The 'next big thing' is small: Expanding Atom's PCI Express lanes

4Intel's latest Atom processors are small by design, which means, like other small packages, they must omit some desired expandability. Although these devices contain a great deal of integrated I/O, many users want more PCI Express (PCIe) ports. Steve offers some tips on how to accomplish this addition.

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In his keynote at the Consumer Electronics Show earlier this year, Intel's CEO Paul Otellini called mobile Internet devices the "next big thing in computing", adding that the world is "going ultra-mobile" with devices and embedded computers based on Intel's new Atom chipset, which began shipping during the first half of 2008. This chipset consists of one chip that handles Ultra-Mobile PC (UMPC) functions and an ultra-low-power processor that is a fraction of the size of previous-generation mobile CPUs.

The Atom chipset was developed for low-power, small form factor applications such as UMPCs, Mini-ITX and COM Express systems, and other small devices not yet introduced. Because of the chipset's compact size, only two PCIe ports are included.

This limitation poses problems, given that systems using Atom processors typically need more I/O than those two ports because they connect to networks and storage devices. Additionally, many systems need PCI connections for various endpoints such as modems and codecs that have not been implemented in PCIe-native silicon.

To resolve this I/O constraint, engineers can implement the latest generation of PCIe I/O switches and bridges with extremely low levels of power dissipation, advanced power management features, and very small physical size.

More ports for COM Express modules

Figure 1 shows a recently announced COM Express form factor card, the Express-MLC from ADLINK Technology. Measuring a scant 95 mm x 95 mm, this COM Express Type 2-compatible card is based on the Intel Atom processor Z500 series with the new Intel System Controller Hub (SCH) US15W. COM Express modules are off-the-shelf PCIe-based building blocks that plug into custom-made, application-specific carrier boards.

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The Express-MLC enables innovative designs in mobile and "light" computing applications, including:

  • Portable and mobile equipment for the automotive and test and measurement industries
  • Visual communication in the medical field, such as home care and video conferencing
  • Entry-level public gaming devices
  • Public points of communication

With a module like this using the low-power (2 W) Intel Atom Z510 series processor and the Intel SCH US15W chipset, developers can utilize a wide variety of mainstream software applications and middleware that will run unmodified with full functionality on a platform familiar to end users.

Connectivity for the COM Express card can be expanded using a PCIe-to-PCI bridge and a five-port PCIe switch (see Figure 2). This expands the I/O capability to four PCIe ports and one 32-bit PCI bus that will support four PCI devices. Thus I/O is expanded by a factor of four to a total of eight potential I/O channels from the two PCIe ports provided on the Atom chipset.

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Engineers selected the PCIe bridge for this application because it features low power dissipation (400 mW) and a small footprint (13 mm x 13 mm). The bridge used in the Express-MLC has SATA/SAS hard disk drives and host bus adapters, as well as five Gen 1 x1 PCIe ports, allowing one upstream port that connects to the chipset and four downstream ports that expand I/O for PCIe-native endpoints such as Gigabit Ethernet.

UMPCs adopt the Atom platform

Figure 3 shows a block diagram of a UMPC that takes advantage of the Atom chipset's small form factor, reduced cost, and low power. This design expands I/O capability using a PCIe-to-PCI bridge to connect to low-cost, low-power, PCI-based peripheral devices instead of using a switch to get more I/O. Because the Atom chipset does not include any PCI connections, the bridge is used to attain up to four additional I/O channels. In this example, two PCI devices (Ethernet and SD memory card controllers) are aggregated on the 32-bit PCI side of the I/O expansion bridge.

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Several UMPCs planned for launch later in 2008 will use the Atom chipset in conjunction with PCIe switches and bridges from vendors such as PLX Technology. In addition to the Ethernet and memory card controllers referenced earlier, applications taking advantage of this expanded I/O include wireless USB (based on ultra-wideband technology), 802.11g wireless LAN, Bluetooth, CardBus, and ExpressCard connections.

These UMPCs have standard feature sets including a 5.6" (1,024 x 600) display, 1-2 GB of main memory, 40-80 GB hard drives, dual cameras, and SD memory card readers. Additionally, these designs are expected to feature four-core batteries that can provide up to seven hours of operation on a single charge.

Looking forward

Other small form factor applications that use the Atom chipset include UMPCs, mobile Internet devices, and Internet-focused machines such as netbooks (scaled-down subnotebooks used primarily for surfing the Internet) and nettops (low-cost desktop devices and other consumer electronics devices).

Now that the Atom chipset has been released, new embedded and mobile computing devices will be hitting the market equipped with the power, cost, and size advantages Atom chipsets deliver. To meet these devices' requirements for expanded I/O connectivity, new small footprint PCIe bridges and switches will provide the additional PCI and PCIe channels needed to deliver rich feature sets while maintaining minimal form factors.

Steve Moore is senior product marketing manager at PLX Technology, based in Sunnyvale, California. His prior experience includes stints in marketing at Mai Logic and IBM. Steve holds a BSEE from the University of California at Berkeley.

PLX Technology