SDXC has a maximum theoretical capacity of 2TB using the exFAT
file system and could eventually
reach speeds as high as 300 MB/s. However, those speeds will only
be realized with the Secure Digital Memory Standard 4.0 specification
due in the spring. The new spec will add an additional serial pin in
order to enable 300MB/s operation, but will still be backwards
compatible with SDHC, SD, and MMC cards.
The current SD 3.0 specification released in April will serve as a
transitionary standard with added support for Ultra-High-Speed 104.
UHS104 enables read and write speeds as high as 104MB/s, and is
compatible with current SD 2.0 specification SDHC readers. There will
be SDXC and SDHC cards using UHS104 during this transition. However,
UHS104 SDHC cards will still be limited to 32GB due to the use of the
FAT32 file system.
reader manufacturers are waiting for SD 4.0 to support SDXC,
although JMicron is a notable exception. However, computer OEMs are
eager to support SDXC and are already
working on designs complying with SD 3.0 and offering support for
Toshiba has disclosed to DailyTech that its UHS104 SDXC and
SDHC cards are already sampling to key OEMs. Its first SDXC offering
is a 64GB card (THNSU064GAA2BC) capable of reading data at 60MB/s and
writing data at 35MB/s. The company is also sampling 32GB and 16GB
SDHC cards with those speeds. Mass production of all three is
slated to start in the spring of next year.