Using FPGAs to solve tough DSP design challenges

DECIDING BETWEEN TRADITIONAL DSP AND FPGA
Conventional digital signal processors have been around for many years, and there are certainly instances where they present the best solution to a particular problem. If the system sample rate is below a few kilohertz and is a single-channel implementation, the DSP may be the obvious choice. However, as sample rates increase beyond a couple of megahertz, or if the system requires more than a single channel, FPGAs become more and more attractive. At high data rates the DSP may struggle to capture, process and output the data without any loss. This is due to the many shared resources, buses and even the core within the processor. The FPGA, however, can dedicate resources to each of these functions.

DSPs are instruction based, not clock based. Typically, three to four instructions are required for any mathematical operation on a single sample. The data must first be captured at the input, then forwarded to the processing core, cycled through that core for each operation and then released through the output. In contrast, the FPGA is clock based, so every clock cycle has the potential ability to perform a mathematical operation on the incoming data stream.

Since the DSP operates on instructions or code, the programming mechanism is standard C or, for higher performance, low-level assembly. This code may have high-level decision trees or branching operations, which may prove difficult to implement in an FPGA. A wide variety of legacy code exists to perform predefined functions or standards like audio and telephony codecs, for example.

FPGA vendors and third-party partners have realized the advantages of using FPGAs for high-performance DSP systems, and today many IP cores are available across most vertical markets including video, image-processing, communications, automotive, medical and military applications. Often it is simpler to break a high-level system block diagram into FPGA modules and IP cores than it is to map it into C code for DSP implementation.

MOVING FROM DSP TO FPGA
Examining a few key criteria may facilitate the decision between conventional DSP and FPGA (see Table 1).


It is widely accepted that software programmers outnumber hardware designers by a significant margin. The same is true for DSP programmers vs. FPGA designers. However, the transition for system architects or DSP designers to FPGA is not as difficult as software-to-hardware migration. Many resources are available to dramatically decrease the learning curve for DSP algorithm development and implementation in FPGAs.

The main hurdle is a paradigm shift from a sample- and event-based approach toward a clock-based problem description and solution. This transition is much easier to comprehend and apply if it is made at the system architecture and definition stage of the design process. It is not unusual for different engineers and mathematicians to be defining system architecture, DSP algorithms and FPGA implementation somewhat isolated from one another. This process is, of course, much smoother if each member has some knowledge of the challenges the other team members face.

In order to appreciate FPGA implementations, an architect need not be highly proficient at FPGA design. A fundamental understanding of the devices, resources and tools is all that is required. These steps can be reduced through the many focused courses that are available.

The exact steps will vary depending on an engineer’s background and expertise. Specifically in the DSP category are classes in algorithm development, efficient implementation and System Generator design. If you wish to become proficient with DSP design in FPGA, a great starting point would be three courses offered by Hardent and other Xilinx Authorized Training Partners: DSP Primer, Essential DSP Implementation Techniques for Xilinx FPGAs and DSP Design Using System Generator.

Hardent also offers general courses describing Xilinx devices, HDL design entry languages, optimization techniques, and design and debug strategies. Specialized courses and workshops focus on high-speed I/O considerations, embedded processing and DSP design techniques.

Consult www.hardent.com/training for more on the Hardent training schedule.

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