RTC Magazine

Born in the early ’90s with the advent of PC/104, the “stackable single board computer” market rapidly evolved into a thriving global ecosystem. Two decades later, PC/104—joined by a handful of stackable descendents such as PC/104-Plus, PCI-104, PCI/104-Express and SUMIT-ISM—remains in widespread use. Despite the many successes of stackable-SBCs, however, the cost and functionality constraints of high-volume products have tended to dictate full-custom designs, complete with high development costs and risks.

In an effort to move off-the-shelf boards into high-volume opportunities, JUMPtec (now part of Kontron) in 2000 introduced a new concept, dubbed ETX. The company’s ETX boards integrated a full set of embedded-PC functions within a compact “computer-on-module” (COM), which plugged into a custom application baseboard through a pair of high-density connectors. 

This effectively transferred the burden of I/O and power connectors—plus additional, application-specific functionality—to the custom baseboard. The COM approach caught on, with ETX soon joined by ETX Express (aka COM Express), XTX, Qseven and several other COM standards. The resulting COM market’s annual revenues are projected to surpass those of the stackable-SBC market by the end of this year, according to a recent report from Electronic Trend Publications.

Interestingly, despite the success of COMs, it took a full decade for the other shoe to drop—namely, “I/O on modules.” Although PC/104-style stackable I/O modules had evolved to encompass new buses and I/O technologies, the COM market’s emergence dictated the need for a smaller, lower-profile, mezzanine-style I/O expansion format more suitable to the needs COM-based systems. It was this unfulfilled need that led to the development of the proposed FeaturePak embedded I/O modules standard.

Introducing the FeaturePak Standard

In March 2010, a group of eight companies debuted an innovative new I/O-on-module concept at the Embedded World 2010 trade show. The FeaturePak standard defines a highly compact, low-profile and inexpensive I/O-on-module format measuring 1.70 x 2.55 inches (43 x 65 mm)—about three-fifths the size of a credit card (Figure 1). 

FeaturePak modules can be used for providing snap-in options or upgrades to both SBCs and COM baseboards. They can also provide building blocks for simplifying the development of custom embedded electronics. The FeaturePak form-factor might also catch on as a consistent format for silicon vendors’ I/O controller evaluation boards and reference designs, instead of today’s hodgepodge of I/O controller evaluation boards.

The mezzanine-style FeaturePak standard is highly synergistic with existing and emerging bus, I/O, chip and board-level technologies. It leverages the latest high-speed serial expansion standards—such as PCI Express and USB—and is compatible with a wide range of current and future processors, including both x86 and RISC architectures. A summary of features and benefits is shown in Table 1.

How FeaturePak Modules Can Be Used

With their compact size and standardized connector, FeaturePak modules are easy-to-use macrocomponents that target a wide range of embedded applications. Embedded computer design can be greatly simplified by treating complex I/O subsystems as components, just as various types of COMs allow designers to treat the core embedded computing functions as a plug-in building block. This macrocomponent approach greatly accelerates design cycles, and also enables the creation of reconfigurable and upgradable products.

A few places where FeaturePak expansion would be beneficial include:

• Single board computer form factors such as EBX, EPIC, 3.5-inch, Mini-ITX, Nano-ITX, etc. 
• Application baseboards for COM Express, Qseven and other computer-on-module formats
• Stackable expansion modules such as the PCI/104-Express and SUMIT-ISM 
• Backplane expansion boards such as standard PCI Express cards 
• Industrial slot-boards such as PICMG’s CompactPCI Express 
• Silicon vendor reference designs and evaluation boards
• Set-top boxes, intelligent transportation systems, medical devices, test equipment, etc.

Figures 2 through 5 suggest a few anticipated FeaturePak applications. The red lines indicate the host interface connections.

FeaturePak Connector Signals

All of the FeaturePak module’s bus and I/O signals appear on a single, high-density, 230-pin card-edge connector. The module inserts into the connector at an angle and then swings down to lie parallel to the host PCB, after which it’s secured with two screws.

The use of a single low-cost connector plus gold fingers on the FeaturePak module results in the lowest possible cost for a mezzanine board interconnect solution. The socket’s MXM connector is rated for 2.5 Gbit/s operation, fast enough for PCI Express and various high-speed I/O interfaces, including USB 2.0 and Gigabit Ethernet. 

In comparison with the venerable stackable PC/104 module and socket, the FeaturePak module is about 1/3rd the size of a PC/104 module, and its socket provides 220% of the signal density in 85% of the board space. Additionally, the FeaturePak socket is lower profile, at 0.31 inch (7.8 mm) vs. 0.43 inch (11 mm) for PC/104’s.

All connections—including power, host interface and external I/O—are carried by the single low-cost, high-density, 230-pin MXM connector, originally designed for use with notebook computer graphics modules. 

Incidentally, the FeaturePak standard utilizes the 230-pin MXM connector differently from the way it is used by the MXM graphics standard, as well as from other standards that employ MXM connectors, such as Qseven COMs.

Height Considerations

Although MXM connectors are available with several board-to-board heights, FeaturePak sockets use the MXM connector options that provide 0.2 inch (5.0 mm) spacing between the bottom of the FeaturePak module and the top surface of the baseboard. Allowed component thicknesses on the top and bottom of FeaturePak modules vary according to two types of modules—“Standard” and “Tall”—as well as by regions on the top and bottom surfaces. 

The maximum topside component thickness on a Standard FeaturePak module is 0.19 inch (4.8 mm), while that on a Tall module is 0.4 inch (10 mm). The Standard height module in a FeaturePak socket on a baseboard can have a PC/104 or similar stacking board with 0.6 inch board-to-board spacing installed above it. Many FeaturePak applications will not require this restriction.

The bottom surface of either type FeaturePak module supports maximum component heights of 2 mm or 3 mm, depending on location. Details are provided in the FeaturePak Specification and Design Guide.

FeaturePak Signal Groups

All of the FeaturePak module’s host and external I/O interface and power connections are carried by a single low-cost, high-density, 230-pin connector. These signals consist of the following: 

• Host interface – includes 2 PCI Express x1 links, 2 USB (1.1 or 2.0) channels, 1 serial port (TX, RX, RTS, CTS), SMBus, reset, JTAG, auxiliary signals, a slot ID, +3.3V, +12V and ground. 
• Primary I/O signal group – includes 50 I/O signals, +5V and ground. Within this group, 34 signal pairs are implemented with enhanced isolation for use in applications such as high-precision analog, Ethernet and optically isolated I/O. 
• Secondary I/O signal group – includes 50 general-purpose I/O signals, +5V and ground. 

The FeaturePak connector interface signals are illustrated in Figure 6.

Two frequently asked questions regarding the FeaturePak connector’s signal assignment are: How are the two groups of 50 I/O signals intended to be used? And why are power and ground pins not included in the two 50-line I/O signal groups?

The answer to both these questions is that these two 50-line groups on the FeaturePak’s MXM connector were intentionally left unspecified in order to maximize FeaturePak flexibility. Letting each FeaturePak define its 100 external interface lines broadens the range of applications that can be supported. Power and ground can be assigned to as many lines as required, in a card-specific manner. 

This lack of specificity in the initial FeaturePak standard does not preclude the possibility of developing multiple market- or application-specific external interface “profiles” in the future, however.

FeaturePak modules operate on 3.3V, although they may optionally use 5V power for auxiliary functions (depending on the specific FeaturePak module). All logic levels are implemented with standard 3.3V signaling.

Based on the MXM connector’s maximum per-pin current rating of 0.5A and the number of defined power pins on the FeaturePak interface, maximum FeaturePak module power consumption is 12W from its 3.3V inputs. Additionally, up to 15W of power can be supplied to a FeaturePak via its optional 5V inputs. The connector’s +12V interface pin is not intended for powering the module.

FeaturePak vs. FeaturePak USB Sockets

The FeaturePak standard defines two types of FeaturePak sockets: “FeaturePak” and “FeaturePak USB.” Each requires a minimum set of functions to be implemented, in order to ensure interchangeability with the open market of FeaturePak modules. The only difference between the two is the minimum number of PCI Express and USB links. The minimum set of functions that must be provided by each type of FeaturePak socket appears in Table 2.

FeaturePak modules, on the other hand, are allowed to implement subsets of the host interface signals, although a future release of the FeaturePak Specification may define certain minimum requirements. 

Thus, a FeaturePak module may communicate with the host board via any combination of PCI Express, USB and serial interfaces. All subsets of these interfaces are permitted—resulting, for example, in possibilities such as a USB-interfaced serial expansion module, a serial-interfaced GPS module, or a PCI Express-interfaced Ethernet switch or video frame grabber.

How Rugged Is It?

Another question often asked is: “How rugged is this standard?” or “Aren’t miniature card-edge finger contacts (230 contacts on 0.5 mm pitch) unreliable?”

With questions like this in mind, Diamond subjected the FeaturePak card and connector design to shock/vibration testing. The company reports that the form factor and its connector successfully passed random vibration testing up to 6.07 Grms over 20 Hz through 2000 Hz in 3 axes, according to MIL-STD-781D Task 401. Refer to Figure 7 for the test profile used.

After more than two decades, stackable single board computers (SBCs) are firmly established as a cornerstone of embedded system design. However, SBCs often are left out of high-volume opportunities due to cost, feature, or spatial constraints. 

A more recent approach—the board-level computer-on-module (COM)—appears to be making better headway in high-volume applications, evidenced by projections that COM market sales are likely to overtake those of stackable-SBCs this year. With the success of COMs has come an opportunity for similarly packaged I/O function blocks—what might be termed “I/O-on-modules.” 

The proposed FeaturePak embedded I/O expansion standard, introduced in March 2010 at the Embedded World trade show in Nuremberg, introduces a compact, low-cost and flexible I/O-on-module format that’s well suited to a wide range of embedded applications, including applications based on stackable-SBCs, COM baseboards and full-custom electronic designs.  

Diamond Systems
Mountain View, CA.
(408) 810-2500.
[www.diamondsystems.com].

[www.FeaturePak.org].