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FPGAs roll with more flexible options for PCIe

Posted: 08 May 2007 ?? ?Print Version ?Bookmark and Share

Keywords:FPGA PCIe? FPGAs flexible PCIe? PCIe-specific FPGA? PCIe-to-Ethernet interfaces?

As the electronics industry shifts from the widely-used PCI standard to PCIe, FPGA vendors are poised to capitalize on the rapid expansion. They are offering products that address a range of system costs and needs in various sectors, including PC, networking, industrial automation, medical and graphics/image processing.

A PCIe fabric consists of point-to-point links that interconnect a set of components. Parts include a root complex, multiple endpoints or I/O devices, a switch and a PCIe-PCI bridge. FPGA vendors are developing products for the full range of PCIe applications, from endpoints to root complexes. (View table of FPGAs with PCIe features.)

Varying flavors, more options
While the major vendorsActel, Altera, Lattice Semiconductor and Xilinxare tackling PCIe applications with products that comply with the specifications of the PCI-SIG industry organization, their products vary mainly at the PHY.

Lattice Semiconductor Corp., for instance, delivers its LatticeSCM and LatticeECP2M products with built-in Serdes blocks, and claims to be the only FPGA vendor to do so. On-board Serdes capability eliminates the need for external PHY devices. The LatticeSCM FPGAs also feature multiple high-speed protocols that are supported in the embedded PCS block, as well as parallel I/Os that enable rates of 2Gbps.

Xilinx's Virtex-5 FPGAs offer low power, reduced area and low cost with a built-in hard PCIe block and PHYs. Its Virtex-5 devices provide compatible LXT and FXT FPGAs with 3.2-G and 6+G PHYs to ensure generation 1 to generation 2 migration. They are designed with built-in low-power blocks for PCIe and 3.2Gbps RocketIO transceivers. Its Virtex-4 and Virtex-II Pro FPGAs are designed with built-in transceivers and soft IP cores for PCIe.

Stratix II FPGAs from Altera offer up to 130,000 logic elements with 20 transceivers on board.

For consumer and other high-volume applications, Xilinx said the Spartan-3 FPGAs deliver the industry's lowest-cost x1 PCIe product. The Spartan-3 FPGAs have soft IP for PCIe with a Pipe interface that works with external PHY from industry-standard vendors.

Stratix II FPGAs from Altera offer up to 130,000 logic elements with 20 transceivers on board. The transceivers deliver PCIe generation 1 and generation 2 performance, and the PHY and PCS layers comply with the standard. The devices are offered with either a Pipe 1.0 or 2.0 interface, with up to 7Mbytes of on-chip memory and up to 750 user I/Os that enable direct connection to external DDR2 memory at up to 333MHz.

As for Actel, its flash-based, high-density ProASIC3E devices can be used in combination with proven IP from Northwest Logic, which supplies the media-access controller, and with Texas Instruments Inc.'s PCIe PHY, the XIO1100. In a bridging application, for example, customers may create PCIe-to-Ethernet 10/100 interfaces and minimize the number of chips on their board. Actel's FPGAs help to minimize board space and power for these applications. Additional logic can also be easily added into its devices.

Key advantages
Despite the differences, FPGA suppliers agree that the key benefits of using FPGAs for PCIe-based systems are their flexibility and faster time-to-market. Customers using FPGAs can implement PCIe functions based on the demands of their end application, they said.

"For example, a customer's PCIe endpoint may require a simple x1 configuration or a high-performance x8 with many of the nice-to-have protocol features," said Steve Mensor, senior director of IP marketing at Altera. "Either endpoint can be easily implemented in the FPGA with highly configurable IP. FPGAs are also ideal for implementation as bridges between PCIe and other communication interfaces such as legacy PCI/PCI-X Ethernet and SPI 4.2."

Remote reconfigurability in FPGAs provides additional longevity to customer end products, Mensor added.

First-grade specs
Customers should look for the following specifications when selecting a PCIe-based FPGA solution: compliance, PCIe-specific characterization, performance and power.

PCI-SIG compliance is the starting point to ensure interoperability with other PCIe-based systems. This includes the ability to support the entire protocol stack and the interfaces within the protocol stack such as the PHY interface for the PCIe architecture, also known as the Pipe interface, which came out of Intel Corp.

Designers should ask a vendor how many generations of its products have passed PCI-SIG plugfests. The more experience, the less the risk, Xilinx said.

Unlike application-specific standard products, PHYs in FPGAs support multiple standards. Designers should make sure their vendor provides PCIe-specific FPGA characterization reports in addition to data sheet information.

They should also review the bandwidth their traffic needs and determine first how many lanes are needed. Does the latency satisfy the data traffic?

Throughput is critical, since not all FPGA performance is the same, nor do all cores have the same efficiency.

- Ismini Scouras

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