TECHNOLOGY IN CONTEXT
Stackable vs. COMs: What's the Best Choice?
Thinking (about What Goes) Inside the Box
Selecting a form factor—be it COM, SBC or stackable—involves a host of considerations. These include size, power requirements, thermal management, connector strategies, upgradability, projected volumes and, of course, cost.
MARTIN MAYER, ADVANCED DIGITAL LOGIC
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Embedded form factors have continued to evolve and become more powerful as new bus interfaces and circuit technologies mature and graduate from mobile to embedded product sectors. Realized improvements in computational power and available I/O resources open new and niche opportunities and expand the embedded market. Any new project must consider how it will realize CPU and I/O requirements, be it in the form of a single board computer (SBC), computer on module (COM), or a full custom design.
Projects with limited engineering time or short initial development times usually benefit from choosing an SBC, while resource-rich parallel development efforts are often required to rapidly leverage COM deployments. Given ample resources, it is not uncommon to see proof of concept and prototype stages implemented using an SBC solution while final market volumes are fueled by COM designs. Other aspects of the project may restrict or limit platform options, and newly available computational platforms may enable projects that were otherwise infeasible.
Computer or Component
Single board computers offer complementary and competitive functionality to fully realized computer-on-module offerings. The chief distinction between these families is that a COM requires a physical host platform, critical additional circuitry and thermal management prior to being able to apply power and bring a system out of reset. Single board computer offerings require only wire-to-board or familiar consumer interconnect before being able to achieve an initial operational state.
SBC solutions require an installation envelope that is large enough to allow for a wiring harness interconnect, while COM options are affected by the carrier board envelope. The design of a custom carrier board is a significant expense, but it can greatly simplify volume manufacture through a reduction in the number of interconnects required. The 3.5” SBC in Figure 1 is sized physically, between Standard and Extended COM Express offerings. Size comparisons of some of the more popular form factors are given in Table 1.
3.5” US15W PC – Connector Ready “Out of the Box.”
Size Matters – Footprint and Board Area Comparison.
Many of the interconnects and some of the circuitry provided on a 3.5” PC must be provided by a custom carrier board for a COM module to graduate from component to computer. The model pictured is designed for automation use and features power-fault ride-through protection. The latest models of 3.5” form factor boards feature PCIe lanes via a riser connector and a flex-print peripheral interconnect for “foldable” stacking options.
It’s about Connections
Traditional SBC applications leverage the benefit of making substantial I/O available to the end user. Cards provide a bus-based “stack” on ATX boards for customization through a range of off-the-shelf or custom produced mission-critical peripherals. Consumer-friendly connectors are commonplace on the ATX platform as well as on the 3.5” platform in Figure 1. When less than the full smorgasbord of flexibility is needed, a cost savings can be obtained by building-out only the necessary I/O interfaces required by an application. A COM solution improves on a depopulation approach by also optimizing fiberglass usage.
3.5” US15W PC – Connector Ready “Out of the Box.”
Single board computers shrink installed volumes of less than a liter while providing full bandwidth expandability alongside consumer-friendly I/O. Integrated protection circuitry, USB, RS-232, video, Ethernet and other user-ready I/O ports provided via board-to-wire interconnect, allow for external connector flexibility. Changing the elements of a cable harness enables Ingress Protection (IP) level 56 and beyond, when required. COM solutions in an application that must exceed IP 30 may no longer benefit from consumer-friendly direct-to-board multi-connectors and the associated printed wire/cable-less system design. In such cases, a designer might implement board-to-wire interconnect on select interfaces, bestowing external connector flexibility to COM-based efforts.
Carrier boards in the COM environment are required to provide all protection and signal conditioning circuitry that a deployed interface requires, and this cost cannot be ignored when considering a COM module. Circuit protection devices are a serviceable element, adding another line item to the total cost tally. In many cases, spare interface channels may be included in the layout, and left unpopulated to keep bill-of-materials costs on target. COM solutions can further benefit by limited protection on interfaces that are not exposed to external connection hazards.
Mechanics Cool COM
Thermal management in the COM ecosystem is accomplished on an application-specific basis. A standard installed height of 18 mm (with a 21 mm option), and a uniform heatspreader with a minimum thickness of 3 mm, provides a consistent physical thermal interface for a given size COM module. Attachment to the spreader is achieved via M2.5-compatible corner holes, which may be threaded or straight through. This arrangement makes the thermal solution size and mounting dependent on the form factor of the COM module. The desired heatspreader—through-hole or threaded, also determines the type of standoff on the carrier board, which is the opposite of the type of standoff in the spreader:
• M2.5 Threaded Spreader → 2.7 mm bore hole 5 mm or 8 mm standoff
• Bore Hole Spreader → M2.5 threaded board standoff, 5 mm or 8 mm
The carrier board interconnect for the COM express ecosystem can be of either 5 mm or 8 mm stacking height, the choice allowing for an additional 3 mm of component thickness for devices that can survive the under-the-module environment. In some designs, complete cutouts in the fiberglass, or intentional placement voids, exist below a module due to the local thermal environment generated by a high-power COM module. Sacrifice of under-module board area forces carrier board footprints to exceed the base footprint of the COM.
The nature of the PC/104 ecosystem has been to assimilate new bus technology in a modular leap-frog manner and preserve simple mechanical mounting of both board and heat spreaders across variants. Designers leverage appropriate buses at the top level and bridge to older bus variants as needed for legacy compatibility. This pushes the PC/104 SBC toward the domain of COM through conductive cooling options and the ability to act as a module atop a multi-board stack, and leveraging the strength of user-ready I/O interfaces. Mechanically, 0.600” (15.24 mm) inter-board spacing offers a peripheral board, below a PCIe/104 CPU card, a component height of 8.76 mm, which exceeds the entire 8 mm under-spreader height of the COM specification as well as the total under-module height when an 8 mm board interconnect is chosen.
Think Out of the Box
The latest offerings in the PCIe/104 family offer reasonable amounts of I/O at user-ready voltage levels with integral onboard protection. No high-speed layout skills are needed to leverage a USB 2.0 or Gigabit Ethernet port with a PCIe/104 SBC system. Connecting a cable is the required skill for integration. Keyed, latching connectors enhance reliability, safety and delivered quality of board-to-wire interconnects.
Use of FCI Minitek Shrouded Vertical Headers and Active Latch Housing crimp-and-poke, discrete wire interconnection helps to satisfy many rugged environmental conditions and allows for use of specialized insulation for enhanced environmental performance (Figure 2). Low-profile, standard 2 mm grid interconnects provide space for a bend radius compatible with 0.600” (15.34 mm) installations featuring conductive cooling by mechanical attachment of the PC/104 SBC to a flat interior chassis wall. Optional 1.000” (25.4 mm) conductive cooling adapters and standoffs can be utilized when board-to-wire interconnect service is needed as part of a product service plan. Flex-print interconnects can also be utilized, outfitted with standard 2 mm headers that will mate easily without interference from header shroud.
FCI Minitek Interconnection on QM67 PCIe/104 CPU.
Recent adoption of the PCIe Type 2 standard has improved the delivered bandwidth of the PCIe 156 pin connector. The first 52 pins of the Type 2 standard are identical to the original (now Type 1) standard, supporting four x1 PCI Express lanes and two USB 2.0 channels. The latest QM67 chipset from Intel supports PCI Express 2.0 signaling at 5 GHz. The QMS/QFS-based connectors for the PCIe interconnect, shown at the top of Figure 3, have been demonstrated to be “eye-open” at 5 GHz signaling, for three additional boards beyond the CPU. PCI Express 2.5 GHz signaling extends to a depth of 5 boards.
High Speed Business – Samtec QFS (top) and Tyco FH pair (bottom).
LVDS propagation through multiple connectors and embedded high-speed lane-switching traces is not a consideration for many COM deployments, but it is key to modular stacking expansion of the extended PCIe/104 form factor. The cost of modular board-to-board interconnect is significant, due in no small part to the impressive -55° to +125°C environmental rating of the Samtec QMS/QFS-based interconnect, and must be considered for any product that requires PCIe bandwidth for the support of a custom, mission-critical peripheral.
COM Express deployments are centered about one or two 220-position SMT board-to-board interconnects from the Tyco/AMP Free Height series shown at the bottom of Figure 3. These are rated from -40° to +85°C, which includes up to 30°C of contact heating at 0.5 mA. Leveraging the documented average 15°C reduction in contact heating by keeping the per-contact current below 0.3A can deliver up to 108 watts on the 30 power pins of a 2-connector solution. This reduced current rating supports +85°C ambient operation within an envelope that enables COM designs to feed Quad Core processors with ample power.
High Speed Business – Samtec QFS (top) and Tyco FH pair (bottom).
By comparison, the published ratings of the Samtec QMS/QFS interconnects of 1.6A per contact and 9.2A per power plane at 95°C offer greater per-contact reliability. The PCIe/104 standard applies a 20% de-rating factor at +85°C and allocates 46 interconnects to ground for a 2.9:1 ratio of GND to Power current. 84 watts at 5V and 100.8 watts at 12V of power may be conveyed by the PCIe connector, making it a viable option for supplying standard operating voltages to a stack-based PCIe/104 system.
Some COM vendors specify variable input voltage ratings for their modules. However, COM units are not power-regulators, so any other voltage level required by the carrier board electronics will need to be supplied.
The PCIe/104 standard specifies both 5V and 12V planes, with coincident tracking being acceptable, with ratio-metric tracking and “early 12V” being viable options as well. In situations were the 12V rail is not utilized by PCIe peripherals, the 12V rail may vary from 4 to 18 VDC without ill effects on the CPU, as only the high current CPU power supply sources power from the 12V bus. All other functions on current PCIe CPU cards require 5 VDC +/-5% and most also support a single 5V suspend plane.
Feeding a modern CPU in the PCIe/104 world is accomplished by a 10-wire single harness for all three power planes (+5V, +12V and +5V suspend) if power is not supplied via the PCIe 15-pin connector. In the case of a COM system, adequate power feed capacity must be designed into the carrier board. The question of using brick-style power modules or external power supplies is valid for both the PCIe/104 and COM, where COM can entertain the deployment of onboard component-level DC/DC converters. Applications in the highest volumes may desire to support this to extract additional margin from system production.
Enhancement of a system is possible in stack-based SBC architectures by trading vertical space for increased functionality. COM architectures will require replacement of the carrier board in order to change the peripheral mix of a system, unless an over-population plus security lock-out approach is adopted. Alternatively, an expansion bus could be provided by the carrier board, where one of the PC/104 variants may be a viable solution. Barring expandability, embedded solutions that leverage a COM standard will require ongoing board design and refinement efforts throughout a product’s life-cycle and subsequent evolution.
Accurate forecasting may provide additional insight, as it has been observed that projects with estimated annual usages approaching 1500 units tend to gravitate toward the economic benefits of optimization and controlled build-out offered by COM solutions. Opportunities with EAU volumes below this threshold appreciate the robust interfaces supported by PC/104 SBC solutions. It has also been noted that the current tight economic climate has shifted this threshold slightly higher (see the “Platform Quiz” p. 28).
Ultimately, it is the actual performance of the product in the market that has the largest impact on design improvements. Successful SBC deployments may see volumes where production synergies are maximized and the per unit price can be improved no further. Such a problem is certainly good to have, as one can then leverage a COM and baseboard implementation of a proven SBC performer as volumes continue to increase. Through this same path, when annual volumes begin to approach the next engineering decade, full-custom, component-level solutions begin to make good economic sense.
Advanced Digital Logic
San Diego, CA.