• Home
• About SD Association
  • About SD Memory Card
  • About SDHC
  • About miniSD
  • About microSD
  • About SD Speed Class
  • About SDIO
  • About Host Controller
  • About SD Application Specifications
  • Downloads
  • Glossary
  • FAQ
• Press Room
• Member Companies
• Membership Application
• Licensing Documents
• Members Site
• HALA Site
• LAMS Site
• SD World Site
• China Product Site
• Japan Product Site

About SD Speed Class

The SD Card Association (SDA) has established a new specification rating the minimum data transfer both SD/SDHC Memory Cards and SD/SDHC host products.

Background

Previously, when writing onto an SD Memory Card, the host product could not determine the storage conditions inside the card. Due to the nature of flash memory, speed and performance depends on card fragmentation. This means that memory cards are not always able to provide the expected performance required by the host product.

Concept

With the new Speed Class specifications, the host product can check the fragmented state in the card and calculate the write speed in each part of the card. The host can then determine where to write the data according to its speed requirement.

* What is fragmented state?
The memory area of a card is divided into minimum memory units. The host product writes data onto those memory units where no data is occupied. The fragmented state reflects how these memory units are occupied. The write speed depends on this state.

The SDA has specified operating conditions for SDHC Memory Cards that guarantee the minimum speed according to the card's speed class. Class logos are used to identify cards that comply with these specifications.

The Class logos must appear near the card's logo on all SDHC Memory Cards (above 2GB).
SD Speed Class logo is optional for SD Memory Cards (2GB and less).

SD Speed Class The following speed classes have been established: SD Speed Class in Actual Use Class 2 Card: A speed of 2 MB/s or higher is guaranteed at the best fragmented state where no memory unit is occupied. Class 4 Card: A speed of 4 MB/s or higher is guaranteed at the best fragmented state where no memory unit is occupied. Class 6 Card: A speed of 6 MB/s or higher is guaranteed at the best fragmented state where no memory unit is occupied. *This capability varies depending on the product manufacturer and the model.

If the host product requires or recommends Class 4 data transfer speed or higher-
	The host product can achieve the best performance with these cards.