Adopting VITA 57 (FMC): Reducing FPGA I/O headaches
No longer just a "sea of gates," FPGAs are now used in just about every type of application you can think of, giving COTS designers the ability to solve myriad problems by using the same board in various systems. The solution isn't always a simple one though, as FPGA I/O requirements vary from system to system. However, the VITA 57 (FMC) standard comes to the rescue with its unprecedented mechanical and electrical flexibility. Meanwhile, IP and interoperability are also vital considerations.
Sometimes a design team will see a really nice COTS FPGA board that was just about perfect for their application – if only that front panel interface were 12 V instead of 3 V, or round instead of square, blue instead of red, or some similar tweak. Engineers who have experienced this scenario are going to love VITA 57 (FPGA Mezzanine Card or FMC). The reason: VITA 57 provides a standard specification for a small mezzanine module designed to adapt an FPGA-based carrier card to different I/O requirements.
Prior to this standard, most vendors were developing their own proprietary mezzanine boards or redesigning them to fit necessary I/O stipulations – a much more costly equation. However, the FMC specification aims to thwart these costly and time-consuming practices by providing a COTS standard for the industry. However, VITA 57 has its share of plusses and challenges. Primary areas of consideration for FMC use and development include mechanical and electrical details, and FPGA IP – along with the issue of whether you'll really be able to put an FMC from one vendor onto the FMC site of another vendor.
The FMC is defined as a small add-on module to provide physical-level I/O drivers for carrier cards with FPGAs. (Also see Sidebar 1 for nonstandard uses of FMC.) Carrier form factor support includes VME, VXS, VPX, CompactPCI, and AMC.
The underlying factor driving FMC is the complexity, and hence time and cost, inherent with developing large FPGA boards conforming to the latest industry standards, whether they be communications focused like AMC or more military-centric such as VPX. Given this complexity, there are several benefits to be gained by allowing these boards to be more easily adapted to different customer requirements with a smaller and much simpler I/O module.
This simpler VITA 57 add-on module allows customers to use the same baseboard for different applications, thus providing the advantages of using a common platform, including all of the associated development tools and supporting software. Vendors can more easily customize their COTS offerings, which should result not only in savings for the customer due to lower Non-Recurring Engineering (NRE) costs, but also in a wider choice of available boards. Finally, customers themselves could become proficient in developing their own FMCs, allowing them to focus on their specific I/O while taking advantage of COTS base boards. An example FMC is shown in Figure 1.
Mechanically, the FMC is about half the size of a PMC, at 76.5 mm (deep) by 69 mm (wide), with an I/O depth of 31 mm. With the default stacking height of 10 mm, component envelopes are 4.7 mm below and 1.3 mm above, with the I/O area allowing for 9.5 mm. Using an optional 8.5 mm stacking height, designers can move 1.5 mm from below to above. The front panel has a PMC-like bezel, though slightly (12.95 mm) narrower. FMC also includes support for conduction cooling.
The small size of the FMC could be a challenge for some applications. With a front-panel bezel narrower than PMC and so many signals available, it will be difficult to find connectors that maximize the full advantage of the FMC signals (see Electricals section). However, allowing for larger FMCs would have meant giving up more precious space on the carrier boards, so the standard attempts to strike a reasonable balance.
To help address this issue, VITA 57 defines a double-wide FMC. As is the case with double-wide PMCs and AMCs, however, we don't expect to see very many of these double-wide FMCs on the market. Double-wides tend to be used in specific programs that need the extra real estate and don't require as much modularity or compatibility with different carriers.
Using a Samtec SeaRay high-speed connector, FMC supports up to 10 multi-gigabit lanes, 80 differential pairs (or 160 single ended), clocks, JTAG, and I2C. The connector comes in low (160) and high (400) pin count versions, with the standard providing mappings for both.
For power supplies, FMC mandates 3.3 and 12 V, along with an adjustable (0-3.3 V) voltage. Maximum current on the three supplies is 3, 1, and 4 A respectively, though the module is limited to 10 W of total power.
FMC provides a standard way of adding different physical-level I/O devices directly to a carrier board's FPGA. This means FPGA IP needs to be developed in the FPGA to deal with these I/O devices. Along with the IP, there will usually need to be some form of control software.
Whereas other mezzanine modules such as PMC usually have onboard control and talk to a standard interface like PCI, there is no standard interface for FMCs because they are designed to tie directly to the carrier's FPGA pins. It's expected that a vendor providing both the FMC and the carrier board would develop and provide this IP, but it still must be done, and, of course, takes time and money. It's also expected that some FMC vendors will provide IP cores for interfaces to their modules, but they still need to be integrated into the carrier board's FPGA. This issue has not gone unrecognized within VITA; consequently, there is a working group in progress discussing it.
Interoperability: Mixing FMCs and carriers from different vendors
Perhaps the largest challenge with FMCs results from the flexibility that VITA 57 provides. With so many available signals and even an adjustable voltage supply, making sure an FMC will work on a carrier will not be as easy as with previous mezzanine modules like PMC, which had a standard PCI bus interface. The VITA 57 standard provides guidelines for mapping signals and for carriers and modules with respect to advertising features and requirements, but designers should expect to have to do some legwork when selecting FMCs and carriers. The specification itself contains a 38-question compatibility checklist as an appendix.
Even if designers find that there is indeed a match between the module and carrier, they still need to develop, or at least port, the FPGA IP and control software for the carrier, as mentioned previously.
For straightforward FMCs – for example, a fiber-optic conversion module that just connects to some multi-gigabit lanes – it should be possible to mix and match between vendors. But for complex modules, it might not be so easy. But is this really a problem? In perception, yes, but in marketplace realities, perhaps not. One would not really expect many companies to be in the business of selling just FMCs; it's more realistic to expect that carrier vendors will develop some FMCs that mate to their carriers and allow their customers to develop their own. Another possibility is that engineering design services firms start gaining expertise in FMCs and bridge this gap.
VITA 57 meets FPGA I/O flexibility requirements
FMC provides a standard method for COTS board vendors and their customers to more easily extend and adapt their standard FPGA-based products. The specification provides a lot of I/O flexibility in an attempt to meet many differing requirements, while also balancing the needs of the carrier and mezzanine. Although there might be some interoperability and IP challenges with FMC, it at least provides a method to allow for potential compatibility and reuse where previously there was none. Several VITA 57 modules and carriers have already been announced. BittWare is following suit, putting FMC sites on its new Altera Stratix IV based AMC (Figure 2) and VPX boards and will soon begin developing FMC modules.
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