To meet their system design goals, designers today must prototype a new idea and integrate its product features into the lowest cost silicon, complete with versatile functionality. Specifically, two of the biggest challenges are to get the design correct in the first place and to fix a problem rapidly. Immediate design solutions are essential for meeting time-to-market pressures, keeping up with changing industry standards, and prototyping quickly.

Even after the design is completed, a need may exist for field upgradeability if a bug is found or a new functionality is available. The Xilinx® Spartan™ FPGA has been a great low-cost programmable platform for lowdensity control logic and system interfaces. However, when it comes to signal processing, designers traditionally purchase a fixed algorithm standard product for high-speed multiply and accumulate (MAC) functions. This requirement demands additional design resources, verification time, system components, and more board space.

With the explosion of the wireless communication market and proliferation of low-cost 90 nm processes, Spartan-3 devices – with plenty of logic, memory, and as many as 104 18 x 18 embedded hard multipliers – are the ideal solution for many signal processing needs.

The challenge for today’s digital processing systems is their large memory requirements and the very fast MACs needed for rapid mathematical operations. Every multimedia system contains an external DSP processor and memory component that reduces system performance and increases component costs. Once you have uncovered a solution that integrates a parallel or semi-parallel system, you can then focus on improving the overall DSP design to eliminate performance bottlenecks.

As a result of digital processing challenges, many companies have focused their efforts on developing the system-on-chip (SOC) concept by adding feature sets to bring additional functionality to a single piece of silicon. Customized ASICs have become very costly solutions in today’s competitive landscape. Traditional DSP processors are capable of carrying out high-speed MAC operations, but have bandwidth limitations. FPGA technology has made tremendous progress in recent years by increasing a large number of intellectual properties to reduce the cost of silicon development in various markets. This is accomplished by optimizing architectures, using leading process technologies, and adding IP cores.

Some of the typical applications for digital signal processing are digital cameras, phones, 3G wireless, video conferencing systems, and high-definition digital televisions. Having a signal processing capability inside an FPGA is the perfect design innovation – the stepping stone to system-on-chip in an FPGA without the high cost of complex customized chip development.

System Generator for DSP
Xilinx expanded its features in the Spartan-3 FPGA by adding embedded multipliers in the architecture. This technological innovation is similar to embedded block memory, clock management, and multiple standards for high-speed I/O circuits, all standard characteristics of the Xilinx Spartan-3 and Virtex™-II Pro families.

Time to market remains critical for companies developing both system hardware and software. With Xilinx System Generator (SysGen), you can simultaneously create behavioral-level hardware blocks and simulate the entire system with just a few tool clicks. The design environment allows you to create block-based systems like digital QAM modulators for software-defined radio, finite impulse response (FIR) filters, image processing functions, mathematical operators, A/D and delta-sigma D/A conversion, and all-in-one silicon for widely used applications.

Using System Generator within the Simulink design environment from The MathWorks, you have unrestricted access to many blocks. You can select both Xilinx and third-party blocks, drag and drop to the Simulink work space, connect, and simulate a system within minutes. The Xilinx System Generator block is used to select various implementation options such as FPGA device, package, speed, system clock, synthesis options, and HDL. The need to write HDL is eliminated, as the tool creates the proper language for you; however, it can write HDL if you prefer.

Simulink also enables you to integrate blocks from many different libraries. Commonly used DSP block sets include math functions, signal management, a variety of filters, transforms, encoders, decoders, and linear feedback shift registers. For example, you can create a fast Fourier transform (FFT) or FIR core with the easy-to-use GUI in System Generator for DSP, customize the core as per your application, and run the Xilinx ISE™ tool in the background to build your signal processor system. This flow can synthesize, place, route, and generate hardware configuration files.

The Simulink environment allows you to verify the functionality of each block or subsystem created with scopes and graphs to view images or observe data.

Digital filters are among the most significant components in digital signal processing applications. The function of a filter is to eliminate undesirable parts of the signal (random noise) or to extract signals in a particular frequency range. Basic FIR filters are used extensively in video broadcasting and wireless communications. A mathematical expression of a basic FIR filter is:

Y (n) = SUM h (k) * x (n-k); k=0 to k =N-1

This design example implements a 43-tap FIR filter with a MAC engine and a dual-port RAM used for data and coefficient storage. The filter is a low-pass filter with a cut-off frequency of 6 KHz. The sampling frequency is 44.1 KHz.

Figure 1 represents the model of the filter. The model has a coefficient width of 12, a coefficient binary point value of 12, a data width value of 10, a data binary point value of 8, and a sampling frequency of 44.1 KHz. Scope shots of the filtered output are shown in Figure 2. An implementation summary displaying the use of RAM and multiplier resources is shown in Figure 3. This design achieved ~ 125 MHz performance in a -4 speed grade of the Spartan-3 device.

The NuHorizons Spartan-3 Board Xilinx, its distributors, and third-party companies offer several boards for prototyping or emulating a DSP-based system. A low-cost prototyping platform from Nu Horizons Electronics Corp. is the Spartan-3 development board (HW-AFX-SP3-2000-DB) (Figure 4). The board comprises these elements (Figure 5):

• Xilinx XC3S2000-4FG676 Spartan-3 device
• XCF08 Flash PROM for configuration
• 4 x 24-character LCD display
• Graphical LCD interface
• 64 Mb SDRAM (2-1 Mb x 16 x 4)
• 32 Mb Flash 2 Mb x 16
• 50 MHz clock oscillator
• PLL clock multiplier
• CAN 2.0B transceiver
• RS232 interface
• PS2 interface
• Audio CODEC
• Two-channel A/D and D/A converter
• 10/100 Ethernet MAC
• 10/100 Ethernet PHY
• Flash memory interface
• SDRAM memory interface
• LED/push buttons
• JTAG configuration header for programming
• 16-bit LVDS I/F with clock and control
• Test point headers for debugging

Besides DSP designs, the Spartan-3 platform is a great tool to implement many other Xilinx reference designs. Several designs written by Nu Horizons’ field application engineers cover topics such as memory controllers, embedded processors, hardware-in-the-loop with digital signal processing, and system monitor design using ADC and DAC on the board. ADC and DAC are very powerful attributes of our low-cost board, and two of the many competitive board features. The Spartan-3 platform can be expanded with the add-on ADC board from Linear Technology.

Conclusion
With fast multipliers and lower cost FPGAs, engineers now have the ideal solution to their signal and image processing requirements. With tools like System Generator, anyone can implement a powerful parallel or semi-parallel customized DSP system-on-chip design within days.

The Spartan-3 board from Nu Horizons is a perfect solution for prototyping signal processing using Spartan-3 FPGAs. The board has all of the interfaces necessary to create designs for wireless and digital imaging applications if you want to:

• Integrate your logic and signal processing capability on a single chip
• Prototype a configurable DSP systemon- chip
• Reduce the cost of conventional serially implemented external signal processors
• Improve system performance

For all of the designs and related documentation for the Spartan-3 board, visit the Nu Horizons website at www.nuhorizons.com/sp3/.

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