RTC Magazine

For many decades, mezzanine boards, also called daughter cards, have proven to be an essential and highly effective strategy for configuring embedded systems to meet the specific needs of a wide range of applications. System designers regularly exploit their modularity by using them to add various types of interfaces to processor boards, thus creating custom systems from standard COTS products. In the past, mezzanine boards consisted mainly of specialized connectors, driver/receiver circuits, modems, UARTS and ASICs dedicated to a particular interface. Board programmability was limited to a few fixed, pre-defined functions.

In recent years, FPGAs have dramatically changed the architectures and extended the functions of mezzanine boards in many different ways. Not only can FPGAs be configured to implement numerous electrical interface standards, they can also implement a variety of protocol engines. In this way, one FPGA-based product can replace several legacy products. Through reconfiguration, that FPGA-based product can also be adapted to new standards and protocols to help safeguard against product obsolescence, at the level of both board and deployed system.

Although these are laudable gains, the real benefits FPGAs bring to mezzanine boards stem from their ability to implement real-time signal processing, high-level local control functions and high-speed interfaces. By doing so, FPGAs have revolutionized both mezzanines and embedded system design.

Advanced Digital Signal Processing Functions

Data rates at the front end of the mezzanine board are often quite high, especially for new network and storage interface standards with multiple channels and gigabit signaling rates. For these standardized interfaces, such as between a host processor and a communication channel like Ethernet, an ASIC is usually the best solution for handling the necessary protocol tasks.

However, acquisition of wideband analog signals for radar, satcom and communication systems requires A/D converters operating at sampling rates of 100 MHz and often much higher. Because of the variety of signal types and frequency characteristics, signal processing tasks tend to be quite unique for each system. As a result, there is no standard ASIC available that can handle a wide range of applications.

This forces the system designer to find the best way to process the tremendous amount of data generated by these data converters. If that data cannot be handled on the mezzanine card, the task of processing it must fall squarely on the shoulders of the CPU or DSP board that hosts the mezzanine card. But performing these data extraction and protocol processing tasks at the interface data rates often consumes a major portion of a processor’s horsepower, thereby increasing the number of processors and causing an immediate impact on system size and cost.

Fortunately, FPGAs provide a nearly ideal solution to this dilemma. Consistent with advances in silicon technology, each new generation of FPGA devices delivers faster speeds, improved density, larger memory resources and more flexible interfaces. One major watershed event for FPGAs was the recent incorporation of hardware multipliers. Because multiplication is an essential operation in nearly every digital signal processing algorithm, hardware multipliers have afforded FPGAs a strategic entry into DSP applications.

Some of the most popular functions of mezzanine boards include digital downconversion for narrowband communication systems, FFT processing for radar applications and coding/decoding for broadband wireless networks. These tasks are tailor-made for FPGAs: they are well-defined, processing-intensive tasks that can take advantage of the parallel structures within the FPGA to achieve real-time processing at rates matching those of high-speed data converters.

Unlike general-purpose processors that must perform multiplications serially, FPGAs can be configured to execute hundreds of multiplications in parallel. This makes them highly complementary companions to programmable CPUs and DSPs, which are far better at handling more complex high-level tasks executing through a C program.

Fortunately, because of their ability to handle diverse logical and electrical interface signals, the natural place for FPGAs on mezzanine cards is between the high-speed converters and the mezzanine bus. This is also precisely the best point for front-end DSP processing, further fueling the adoption and acceptance of FPGAs for performing signal processing on mezzanines.