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Designing a Universal Carrier

COLIN MCCRACKEN

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At last. Through an unprecedented spirit of cooperation, the industry has come up with the Holy Grail for computer-on-module (COM) users. A true “universal” carrier board that allows system OEM customers to easily qualify multiple COM suppliers. It supports all of the various modules, processors and form factors that OEMs want. It uses all of the popular (though incompatible) connectors and pin assignment “types.” There are more switches on it than a Manhattan telecom central office. The carrier board requires larger fabrication and pick and place equipment, because it’s as big as an ocean-going vessel. So it has earned the name “Aircraft Carrier.”

Okay, maybe including every module form factor is impractical. How about even just one? COM Express is popular for a wide range of processor performance within Intel’s and AMD’s mobile and ultra-mobile series roadmap. Perhaps a universal COM Express carrier is feasible. It could use one or both connectors, support all of the various module sizes and mounting holes, and connect the signals that are common to all of the popular pin-out types. Not bad. The board isn’t large, and it has more holes than Swiss cheese in the middle. It’s limited to x86 processors, and most (but not all) of the type 2 power and ground pins are hooked up, along with a few PCI Express lanes and USB ports. The LPC bus can’t be used in order for all modules to have a chance of booting on the first try without BIOS customization fees and minimum order quantities. To be safe, be sure to disable the ceiling smoke detectors when powering up the second module.

On a more serious note, COM Express carrier boards have indeed become much easier to design in the past few years, thanks to a rare free publication from PICMG called the Carrier Design Guide. A painstaking effort by multiple suppliers who knew they had to cooperate for the survival of the standard resulted in this very thorough document that contains circuit examples for terminating most module interfaces. You might need to ask a board supplier for the Compact size (95 x 95 mm) mounting-hole positions relative to the carrier board connectors and board outline, since the document predates R2.0 and R2.1 of the module specification.

As far as selecting a pin assignment type, resist the temptation to select based solely on I/O required by that application. For example, you may want to connect to Compact Flash, LVDS or SDVO for display, PCI for serial ports, etc. If you want to maximize the “life” of the carrier board before re-spinning it, such as for medical and mil/aero applications, consider type 6 modules instead of type 2, even if it means adding circuitry to the carrier board. It seems counterintuitive at first, but nearly all new modules introduced starting in June and going forward with performance that scales up to Core i7 are only available with type 6 pin-out. Finally, I/O that is considered “legacy” by the desktop/laptop PC world must be attached to one of the module interfaces; select chips according to device driver availability.

For OEMs who are focused on the low-power SoCs (< 10 watts), some form factors like Qseven have supported ARM and x86 processors for years. It’s important to review user manuals to confirm not only which features are populated, but what port numbers are provided on what pins. For deeply embedded headless designs or applications with modest graphics requirements running Linux or certain Windows variants, such modules are commonplace now and are based on free open standards, unlike their proprietary counterparts from 10 years ago.

Although a true universal carrier board remains impractical, a “close enough” carrier board is fairly straightforward for system OEMs and third parties to design. OEMs who are brand new to COMs can celebrate their blissful ignorance of how messy it was to wallow through the mire of specification revisions and the shedding of parallel interfaces to arrive at the current form factors and pin-out types. A ground-up new design just takes enough patience to pour through the documentation of every single module you want to support, and the willingness to add circuitry to the carrier as needed after the lowest common denominator of module features is determined.