SoCs power legacy I/O

One of the largest input/output (I/O) board ecosystems in the small form factor (SFF) marketplace uses a board-stacking architecture. Unlike I/O cards that are plugged independently into different slots of a motherboard, stackable I/O boards are mounted one on top of another like a stack of pancakes, except that the middle boards cannot be removed without first unplugging all the boards below or above them. In terms of production quantities, the majority of such stackable I/O modules still use parallel PCI or ISA bus interfaces and are designed into rugged and demanding applications that don’t need to migrate quickly to new architectures. Besides, the I/O chips and components themselves are rarely affected by obsolescence.

Most of the processors and chip sets that used to directly drive those parallel PCI and ISA buses were on an obsolescence cycle of every five years. In this new era of powerful all-in-one systems-on-chip (SoCs), however, the good news is that processor chip life cycles have stretched out to seven years for Intel and 10 years for AMD. The “other” news is that the PCI and ISA buses are not available in those SoCs. Their primary markets – such as high-volume tablet and modular PC markets and in-vehicle “infotainment” – simply don’t need the legacy buses and I/O.

Long live legacy

Hope arrives in several forms. Still available at the entry level is a low-cost, legacy-friendly SoC from DM&P Electronics called the Vortex86DX. The processor performance is modest, as the whole point is to meet the legacy application software and I/O where they are. SFF processor modules are available from multiple suppliers with stacking PCI and ISA buses. Legacy I/O also includes universal asynchronous receiver/transmitter (UART) serial ports, which are still essential in many embedded systems, and are included in the Vortex86 SoC.

Also legacy-friendly and still available is the AMD Geode LX800 processor. The built-in PCI interface only needs the external ISA bridge chip to produce both of the stacking legacy buses. Like the Vortex86 based boards, the Geode-based boards are available from many suppliers.

Boosting performance

Some embedded applications, however, need to move up substantially in processor or graphics performance or both, while keeping the legacy buses and I/O. While the new Intel Atom E38xx, Intel Celeron J1900/N2930, and AMD GX-2xx & GX-4xx SoCs fit the performance bill, the processor module designs need to add back in the serial ports and bus bridges that were eliminated from the SoCs. Plenty of PCI and LPC bus chips are available for the task. Some legacy features like I/O space, memory hole, and interrupt requests are limited, so be sure to dig into these details before porting over legacy code.

If there are no legacy code “gotchas,” the huge jump in performance to quad-core 1.2 GHz with great graphics, or to 2.0 GHz with good graphics, can provide a needed facelift for old systems. Part of this stems from the ability to run modern GUIs and OSs that require substantial compute and memory resources, from the various embedded Windows platforms to Linux and Android. Certain legacy applications may benefit from 64-bit register size for data handling or overall physical memory address space.

Upgrade the CPU, but don’t forget power

Now that several vendors are featuring the latest Atom processor family on their boards with stacking PCI and ISA expansion and serial ports, the old stacks can be upgraded. Simply unplug the obsolete processor module and replace it with one of these new ones. The good news doesn’t end there: The low-power, high-performance SoCs deserve the latest switching power-supply technologies to finally replace some of the ancient designs that have been on the market for decades. (See example in Figure 1.) Efficient power-supply design for embedded is a topic for another column, though.

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Figure 1: WinSystems’ PCM-PS394-500 complements the latest SoC-based processor modules.

Here we are in 2015, still talking about the usefulness of stackable processor boards with ISA bus expansion. The bottom line is the staying power of the installed base of I/O cards, more than that of the CPU cards. The biggest surprise of all is the continued availability and even brand-new introductions of x86 SoCs on SFF boards that bring out PCI and ISA interfaces. While these boards provide the legacy signaling that the I/O cards need, don’t forget to update the power supply card while you’re at it.

Small Form Factor Special Interest Group

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