In part one of this two-part article, we discussed the field-programmable gate array (FPGA) mezzanine card standard, highlighting the differences in capabilities from conventional PCI mezzanine cards and their implications for military and aerospace applications. In part two, we will discuss how to choose the appropriate solution, focusing on the benefits and pitfalls of the technology.
FPGA mezzanine cards (FMCs) provide design teams with a simple solution for maximizing I/O bandwidth while still being able to change I/O functionality. FMC modules only feature I/O devices such as analog-to-digital converters (ADCs), digital-to-analog converters (DACs), or transceivers. Unlike conventional peripheral component interconnect (PCI) mezzanine cards (PMCs)1 or the later release of XMC2, which replaces the parallel PCI or PCI-X bus3 with a serial interface like PCI Express4, FMC modules do not include on-board processors or bus interfaces. Instead, the modules take advantage of the intrinsic I/O capability of FPGAs to separate the physical I/O functionality on the module from the FPGA board design of the moduleís host, while maintaining direct connectivity between the FPGA and the I/O interface.
For the right application, an FMC approach can be an ideal solution. Let's take a closer look at how the technology fares when applied to various parameters (see table).
Table: Relative comparison of mezzanine capabilities.Key benefits of FMC compared to PMC/XMCLatency
In determining which applications best suit an FMC approach, the decision really hinges on whether there is a benefit from using an FPGA. Latency is very low for FMCs. If an FMC supports both input and output, then a solution can be realized in which one or more analog signals are digitized and read directly and processed by an FPGA, with the data then transmitted back out through the same FMC. This implementation requires no busses, not even between FPGAs. The only delay is the FPGA processing which can be as low as a few nanoseconds. This type of solution is ideal for applications like electronic counter measures and electronic support measures, in which processing response time is critical. Bandwidth
XMCs might get around the bandwidth problem, compared with FMCs, through decimation or using newer generation serial interfaces operating at ever higher speeds. Some applications may not be able to tolerate this reduction in data off the mezzanine, however. This category can include beam-forming applications, in which high-bandwidth data, from potentially a large number of channels, must be shared between processing elements. High-bandwidth beam-forming is thus another good application area for FMC technology because it would not be exposed to data reduction problems.
Figure 1: Latency and bandwidth requirements vary for each application.
FMCs by nature promise a faster development cycle through lower complexity and a focus on the I/O itself within the design. This makes FMCs a good choice for system upgrades as newer, faster, and higher resolution devices come onto the market.
The approach is well-suited for software-defined radio (SDR) applications. FMCs and FPGA processing make an ideal platform for a wide range of digital receivers. There is a great deal of flexibility afforded by implementing different modulation and coding schemes that can be performed in a common FPGA host. Additional flexibility results from being able to upgrade to new, higher-resolution ADCs as they become available, without the need to develop complex interface structures and new power supplies. Flexibility
Radar has a thirst for increasing resolution and bandwidths, and benefits from direct RF digitization for coherent sampling. Direct sampling of up to X-band frequencies, with appropriate digitizers, means that applications such as marine, ATC, weather, and surveillance radar are well within the capabilities of FMC bandwidth. Faster ADCs are continually being developed, so upgrading is more straightforward because changing the FMC means that only the ADCs are changed in the hardware. However, there are HDL changes required to support this.Customization
If the appropriate FMC function does not exist, it is now easier for customers to design their own cards to fit on a third-party FMC host and more easily track new technology, such as higher resolution ADCs as they become available. The conceptual simplicity of FMC makes this approach more viable and reduces system risk. Because latency and bandwidth requirements vary for each application, this flexibility speeds the development cycle.Limitations
No mezzanine specification is perfect because its target markets are usually varied, and therefore a compromise. This holds for FMCs just as much as PMCs or XMCs. If the compromises are few, however, then these imperfections can be overlooked.