• New information on next commands may be sent to thecard during current command execution and during
data transfer

Cache function
In order to overcome the relatively limited write speed
operation of the flash memory, the Cache function allows
the card to accumulate the data accepted by the host in ahigh-speed volatile memory (RAM) first, release the busy
line and perform the actual write to the non-volatilememory in the background or upon flush command (refer
to Figure 3).
Card may cache the host data during write and read
operation:• Cache size is card-implementation specific• Flushing of contents stored in cache shall not take morethan 1 second.
• Supported by OSs today (for embedded)• Easy to implement on cards
Data loss is allowed if the data is not moved to non-volatilestorage. In order to minimize data loss, the flush cachefeature shall be supported and implemented by host before
power down operation. The data is not guaranteed as longas the flush operation is not completed.

Figure 4 Data transfer with Command Queue – Example of flow

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www.sdcard.org | ©2017 SD Association. All rights reservedSelf-Maintenance function
The Self-Maintenance operation of the card is an internal
data management operation done on the memory to
improve its performance (i.e. defragmentation). There are
two cases when the Self-Maintenance may start its
operation: card initiated and host initiated.
Card-InitiatedCard-initiated operation allows the SD memory card to
perform internal maintenance operations while the hostbus is idle. When a new command is received, the card
must ensure that new command is serviced without
violating SD standard timing (Read Command, 100ms; Write
Command, 250ms).
Power-off notification function support is mandatory toensure graceful shutdown, allowing the host to tell the cardin advance about its intention to power off.
Host-Initiated
In host-initiated operation, the SD memory card indicates tothe host the self-maintenance urgency level through fourlevels of urgency. The memory card could notify the hostvia Card Status when it’s at middle or urgent level.
Maintenance operation is triggered by the host via a
specific command.
After the card’s Self-Maintenance is initiated, it sets the busy
line (prg state) for up to 1 second and then moves back tonormal (tran state).
The self-maintenance urgency level may still be the sameafter a maintenance operation.

Low Voltage Signaling – Another new SD
6.0 specification feature
Silicon foundries have been fine-tuning system on a chip(SOC) integrated circuit technology for use in devices suchas smartphones, with recent progress pushing thepowerful chips to the 7-10 nanometer range. These SOC
developments will enable smartphones to offer higher
performance with less power consumption. Designing
SOCs, which use finer processes and lower voltage supply,is easier when restricted to 1.8V signaling.
SD memory cards have used a 3.3V signaling interface
since the SD standard was introduced in 2000 and throughSD Specification 3.0, when 1.8V signaling was added with
the UHS-I bus mode, the ultimate removable single-ended
interface. UHS-I adopted 1.8V signaling because it issuitable for faster rise/fall time and lower electromagnetic
interference. However, UHS memory cards still require 3.3V
signaling to initialize the card so hosts need to support
3.3V signaling, too. UHS-II is another low-voltage interface
option for host devices that implement an SD Connector
with a second row and offer a 1.8V power supply.
Therefore, UHS-II is a solution for higher performance
applications which require more than UHS-I performance.
Now, SD Specification 6.0 introduces the Low Voltage
Signaling (LVS) memory card that may support either 3.3V
or 1.8V signaling with an auto detection mechanism of thehost’s operating signaling level. The new specification
allows LVS cards to be backwards compatible toconventional 3.3V signaling hosts. It coincides with new
SOC designs for LVS host devices that can start in UHS-I
1.8V mode directly after power up on the LVS card.

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Low Voltage Interface Supported Card Types
LVS memory cards are compatible with conventional host devices, and may also support UHS-II andUHS-III host devices. Table 2 shows LVS card types defined by SD Specification 6.0.

Table 2: LVS Card Types

LVS cards are designed to start command/response communication either with 3.3V signaling
modes or 1.8V signaling modes. An identification sequence after power up, called LVS Identification,
is used to select either signaling mode. LVS cards can identify LVS hosts via the LVS Identification bychecking the specified clock period on SDCLK pin and signal lines level at clock edge. If an LVS host is
identified, the LVS card selects 1.8V signaling; otherwise, it selects 3.3V signaling. An LVS host canidentify an LVS card by checking the specified card signal level during the LVS Identification.

Host and Card Combination
Figure 5 shows host and card combinations:• The LVS card is fully backwards compatible on connection. It is usable either by conventional hosts
using 3.3V signaling or by LVS hosts that operate only with 1.8V signaling.
• An LVS host may operate only with the new LVS cards. However, conventional UHS-II cards may
also operate if a UHS- II mode is available by the host and chosen by the user.
An LV symbol identifies LV Interface supported SD products for users. An LVS host will bear the LV
symbol somewhere on the product, package or manual. LVS host device users need to use acorresponding SD memory card with an LV symbol for their LV-marked device. On the other hand, an
LVS card is usable by any host, including hosts that do not carry the LV symbol.

Figure 5: Host and Card Combination

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Summary
Mobile use of SD memory cards continues to be robust and innovations found in SD 6.0
specification give the market more flexibility and capability. With the expanded usage of SD
memory cards for storing applications and application data, there is a growing need for a
combination of random and sequential performance levels. This demand becomes evenstronger with the introduction of Android’s Adopted Storage Device capability. SD 6.0
specification introduces the next Application Performance Class, A2. The new class assureshigher levels of minimum performance under given conditions and expands SD memory card
use for a wider range of higher performance devices and applications. It gives device
manufacturers the ability to match the most effective cost-performance SD memory card totheir needs.
Further, introducing LVS cards further increases manufacturers’ options and flexibility inmeeting the new low-power SOC needs while keeping full backwards compatibility toexisting products.
The SDA has also created a clear method of communicating all SD memory card requirements
so consumers can choose the right card for their device at the point of sale.

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