Choose portable memory for embedded systems

Paul Boughton

Victoria James provides a series of guidelines to follow when choosing portable memory devices for use in embedded systems.
While 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.  [Page Break]
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.[Page Break]
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 memory device. For example, a memory token must be able to be easily inserted and removed by a user wearing arctic gloves in certain military applications.
As you can see, while consumer memory products are not explicitly designed for OEM designs, they do offer unique benefits. However, these benefits may also have unintended consequences.
With hundreds of different models of consumer memory products available, OEMs can’t possibly test, approve and support every device that physically fits. Despite memory manufacturers’ attempts to ensure compatibility, some models will work in an OEM device and others won’t. A lack of control over which devices will work can increase support costs and can be inconvenient for end users.[Page Break]
One must also bear in mind that technology standards change. The transition from SD to SDHC is a recent example. A system designed to use SD cards prior to the release of SDHC will not work with SDHC cards unless the system’s firmware is updated. Likewise, the fast-changing consumer electronics market virtually guarantees that a new technology will emerge in the future, bringing with it new compatibility issues.
Finally, using a consumer memory device may increase the risk of data theft. A misplaced or stolen USB flash drive is not protected from an unauthorised user accessing contained data from a PC, for example. Similarly, using a USB receptacle in an embedded design allows any USB flash drive to plug in, which can increase the risk that information could be pulled off the system or that a virus or other piece of malware could be transferred to the embedded device.
Another drawback is that most consumer memory devices today start at 1GB or higher. As such, embedded systems that require only kilobits or megabits of memory to perform a desired task are essentially paying for capacity they don’t need.
While not always apparent in the early stages of the design process, these considerations should be examined before deciding whether consumer memory meets both the design criteria and the expected use.[Page Break]
Specialist OEM focussed memory
Non-consumer, OEM systems are usually manufactured and designed to last years. As a result, portable memory used in these systems must provide long-term availability and reliability. The environmental conditions in which OEM designs operate can be demanding as well, and there is often an increased emphasis on secure access. Requirements that are unique to OEM designs illustrate why consumer memory devices’ light-duty construction, low cycle life connectors, lack of security features and short product life cycles can be detrimental to non-consumer applications.

Rugged construction is another characteristic required by OEM applications, especially for use in harsh environments. Some outdoor applications such as the ones used in the military, construction or agriculture sectors, use portable memory devices to transfer operation and maintenance data between the base and vehicles in the field. The memory device may be exposed to vibration, dirt, moisture, shock, extreme temperature and rough use. Light-duty consumer memory devices and receptacles do not provide sufficient protection or the environmental ratings needed for harsh environments.
Frequent use and long-lasting designs require durable connector systems. Vending machines, for example, may use portable memory to provide cashless vending in non-networked environments. A customer can use the memory device to purchase merchandise from the machine, such as snacks, drinks or even industrial supplies. These machines often see 50-60 cashless transactions per day. Rated at just 1,500 cycles, a USB mating receptacle could wear out in as little as 25-30 days.[Page Break]
Furthermore, many OEM applications require increased security. The widespread availability of devices and systems with USB and SD card interfaces leaves sensitive data vulnerable to data theft and malicious viruses. Accidental data loss and virus uploads have been under the microscope in the Government and commercial sector for some years. In response, some Governments and corporations have banned the use of consumer memory devices to decrease security risks.
Long-term availability for the life of the system is another key issue to be factored in when choosing memory devices. Consumer memory is driven by the consumer electronics market and therefore subject to short product life cycles and frequent obsolescence. Traffic light controllers are an example of long-lasting OEM designs that need a portable memory solution offering long-term availability.
In these applications, technicians use portable memory to upload setup parameters to the controllers. These important pieces of transportation infrastructure are fielded for a decade or more, so a consumer-based memory device would likely become obsolete over the life of the controllers.
The inability to meet demands for rugged construction, a high mating cycle life, secure form factors or long-term availability best illustrate why consumer memory devices should not be used in embedded designs.
As a result, OEM portable memory was developed in response to the unmet needs of many embedded systems. These devices provide the rugged construction, high mating cycle life, secure form factors and long-term availability that average commercial memory products do not deliver.
Some manufacturers of portable memory produce ‘industrial’ versions of consumer memory products, such as industrial CompactFlash cards or industrial SD cards. These so-called ‘industrial’ devices may offer an extended temperature range, use slightly more robust materials and support a higher number of write cycles. However, they still share the drawbacks associated with consumer memory devices. They are non-robust, they use low-cycle-life connectors and non-secure form factors; they come hand in hand with potential obsolescence issues and they lack harsh environment ratings.[Page Break]
What is OEM portable memory?
For the purpose of this guide, true OEM portable memory devices are defined as purpose-built systems that provide unique form factors for controlled availability, and baseline physical security. OEM portable memory is most accurately described as a memory system as opposed to a singular device.
Another key requirement for a product to truly be considered to be specialised OEM memory is a unique form factor. This prevents most unauthorised data transfer, helps protect the host system from malicious files and provides a new potential revenue stream when an OEM becomes the exclusive source of new and replacement keys or tokens.
Truly rugged portable memory can withstand extreme temperatures, rough use, sterilisation, chemical exposure, static electricity, dust, dirt, moisture, shock and vibration. To meet these harsh environment requirements, some manufacturers use a solid over-moulding process that encases electronic components in specially engineered composite plastic materials.
However, the environmental capabilities of a portable memory device do little good unless the associated receptacle is also protected. Receptacles in OEM memory systems should also carry ratings consistent with the harsh environments where they will operate. These environmental ratings may include IP ratings, such as IP65 or IP67, or military environmental ratings such as MIL-STD-810. These ratings allow OEM memory systems to operate in harsh environments where consumer memory devices would not survive.
OEM memory systems offer a higher number of mating cycles between the memory device and its mating receptacle: ranging from 50,000 cycles to 200,000 cycles for some OEM receptacles. For example, an OEM memory receptacle rated at 50,000 insertion cycles would last a minimum of thirteen-and-a-half years if used 10 times per day. To put that into perspective, a typical USB connector (rated at 1,500 insertion cycles) that sees the same 10 insertions per day will reach its rated cycle life in just five months.
Many OEM memory products have been available unchanged for over 20 years. Lower capacity devices that use EEPROM or NOR flash non-volatile memory technology have a proven track record of long-term availability and are available from multiple manufacturers. This longevity significantly reduces the likelihood that OEMs will have to re-qualify new memory devices when the old one becomes obsolete.
While OEM memory systems typically do a better job of meeting the unique requirements of OEM applications, there are some design considerations that should be weighed against the benefits offered by consumer memory devices.[Page Break]
For instance, is OEM memory affected by the consumer memory market? Unlike low-capacity OEM memory, high-capacity OEM memory typically utilises NAN D flash technology. As a result, the long-term availability of high-capacity devices is reliant on the consumer market. However, OEM memory systems based on NAN D flash technology typically offer better long-term availability than consumer products by using qualified and consistent sources, featuring configuration control and providing built-to-last designs.
Many OEM memory systems are geared towards the low-capacity applications that are more prevalent in non-consumer embedded systems. However, as previously stated, some newer OEM memory systems offer higher capacities that match those of consumer memory products. Most important to engineers is that the memory solution specified be offered in a range of memory capacities that best meet the specific memory requirements of the system.
Deciding between consumer and OEM is the likely first step in choosing a portable memory solution for an embedded design. In many cases, both will work. However, for successful OEM designs, engineers need to look beyond ‘will it work?’ and consider the long-term implications and the environment in which they will function. [Page Break]
While consumer-focused components are less prevalent in non-consumer OEM designs, it is hard to argue with the convenience, high memory capacity, small size and low cost that USB flash drives, SD cards and other consumer devices can provide. Nonetheless, the short life of consumer memory products is inescapably tied to the fast-paced consumer electronics market.
Manufacturers of consumer memory products are not anti-OEM; they are just meeting the demands of a consumer market that simply does not care about or have a need for performance requirements that are unique to non-consumer, OEM applications. When rugged construction, a high mating cycle life, secure form factors and long-term availability are higher priorities, a purpose-built OEM memory system is likely a better fit.
Victoria James is with Nexus GB, Midhurst, West Sussex, UK.
Fig. 1. A data key from Nexus GB in use in fuel management application on behalf of SITA Waste.
Fig. 2. Nexus GB has supplied data keys to Bombardier as part of a project to help monitor and manage location of trains on the track.

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