Instrumentation • Electronics
4 Victoria James provides a series of guidelines to follow when choosing portable memory devices for use in embedded systems
4 Victoria James fournit une série de directives à suivre lors de la sélection de dispositifs de mémoire portables pour l’utilisation dans les systèmes intégrés.
4 Victoria James mit einer Reihe von Richtlinien zur Auswahl von tragbaren Speichermedien für eingebettete Systeme.
Choose portable memory for embedded systems
W
Fig. 1. A data key from Nexus GB in use in fuel management application on behalf of SITA Waste.
hile most embedded system components are sourced from OEM focused manufacturers, portable memory devices are a grey area. The high-
volume consumer electronics market is estimated at £129 billion annually. This creates significant demand for USB flash drives, SD cards and other consumer-focused portable memory devices and for embedded system designers the economies of scale these devices offer can be appealing. Manufacturers of consumer memory devices are
primarily focused on the needs of the fast-changing consumer electronics market and this does not suit the design engineer. In response, a lesser-known category of OEM focused portable memory systems is providing alternatives that meet requirements unique to non-consumer embedded designs. These two categories of portable memory
devices offer unique advantages and disadvantages. The most important thing for engineers though, is how the two categories perform when integrated into embedded designs in commercial, industrial, government, military, medical and other demanding OEM environments. Typical applications for portable memory in
embedded systems include access control and rights management, usage monitoring, data logging, in-field firmware updates and product authentication. The environmental factors and
typical usage of the system influence which kind of portable memory solution is the best fit for the application.
Consumer memory
As a result, the potential design considerations for portable memory systems are extensive, ranging from the communications interface/protocol, connector durability, cost and data throughput speed to product life cycle and ruggedness. Some of these design considerations may be
mutually exclusive. For example, small size typically precludes rugged construction. Prioritising these design considerations makes it easier to determine which features are most important for a particular application. In addition to SD cards and USB flash drives,
other consumer memory devices used in OEM designs may include SDHC, microSD and CompactFlash cards. While these NAND-flash- based devices are primarily designed for PCs laptops, digital cameras, mobile phones and MP3 players, the principal benefits that are intended for consumer electronics can also apply to non- consumer embedded devices: The widespread availability of USB flash
drives and SD cards makes consumer memory a convenient option. Most users are already familiar with their operation, and it is easy for OEMs and end users to obtain these devices from electronics retailers.
Another issue to consider is the need for high
memory capacity. Transferring data to and from embedded systems often only requires kilobits or megabits of memory capacity, but the higher capacities available in consumer memory devices may be required in some cases. Video logging, for example, can require gigabytes of memory capacity. However, some OEM memory devices are now offered in memory capacities of up to 32GB, so finding devices with high memory capacities is no longer limited to consumer memory. In terms of costs, the upfront cost-per-bit of most consumer memory devices is relatively low. But if an OEM application requires thousands of portable memory devices over the life of the design, then the expenditure over the lifetime of the application is considerable. Handheld embedded designs often require a
small portable memory device to fit in the system. In these applications, a microSD card may be the most attractive option due to its small size. Equally, some designs may actually require a larger portable
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