Small Form Factor (SFF) embedded systems have been gaining popularity due to their small size, weight, and power (SWaP) advantages. The SFF architectures are typically purpose-built, providing just a few specific functions. But, the performance level is often limited and the versatility and re-usability across multiple applications is typically very low. Is a balance available where there is high performance and versatility in an open standard architecture, but a compact size that is optimized for SWaP-C (C stands for cost)?

Figure 1. MicroTCA is more much more compact than the Eurocard form factor and offers significantly better performance than legacy 3U and 6U bus-based systems.

Open Standards

There are many SFF solutions in the market, but many are not based on an open standard. Why is this significant?

The critical importance of open standards is often overlooked, but there are many benefits to a Modular Open Standard Architecture (MOSA) design. One of the key reasons is to reduce the risks posed by a single source technology and obsolescence. With several vendors in the industry supporting an open standard, the design engineer is not putting all of their eggs in one basket. The Mil/Aero community has learned the lessons of choosing a server blade approach from a Fortune 100 corporation and getting locked into a single vendor solution. It backfired when that corporation sold their business unit to the Chinese. Had the program gone to a MOSA solution, even if their prime supplier sold the business, they would still have had many other vendors to choose from and an open specification available. Additionally, with competition between the various MOSA hardware vendors, the constant drive to innovate, upgrade/improve, and reduce costs is prevalent.

Finally, the scalability of open standards is a key element. There is tremendous cost and time savings in upgrading on a scalable architecture versus a stand-alone, purpose built approach, which may not leverage much in forward compatibility.

What is SFF?

What size do we consider SFF? Comparing it to the traditional VME and CompactPCI form factors of Eurocard (3U and 6U) may be the best start. Those are the most common sizes for legacy open-standard architecture equipment in the embedded market. A 3U Eurocard board is 100 mm high (133.5 mm including subrack) by 160 mm deep, and a 6U is 233.4 mm (266.70 mm with subrack) × 160 mm. Nearly half that size is MicroTCA, a high-performance embedded architecture that offers significantly more performance than most SFF systems. MicroTCA is 75 mm by 180 mm in the single module size and 150 mm × 180 mm in the double module size. See Figure 1 for size comparison of MicroTCA to Eurocard.

Comparing Open Standard Architectures

Figure 2: Performance and feature comparison of OpenVPX, VITA 74 SFF, and MicroTCA.

AdvancedTCA is often used for Telco applications where massive throughput is required and size, power, and cooling limits are much more flexible. The modules are a large 8U (355.6 mm x 280 mm) size with a 30.48 mm backplane pitch. They typically have dual socket high-end processors that require more board space and cooling. The other area for AdvancedTCA is Mil/Aero where the processing power outweighs the architecture’s SWaP limitations.

MicroTCA is an excellent fit for a wide range of applications because of its high performance-to-size ratio and versatility. The COTS (Commercial Off The Shelf) architecture has benefits over other form factors due to its robust system management, e-keying, and highreliability features. In addition, the open-standard COTS architecture is typically more interoperable and lower cost than competing standards.

VPX is mostly a MOTS (Military Off The Shelf) architecture, serving one market. The architecture was billed as the “next VME”, although ironically it is not standardly backwards-compatible. It does, however, share the Eurocard form factor. Like MicroTCA, it can easily be ruggedized for hardened applications. Figure 2 takes a closer look at MicroTCA, VPX, and one of the open standard SFF form factors.

As shown in Figure 2, MicroTCA modules are close to half the size, weight, and cost of VPX. For the power, it obviously depends on the application. But, MicroTCA systems often have lower power consumption as well.

The SFF specification shown in the chart is roughly the same board height as MicroTCA, but about half the depth and a thinner pitch. But, there is a big tradeoff for the small size and architecture – lower computing performance. With a 20W limit for even the larger SFF conduction-cooled modules, that limits the processor selection to mostly single core, lower-performing Atom™ and G-Series ™ types of chipsets. For graphics modules, many of the upper-end chipsets will exceed 30W. Most higher-performance FPGAs such as Stratix-IV, Stratix-V, Virtex-6, Virtex-7 and digital converters are also above the threshold for the open standard SFF.

Now, purpose-built SFF systems are available that can use some 3rd Generation Core i7 processors, but you are losing the many advantages of an open standard architecture and its vast ecosystem.