Mezzanine cards cope with changing I/O requirements

More I/O flexibility is being provided through the FPGA Mezzanine Card – or FMC – specification, as defined in VITA 57. Connecting to an fpga or any other device with reconfigurable I/O, FMC modules can be used on a range of popular board formats, including VME, VPX, CompactPCI, AdvancedTCA and MicroTCA. The FMC standard, which is fpga architecture independent, is designed to: * Maximise data throughput * Minimise latency * Reduce fpga design complexity * Minimise system cost, and * Reduce system overhead, including power consumption and cooling needs. VITA 57 introduces an electromechanical standard that creates a low overhead bridge between front panel I/O on the mezzanine module and an fpga on the board carrying that module. Essentially, FMC removes the need to inject protocol data into the raw data to be processed, something which happens with a defined bus interface. By assuming the fpga has a relationship with the I/O mezzanine module, the FMC standard can capitalise on the ability of fpgas to process raw data formats. Rather than defining pin functionality, the FMC specification simply defines the upper limit of connections for both parallel lines and multigigabit serial signals. The FMC specification has two defined module sizes – single and double width. The single width module is 69mm wide, while the double width module is 139mm wide. Both are 76.5mm deep. The FMC standard specifies a 400 pin high speed array connector, but a mechanically compatible connector with 160 pins can also be used. FMC modules can handle up to 80 differential signal pairs with a signalling speed of 2Gbit/s. However, up to 20 differential pair signals can be connected to 10 multigigabit transceivers running at up to 10Gbit/s. Together, they deliver a bandwidth of more than 40Gbyte/s. "FMC has rapidly become the de facto standard for daughtercards in the fpga industry," said Raj Seelam, senior marketing manager, Platform Solutions, with Xilinx. "The FMC standard reduces design costs and time to market by bringing the benefits of modular design much closer to the fpga user community." Dave Lautzenheiser, from Faster Technology, noted: "With all the standards out there, FMC makes it a lot easier for the system designer to build something, becoming a component choice at the board subsystem level. They can then focus on the application." FMCs are most commonly used in applications that need low latency I/O. Getting the high speed data to fpga quickly for processing and then turning it around as output without the need to transfer around the buses in a system has great advantages. "The speed and number of connections that an FMC format module uses, together with direct fpga-to-I/O devices, means the FMC format is particularly suited to applications benefitting from multi gigabyte per second I/O with low latency," said Jeremy Banks from Curtiss-Wright Controls Defense Solutions. There are complementary specifications to ANSI/VITA 57.1. For example, VITA 57.2 defines an 'electronic datasheet' metadata standard to provide automated validation of FMC configurations and performance capability. In short, VITA 57.2 helps users to determine the compatibility of FMC products from different vendors before products are purchased. It also allows pin description files to be created automatically, which can then be loaded in fpga design tools. Meanwhile, VITA 57.3 defines logic interfaces for the firmware that resides in the carrier card fpga that communicates with the FMC module. Effectively, this acts as a 'device driver' layer. While FMCs can ease the pain of system development, using them is not without challenge. "Many system designers will ask which carriers to use with a specific FMC," said Lautzenheiser. "They want a level of assurance that they can get past the integration issues quickly, getting onto the challenge of working on their application software." Seelam agrees that compatibility is the biggest challenge. "With so much flexibility, there is an inherent increase in risk factor: is this going to work with this or that card?" Lautzenheiser feels there is something missing at the application level. "Is there a way to define clearly and consistently what the carrier can support?," he asked. He says he is looking for something that would package and deliver a known reference point, but doesn't know exactly how that might be done. While the physical level is covered in the work of the VITA 57.2 committee, he believes 'something above that' is missing. One way of seeing how various devices interoperate is through PlugFests. FMC PlugFest 2012, held in August in California, gave developers the opportunity to find out just how compatible their parts were. The event was also the first outing for FMC Check, a set of tools intended to help system developers, integrators and module designers to validate FMC module and carrier designs and their interoperability. At the PlugFest, a combination of 23 FMC modules and 13 fpga based boards were used to test a range of applications. Altera, one of the more recent companies to join the FMC Marketing Alliance, took part in the PlugFest. Charles Pryor, manager of the company's high speed board group, said: "Our first two FMC carrier cards passed 100% of all electrical testing with three modules from Faster Technology. Altera looks forward to broadening its FMC testing with more module vendors, both using the VITA57.2 FMC Check tool, as well as full electrical testing." Although FMCs are supporting the demands of current generations of fpgas in terms of I/O and power, it is important that the specification is developed. For example, companies such as National Semiconductor that launched 1GHz digitisers are now sampling parts running at 5GHz and even 10GHz. While the current revision of VITA 57 can handle these data rates, the standard will need to look forward continually in order to match the increasing capabilities of fpgas and external I/O interfaces. Marc Couture, Mercury Computer Systems' director of product management, microwave and digital solutions, believes that, in the future, there will be an FMC 2.0, with more pins and/or with pins that can sustain higher digital bandwidths. "FMC 2.0, I believe, will need to be faster and wider in terms of the bandwidth that can be sent from the mezzanine card to the baseboard," he said. Curtiss Wright's Banks added: "More serial paths will need to be defined, either through a more dense connector or by redeploying parallel lines to serial." However, a different approach is being pursued by French company Techway. "Stacking multiple FMCs to gain more front panel space or creating an extended pcb with FMC connectors, making a wider FMC, is something that Techway is exploring," said Patrick Mechan. While this does not impact the electrical specification, it does offer a creative way of getting more I/O into the system. Author profile: Malachy Devlin is chairman of the FMC Marketing Alliance. For more information, go to www.vita.com/fmc.