Part 1 of 4
Programmable logic is about to evolve into a new role in the electronic system. Since the advent of the FPGA over 15 years ago, the number of gates available on a single device is greater than ever before. The portable device market is exploding with a vast array of functions previously unimagined. Complex peripherals are now commonly required to fit into a size no larger than a PCMCIA slot. Advanced broadband redundant switches today fit into a one-width rack unit and operate at 3GHz and higher data transfer rates. And while meeting these demands, the product must still hit the market window faster than ever before. The consequence to the designer: more functions, packed into smaller spaces, operating faster, and designed quicker. These time to market pressures are driving FPGAs to deliver more design solutions and offer more available complexity.

If you're a digital designer that keeps up to date, you already know this. But exactly what does this mean to the way you've come to design? Or if you're a design manager, how will you control this looming environment to your advantage? If any of the above comes close to describing your company, you're about to launch into the world of high-density programmable design. This article is the first of a four part series introducing high-density programmable design, what defines it, what makes it unique, what differentiates if from other logic design methods. We'll spend time looking at software design developments that help ease the transition to high-density design, and get your products out the door faster than ever before possible. We'll follow a typical design process from creation, through capture and source options, synthesis and implementation, to chip verification both during design, and finally when the FPGA is in the system.

Platform FPGA
In November, Xilinx announced the Platform FPGA initiative, recognizing that FPGAs have matured beyond the original glue-logic function of programmable logic. With Platform FPGA, programmable devices can now be the central driving point of the primary board functions.

The Xilinx Virtex-II architecture will push the definition of "high-density", by packing more functionality, I/O choices, clock management, and memory into our programmable devices. It? easy to define "high-density" by gate count and available memory alone; but to the designer "high-density" design is primarily defined by what those possibilities do to the design process.

Low-Density vs. High-Density
Low-density logic design is typically about delivering a small set of logic functions, often only one. The FPGA or CPLD is used to provide an ancillary but vital function of the overall board. If IP is used, it's usually a combination of math processes or simple to moderate functions, and used in a limited way. And importantly, low-density designs often can be completed by a single engineer.

The needs that drive customers to high-density design dictate that the function of the chip on the board has now changed. In high-density design the FPGA has now become the heart of the system providing multiple functions, key bus management, processing, and interface tasks. IP is likely to be used in the design, and is further likely to drive the critical processes (PCI-bus controller, DSP functions, etc...). IP becomes a way of solving critical design problems quickly. Most importantly, only the most masochistic engineer would want to complete a high-density design alone. Team design moves from being an interesting concept, to a critical success factor in order to bring a high-density device to successful delivery.

Start by partitioning
Partitioning the device by functionality is our starting point, making the design process much more manageable. We start by dividing the design into hierarchical modules. Many factors will dictate how these modules should be defined: division points of the available designers; where can prior design work be utilized; how many functions can be realized through core or purchased IP blocks; which areas of the design will require unique or intensive design work and re-work? Most importantly, partitioning the design correctly can accelerate timing closure by keeping critical gates and paths together, and also minimizing the number of interface ports between modules.

Figure 1 shows an example of a device being partitioned using Xilinx High-Level Floorplanner. The hierarchical modules are now constrained to specific physical areas of the device. In the process the design manager can also map I/O if known, or reserve I/O blocks for future use. Floorplanning up front serves two main purposes. The first is to break a large device down to manageable sub-designs. Out of floorplanning, the design manager can create black-box definitions to define the HDL modules. This allows each designer to receive the correct design definitions of their respective module, while the design manager can still perform analysis functions at any time from the top block of the device without requiring all the modules to be complete.

The second is to map those modules onto the actual chip for fitting, as shown in Figure 1. As each designer works on only one module of the design, completion is now about finishing logic design and reaching timing closure for that module only. The full range of design functions can be utilized on any level of the device, from the top, on down to any given module. And once timing closure is reached, the timing for that module remains "locked" while the remaining work is still being completed. Future articles in this series will go into the importance of this breakdown to Incremental Design, and how the design team can now see vast improvements on implementation times.

Figure 1 - Xilinx Floorplanner

Next Month
Now we?e ready to begin the actual logic design work. We have a high-density device partitioned into manageable modules for design capture. Those modules reflect the different design sources we'll be using: purchased or core IP, reusable design, or custom HDL code. Next month we?l talk about design capture and IP options, and getting logic design done and ready for implementation.

For More Information
The xilinx.com website contains more information on the FPGA Platform Initiative, Xilinx High-Level Floorplanner and Modular Design; and also the Xilinx Foundation ISE and Alliance family of software design products.

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