Micro SD cards are most often used with mobile electronics, including smartphones, music players, and cameras. Their universal adaptation means that users no longer have to open the unit’s case to upgrade memory.
Micro SD cards are built of flash memory, which means that even when the devices they support are turned off, the stored information is retained.
Users can plug the tiny micro SD cards into recessed sockets that are usually placed unobtrusively alongside USB ports or power jacks. The physical size of a micro SD card is 11 mm by 15 mm and they are only 1 mm in height. The geometry of the device is such that, unless the user uses undue force, it is impossible to incorrectly insert the unit into its socket. And that’s about the only thing set in stone in this product class. They come with a variety of memory capacities, operate at different I/O speeds, and are governed by internal standards that require care in matching a device to a compatible memory unit.
Micro SD Card Standards
The SD Association (SDA) is a non-profit group composed of many of the leading manufacturers of micro SD cards. This group sets the standards that are followed by most of the producers of micro SD cards, as well as other types of devices. These standards released by the SDA are evolving, so it is important for both manufacturers and users to make sure that decisions are made based on the most up-to-date information.
Table 1: Micro SD card standards as set forth by the SD Association; last updated September 2015. (Source: Author Rendered)
Compatibility
SD cards will work on devices built to accept the two larger sizes, but SDHC ™ cards won’t work on SD devices, and SDXC ™ cards aren’t backwards-compatible either. While the micro SD cards aren’t backwards-compatible, the devices they support are. Specifically, devices built for SDXC ™ cards will support all three micro SD cards, and devices built for SDHC ™ will support SD cards.
Speed
The speed at which the combination of a micro SD card and its host device can operate is especially critical when performing video recording, as an actual live-stream of data must be recorded as it is happening in real-time. There are a daunting array of explosively evolving standards and capabilities now in force and soon coming online.
The newest and fastest designation is the UHS-II interface standard for SDHC and SDXC devices, with an upper limit of a blistering 312 MB per second. There is also UHS-I, which tops out at 104 MB/s. An important point to note is that micro SD cards designed for these higher speeds will operate with hosts not designed to take full advantage of them. Of course, in those cases they can only run as fast as the host device will allow.
The implementation of UHS-II has required a new pinout. Figure 1 details the new UHS-II Micro SD Card pinout on the left and the classical pinout on the right.
Figure 1: Physical appearances of micro SD cards. (Source: MYCE)
There is considerable controversy over the issues of speed and reliability. Many in the industry contend that the faster micro SD cards sacrifice too much reliability for the sake of increased speed, and this seems to be very much an open issue.
Types of Flash Memory
Micro SD cards can use a variety of flash memory types, but two common varieties are multi-level cell (MLC) and single-level cell (SLC). MLC is cheaper; however SLC—though more expensive—completes faster write operations. SLC also holds up better in environments where there is a heavy writing demand, and they are sturdier in general, even at greater temperature extremes.
In situations where the host device calls on the micro SD card to do more writing as opposed to mostly reading, SLC might be the better way to go. This technology is frequently being touted as the “industrial-grade” micro SD card technology.
SanDisk’s SDSDQAD-064G is available from Arrow Electronics. A look at the data sheet for this micro SD card reveals that all members of the SanDisk family of micro SD cards have on-board, built-in controllers. This frees the host system from involvement in the internal details of how the flash memory itself is written to, read, or erased. This arrangement ensures that—as micro SD cards evolve—there will be less need to modify the host’s software.
Members of this family of devices also have a sleep mode. This is an important feature for any mobile application, given the overriding need to conserve as much power as possible to allow the host device to continue operating as long as possible without a recharge. In the UHS-1 mode, members of the family draw 400 mA while reading and writing. In sleep mode, only 350 μA is required.
Because micro SD cards are likely to be operating in outdoor environments, it is important to know that these devices are operational over a temperature range of –25 °C to +85 °C.
Top 5 Considerations When Selecting Micro SD Cards for Your Next Application
1) Will the device be used for recording very fast live video streams?
2) How much memory capacity will be needed?
3) Will socketing for the new UHS-II interface be required?
4) Will your device be doing a lot of writing as well as reading?
5) Is speed or reliability a greater concern in your application?
What’s New in Micro SD Cards?
Micro SD cards are evolving at an extremely rapid pace. Both memory capacity and I/O transfer speeds are improving almost too quickly to tabulate. As of this writing, the highest widely available mainstream capacity is 128 GB, and the first tentative 200 GB micro SD cards are beginning to appear. Most mobile devices today can only access 128 GB at best. The big question is: will mobile devices be designed in the future to accommodate ever-larger memory capacities?