Specifications

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Physical Layer Simplified Specification Version 4.10

4.13 Speed Class Specification

The Speed Class Specification classifies card performance by Speed Class number and offers a

method to calculate performance. The specification enables the host to support AV applications to

perform real time recording to an SD memory card. The following sections describe the Speed Class

specification for the card. Refer to the Application Notes for an example of host implementation.

Figure 4-46 shows an overview of the Speed Class Specification. Class 2, 4 and 6 are defined and

achieve in Default Speed Mode while Class 10 requires High Speed Mode. Speed Grade 1 is defined in

UHS-I and UHS-II modes.

Figure 4-46: Overview of Speed Class Specification

Section 4.13.1 describes Speed Class Specification for SDSC and SDHC.

Section 4.13.2 describes Speed Class Specification for SDXC.

Section 4.13.3 describes Speed Grade Specification for UHS-I and UHS-II Card.

4.13.1 Speed Class Specification for SDSC and SDHC

4.13.1.1 Allocation Unit (AU)

The User Area is divided into units called "Allocation Unit (AU)" (Refer to Figure 4-47). AU is physical

boundary in User Area of a card and is not defined by the file system boundary. Each card has its own

fixed AU Size (S

) and the maximum AU Size is defined depending on the card's capacity. The host

should manage data areas with the unit of AU. If the first AUs in the card contain file system information

then they should not be used for real time recording. An AV application should start recording from the

first complete AU, to which only user data can be recorded. Note that this specification does not apply

to the Protected Area.

Figure 4-47: Definition of Allocation Unit (AU)

SDSC

(AU =<4MB)

SDHC

(AU =<4MB)

SDXC

(Large AU)

Class 2, 4, 6 (0<=r<1)

Default Speed Mode

High Speed Mode

Class 10 (r=0)

Class 2, 4, 6 (r=0)

UHS-I and UHS-II

Speed Grade 1 (10MB/sec, r=0)

Standard Capacity

Logo

AU1

AU2

AU3

AU4

.......

AUn

RU1

RU2

RU3

.......

RUm

User Area

Recordable Area

m =N

= S