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What is SDHC Card?

SDHC card

SDHC card

SDHC Card (secure digital high capacity card) is a flash memory device that easily saves between 4 and 32 gigabytes (GB) of data. These cards work as a removable memory that can be used with many different digital devices, including camcorders and computers. SDHC cards were designed to be an improvement on the original Secure Digital (SD) memory card developed in 1999. The new card helped satisfy the growing demand for high-capacity storage caused by the increasing use of high-definition video and high-resolution digital photography.


SDSC card only available in 2 GB capacity. In standard-capacity cards (SDSC), 12 bits identify the number of memory clusters (ranging from 1 to 4,096) and 3 bits identify the number of blocks per cluster (which decode to 4, 8, 16, 32, 64, 128, 256, or 512 blocks per cluster). The host device multiplies these figures with the number of bytes per block to determine the card's capacity in bytes.

In SD version 1.00, the number of bytes per block was assumed to be 512. This permitted SDSC cards up to 4,096 × 512 × 512 = 1 GB, for which there are no known incompatibilities.

Version 1.01 let an SDSC card use a 4-bit field to indicate 1,024 or 2,048 bytes per block instead. Doing so enabled cards with 2 GB and 4 GB capacity. Early SDSC host devices that assume 512-byte blocks therefore will not fully support the insertion of 2 GB or 4 GB cards.


SDXC format supports cards up to 2 TB (2048 GB), compared to a limit of 32 GB for SDHC cards in the SD 2.0 specification. SDXC host devices accept all previous families of SD memory cards. Conversely, SDHC host devices will accept SDXC cards that follow Version 3.0.

On March 6, 2009, Pretec introduced the first SDXC card a 32 GB card with a read/write speed of 400 Mbit/s. But it was not until early 2010 that compatible host devices came onto the market, including Sony's Handycam HDR-CX55V camcorder, Canon's EOS Rebel T2i Digital SLR camera, a USB card reader from Panasonic, and an integrated SDXC card reader from JMicron. In September 2011, SanDisk released a 64 GB microSDXC card. Kingmax released a comparable product in 2011.


A SDIO (Secure Digital Input Output) card is an extension of the SD specification to cover I/O functions. Host devices that support SDIO (typically PDAs like the Palm Treo, but occasionally laptops or mobile phones) can use the SD slot to support GPS receivers, modems, barcode readers, FM radio tuners, TV tuners, RFID readers, digital cameras, and interfaces to Wi-Fi, Bluetooth, Ethernet, and IrDA. Many other SDIO devices have been proposed, but it is now more common for I/O devices to connect using the USB interface. SDIO cards support most of the memory commands of SD cards.

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