PCI has been the most widely used bus standard in the PC, server, and embedded markets for the past decade. Because PCI is limited by its shared, central arbitration-based architecture and system-synchronous clocking scheme, current and next-generation processors are outstripping its ability to keep up.

PCI’s emerging replacement is PCI Express, a new connectivity standard that preserves the flexibility and familiarity of PCI while dramatically increasing bandwidth and performance. The controlling body for the PCI specification, the PCI SIG, has ratified PCI Express as the nextgeneration PCI. PCI Express-based products are now becoming available; shipments are expected to achieve high volume as early as 2006. Figure 1 shows the adoption forecast for PCI Express.

PCI Express uses serial I/O technology to create point-to-point connections and is reverse-compatible to PCI, preserving many original PCI advantages. It scales from a single lane (1x) to a 32 lane (32x) architecture, offering a bandwidth of 2.5 Gbps per lane. PCI 32/33 has a bandwidth of 1 Gbps, while PCI 64/66 has a bandwidth of 4 Gbps.

The 1x PCI Express implementation matches up very well with PCI 32/33, the most commonly used PCI interface across all markets. A two-lane implementation (5 Gbps) is an incremental improvement over PCI 64/66. At the high end, a 32-lane PCI Express implementation supports a total of 80 Gbps, providing more than enough bandwidth to support the vast majority of next-generation applications.

Implementation Details
PCI Express is a three-layer specification: physical (PHY), logical, and transport, all defining separate functionalities. Also included in the specification are advanced features for hardware error recovery and system power management. (For more information about PCI Express, visit www.pcisig.com.)

Since 2000, Xilinx® has offered a line of PCI 32- and 64-bit solutions for Spartan™ series FPGAs. The most logical successor is a PCI Express solution using an external PHY chip paired with a Spartan-3 or Spartan-3E device. The PCI Express specification defines an interface to hook a PHY chip up to a separate device that houses the logical and transport layers (called a PIPE interface – a white paper about this is available from Intel).

In the two-chip solution, the transport layer resides in a dedicated PHY chip, and the logic and transport layers reside in a Spartan FPGA. A broad range of PHY devices are available from manufacturers such as Genesys Logic, Philips Semiconductor, and Texas Instruments. PHY pricing will be less than $10 for high volumes (250,000 units per year). (See the sidebar, “PHY Vendors,” for contact information.)

To implement the interface, Xilinx and several of our IP partners (including Eureka, GDA, and Northwest Logic) provide PIPE IP cores for Spartan-3 and Spartan-3E devices. A single-lane PCI Express controller requires approximately 500,000 gates (50% of a Spartan XC3S1000) for the logical and transport layer core, leaving the rest of the FPGA available for the user application (see the “PCI Express Core IP” sidebar for details on Northwest Logic’s product and www. xilinx.com/pciexpress/ for details on PCI Express IP from our other IP partners.) Figure 2 shows the implementation of a PIPE interface using a Spartan FPGA and external PHY.

Figure 3 illustrates a range of options to implement a single-lane PCI Express interface. The cost of a standard-product option is fairly high (>$40), making it tenuous for high-volume/low-cost applications. The Spartan options drop that cost substantially, and add the flexibility of programmable logic to integrate and implement other system capabilities. In 250K quantities (reasonable for typical consumer applications), the Spartan-3E version will cost approximately $17.

Conclusion
In addition to reducing total costs, the Spartan FPGA + PHY option gives you substantial flexibility to build “PCI Express-to-anything” bridges and integrate other circuit elements. As most systems have a range of bandwidth requirements, preserving flexibility is important so that you can add lanes without dramatically changing the layout.

Spartan-3 and Spartan-3E FPGAs are available in a wide range of densities, and preserve migration up and down in overall bandwidth. And because FPGAs are fully reprogrammable post-deployment, they eliminate the risks associated with first-generation ASSPs and ASICs.

If you are currently using PCI for your interconnect standard and are architecting your next-generation designs, you should consider the PCI Express option from Xilinx. We encourage you to find out how Spartan-3 and Spartan-3E FPGAs will help you meet your current and future design requirements. More information about Spartan-3 and Spartan-3E FPGAs, PCI Express IP, and compatible PHY devices is available at www.xilinx.com/pciexpress/.

PCI Express IP PCI Express IP cores are available from multiple vendors including Xilinx and our partners. One such core from Northwest Logic is featured below. Northwest Logic’s PCI Express Core is specifically designed for low-cost Spartan-3 FPGAs. A Spartan-3 based PCI Express design uses the Spartan-3 device with a low-cost physical interface for a PCI Express (PIPE)-compatible PHY chip. The PHY chip implements the low-level PCI Express physical layer, while the device takes care of the upper-level data link and transaction layers. Another version of the PCI Express Core uses the internal MGTs in Virtex-II Pro and Virtex-4 FX FPGAs to provide a fully integrated PCI Express solution. Northwest Logic’s PCI Express Core is one of the smallest PCI Express cores available, enabling you to target the smallest and consequently lowest cost FPGA. The core is provided with a comprehensive verification suite and expert support to ensure rapidly developed and validated designs. Also available is a PCI Express Development Board for quickly prototyping a complete PCI Express System. A demo GUI, drivers, and PCI Express FPGA reference design are also included. For more information (including pricing and core size for a particular FPGA family), visit the Northwest Logic website at www.nwlogic.com.

PHY Vendors
Genesys Logic	www.genesysamerica.com
Philips Semiconductor	www.semiconductors.philips.com
Texas Instruments	www.ti.com/pciexpress/

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