Secure Digital

Secure Digital was first introduced in 1999, evolving from MultiMedia Cards (MMC), primarily as a consumer-based storage medium for the photography market. This technology has been leveraged into the industrial space due to the advantages of size, low power consumption, simplicity of integration and availability. Governed by the SD Association (www.sdcard.org), the SD platform has continued to expand as capacities and bus speeds increased, while smaller form factors were developed.

The two most prevalent industrial sd card formats are the full size SD card and the microSD card, however, understanding the different capacities, bus speeds, speed classes and associated compatibility with host devices is critical to successfully implementing the SD interface.

Industrial SD cards and microSD cards are marked as SD, SDHC or SDXC – which is a capacity-related classification – as detailed in the table below:

			SD Standard	SDHC Standard	SDXC Standard
Capacity			up to 2GB	more than 2GB	more than 32GB
				up to 32GB	up to 2TB
File System			FAT 16	FAT 32	exFAT
SD Memory Cards / Host Marking
Card form factor	Full Size SD		32 x 24 x 2.1 mm, Approximately 2g
	microSD		11 x 15 x 1.0 mm, Approximately 0.5g
Bus Speed			Normal Speed (NS)	NS, HS	NS, HS
			High Speed (HS)	UHS-I	UHS-I
Speed Classes	NS, HS mode		Speed Class (optional)	Speed Class (Mandatory)	Speed Class (Mandatory)
	UHS-I mode		----	Speed Class (Optional) UHS Speed Class (Optional)	Speed Class (Optional) UHS Speed Class (Optional)

Industrial SD Cards are only compatible with hosts that are rated for the same capacity grade or higher, for example, SDHC cards are only compatible with SDHC or SDXC hosts, not SD hosts, as illustrated in the graphic below.

The second classification of cards is related to the bus interface, designated as Normal Speed, High Speed, UHS-I (Ultra High Speed) or UHS-II, which are differentiated by the Bus Speed. Bus Speed indicates the maximum theoretical read & write speed for a card, however the actual performance is affected by many other factors, such as the card configuration, operation being performed, organization of data on the card, etc. The UHS Bus Interfaces are only applicable to SDHC and SDXC capacity grades, and while UHS memory cards can be used in non-UHS hosts, the performance will always be best in a UHS host.

Bus Interface	Card Type	Bus Mark	Bus Speed	Spec Version
Normal Speed	SD, SDHC and SDXC	---	12.5MB/s	1.01
High Speed	SD, SDHC and SDXC	---	25MB/s	2
UHS-I	SDHC and SDXC		50MB/s (SDR50, DDR50) 104MB/s (SDR104)	3.01
UHS-II	SDHC and SDXC		156MB/s 312MB/s	4

Speed class ratings indicate the minimum sustained write speed of a given card, originally developed for video recording. Normal Speed bus interfaces utilize a Class 2, 4, or 6 system to indicate that the card will record a minimum of 2 MB/s, 4 MB/s or 6 MB/s, respectively. High Speed bus interface supports Class 10 speed class, indicating 10 MB/s minimum write speed.

UHS speed classes are a completely different set of write performance ratings for UHS devices when used in UHS hosts. UHS Speed Class 1 (U1) indicates a minimum write speed of 10 MB/s, whereas UHS Speed Class 3 (U3) indicates a 30 MB/s minimum write speed, specifically targeted for 4K video recording.

Delkin offers full size SD and microSD cards in a range of capacities, bus speeds and speed class ratings, as well as multiple flash configurations, to allow selection of the optimum card for the host and usage model.