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Buh Bye, PCI

COLIN MCCRACKEN, CONTRIBUTING EDITOR

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The long run of the parallel PCI bus may be over from a design point of view. We’ve reached the beginning of the end, at least.

While the desktop PC market moved quickly from PCI to PCI Express many years ago, certain processors and chipsets are kept in production for extended lifecycles—5 or 7 years or more. FPGAs and specialty I/O cards give embedded system OEMs added longevity and control of their own destiny. PCI even became widely used in other architectures too like PowerPC and MIPS for networking applications such as core routers running RTOSs like VxWorks.

Often, reliable connectivity and real-time precision weigh in as higher priorities than raw I/O bandwidth. Even overall board space and sometimes power efficiency are relegated to nice-to-have goals, when push comes to shove. Unique I/O requirements for each application in medical, communications, military, transportation and other markets means that “perfect fit” boards rarely exist. Outside of simple kiosk and signage and touch panel systems that use vanilla motherboards, the challenge to designers becomes how to round out the I/O.

It took so many years to build up large ecosystems of special PCI cards and carrier boards. Embedded Rome wasn’t built in a day. Usually there isn’t a direct replacement that uses PCI Express. Many choices of form factors and bus connectors exacerbate this problem. Cobbling together a solution with bridge cards and adapters isn’t going to make the pointy-hair boss happy.

Changes involve extensive redesign, requalification and recertification cycles. Fortunately, many embedded CPU card vendors are sympathetic to their customers’ needs. As long as chips are available, and even beyond that thanks to last-time buys and stocking programs, these vendors can keep the system OEMs in production. As an example, even though the Intel fourth generation Core i7 “Haswell” modules are only in Type 6 pinout, last year’s third generation “Ivy Bridge” modules are available in Type 2 pinout with only one year depleted of the 7-year lifecycle. A brand new design start with a Type 2 module is just fine for most OEMs if PCI is required, even for some medical and military product design cycles. Some board vendors go the extra mile by implementing legacy I/O (Compact Flash) and PCI bridges in FPGAs on their boards. This level of commitment and investment gets rewarded by customer loyalty for many more years to come. It’s truly a win-win scenario when the heartbreak of obsolescence is redirected toward longer-term healthy relationships.

With a few more years of supply arranged, designers can turn their attention toward next-generation platforms and architectures. From a software point of view, moving from PCI to PCI Express is straightforward. Moving from PCI 10/100 LAN to PCIe GigE has been very smooth, almost transparent, and the same is true for other PCI devices. Firmware initialization enumerates peripheral devices and circuits and assigns resources in much the same way as before—“config space,” interrupts, base addresses and so on.

The hardware often takes longer to sort out, so “buying time” is the key. Sometimes the hiccups include availability of industrial temperature versions of a chip, power consumption, BGA ball pitch, noise from PLLs, additional or unusual core voltages, or other things that don’t affect software. Another glitch to watch for is low-volume specialty SBCs that are much more expensive with PCIe than their predecessors, so researching prices and getting quotes is imperative up front, before getting too far down any one path. With COM Express, the more “legacy free” (i.e., Type 6), the lower the price due to fewer bus bridges and legacy I/O controllers.

Full custom SBCs take the most time to redesign. Fully off-the-shelf board stacks can be much simpler as long as the right processors and I/O exist, which is not a given. The middle ground of using an off-the-shelf processor module with a custom carrier allows OEMs to get to market quickly while focusing only on the I/O migration without re-inventing the processor, chipset, RAM and LAN wheels.

In the embedded market, “buh bye” doesn’t have to mean the bitter end, but it is a signal that it’s time to start planning for a smooth migration that might take years to complete. The sooner you start, the less obligated your purchasing department will be with supply chain commitments and inventory. You just might make a new best friend.