Actel

-- EDN, 8/17/2000

You probably know Actel as an antifuse-based FPGA supplier, but recent acquisitions have rounded out the company's technology portfolio. Three main device families form the foundation of today's Actel product line: the 5V MX, 3.3V SX, and closely related 2.5V SX-A architectures (Picture). Older product families—the 1200XL, 3200DX, Act 1, and Act 3—round out the antifuse team.

Antifuse technology delivers low-impedance—therefore, low-power and high-speed—signal interconnection and more robust immunity to high-radiation operating environments than other configuration technologies provide. Actel devices are available in both standard commercial- and extended-temperature options and in screened high-reliability, radiation-tolerant, and radiation-hardened versions.

Actel's parts are also nonvolatile. Unlike SRAM-based FPGAs, they require no separate memory to store their configuration data, and their functions are immediately available upon system power-up. The single-chip nature of antifuse FPGAs also makes them nearly impossible to reverse-engineer or clone. This characteristic becomes increasingly important as decreasing costs and higher capacities result in the parts finding use in high-volume consumer products. Such products include the highly successful Rio MP3 player, which uses Actel's MX devices and might formerly have employed ASICs.

The company's first-generation PLICE (programmable low-impedance circuit element) antifuse-technology approach employs a metal-to-metal interconnect structure comprising polysilicon and a diffused N+ region, separated by a high-impedance oxide-nitride-oxide barrier. Applying high voltage during programming ruptures this barrier. The PLICE antifuse structures, which reside on the same base layer as active circuit elements, take up die area that could otherwise find use in constructing additional logic blocks, embedded memory arrays, and other circuits.

As a result, beginning with the SX family, Actel has migrated to a second-generation-technology approach that locates antifuses directly between metal layers, above the logic. Whereas MX-series FPGAs use a relatively generic multiplexer-plus-register logic block, SX and SX-A devices employ a sea-of-modules ratio of two logic structures: C-Cells and R-Cells. C-Cells contain a dual-level, two-input multiplexer structure plus input-inversion capability, and the company claims that the C-Cells can implement more than 4000 functions of five inputs. R-Cells contain multiple-function flip-flops with numerous signal-input, clocking, reset, and clear options.

The high voltage necessary to configure an antifuse FPGA usually means that you program it before installing it on your system board, and, because antifuse creation is irreversible, in-system reconfiguration is impossible. In response to customers' requests for a more flexible FPGA technology (in the lab, the manufacturing line, and the field) that retains antifuse's fundamental benefits, Actel first partnered with, then acquired, Gatefield Corp and its ProASIC line of flash-memory-based FPGAs.

In addition to being onboard-programmable and -reprogrammable, ProASIC devices use an extremely fine-grained three-input and one-output logic cell. Actel claims that this logic cell not only provides you with an intuitive ASIC-prototyping vehicle but also delivers a smooth learning curve for budding FPGA designers who are already experienced with ASICs and their design tools. ProASIC also brings embedded-SRAM and FIFO capability to Actel's arsenal—a feature missing from all but a few of the company's antifuse FPGAs.

Actel has experimented with SRAM-based FPGAs since the mid-1990s but has never brought them to market. However, the company recently acquired Prosys Technology, an IP provider that has developed an SRAM-based FPGA core suitable for embedding inside ASICs. For now, at least, Actel says it's content to license the core to interested third parties and has no plans to introduce its own SRAM-based programmable-logic devices. The company also has embedded-ProASIC-core ambitions; stay tuned for details after the technology ramps into volume production.

Actel's DeskTop and Designer Advantage tool sets support antifuse FPGAs, with several variants supplying combinations of device and family, design entry, synthesis, simulation, placement and routing, and programming support. The Silicon Explorer II verification and logic-analysis tool enables you to observe and analyze internal device nodes without, in most cases, iterating your design. For ProASIC FPGAs, the vendor-supplied tools include ASICmaster (for placement and routing) and Memorymaster (for embedded-memory-function generation). IP comes both from Actel and from the company's strategic partners.