Mezzanines continue to evolveStory
February 21, 2011
SBC designers need to anticipate both a large number of mezzanine vendors and future growth in performance and capability, and must address overspecification and unused resource issues to create a standard, long-term future.
The ability to customize the functionality of off-the-shelf embedded computing modules by the addition of mezzanines has become an essential element in the battle to get the most from the least space at an affordable price. Mezzanine standards abound, one of the most common being “IEEE1386.1, PMC/XMC,” which can be used with many types of architectures such as VMEbus, VPX (VITA 46), and CompactPCI, as well as the desktop PC using PCI carrier cards.
The PMC standard was introduced more than 15 years ago and offers an ideal amount of real estate. It has been continuously improved with greater connectivity, an extended PCI bus, better power dissipation and, most recently, support for high-speed serial signaling such as PCI Express between mezzanine and base card. Continuous improvement has yielded a quadrupling of the PMC’s power dissipation capability compared to the original IEEE1386.1 specification, as more functionality and performance get packed into its limited space.
There are many different military applications for PMC modules, ranging from flash storage, avionics bus interfaces, and general-purpose I/O, to Network Interface Controllers (NICs) and switches to sensor processors. Some of the most challenging of these PMC applications are sensor front-end processors using either an FPGA or the emerging GPGPU-based processing arrays. Using these types of devices pushes the boundaries of power dissipation. Typically, such a sensor processing PMC incorporating, for example, a Xilinx Virtex-5 FPGA and high-speed analog I/O will dissipate almost 30 W. These types of front-end processors are often used as digital downconverters, video processors, or radar pre-processors. With the addition of analog O/P, a complete Electronic Support Measures (ESM) subsystem can be configured on a single PMC module, in many cases able to operate without runtime processor intervention.
Multiple PMC/XMC modules
Because PMC/XMC modules offer such efficient use of space, subsystems often get configured with more mezzanine modules than can be fitted to just one SBC. A 3U-sized SBC is limited to one mezzanine slot, whereas 6U accommodates two. Carrier cards provide the means of adding further mezzanine slots within a chassis, but must be connected to the SBC via PCI, PCI-X, or PCI Express, generally using the SBC’s PMC/XMC site to make this connection. In a VPX environment, adding further XMC modules has been simplified. They can be plugged directly onto a passive carrier card into any backplane slot supporting PCI Express. However, where PCI Express is not so readily available, other configurations and permutations might require PCI bridges, for example, to support a PCI Express mezzanine in a PCI-X environment.
The myriad embedded products vendors supporting PMC/XMC mezzanines recognize the ways that the original specification has been stretched to accommodate today’s levels of performance and functionality. This is reflected in their SBC designs, which account for the additional power needs and provide well-managed signal routing and noise containment through the use of best practices. This is not always the case in the prototyping laboratory where critical PMC/XMC modules might be incorporated into a desktop PC environment using a PCI carrier.
Compared to a rack-mounted modular system, a desktop’s PCI and PCI-X slots can be electrically noisy and lack sufficient power or heat management to support leading-edge PMC/XMC modules. In this case, the PCI carrier card can be vital to remedying these shortcomings. One viable example of this is the soon-to-be-announced short form factor PCI Express carrier card from GE Intelligent Platforms (Figure 1) supporting Gen2 x8 PCI Express to the host and including cooling for high-power PMC or XMC modules.
Figure 1: Short form factor PMC carrier card for PCI applications from GE Intelligent Platforms
(Click graphic to zoom by 1.9x)
New mezzanine proposals
Just as VPX provides a step increase in capability compared to its VMEbus forebear, significant evolution of mezzanine standards will be needed to maintain future technology leadership. A VITA 71/Rugged Mezzanine working group has been formed to evaluate and create standards for a new generation of mezzanines for embedded computing users. Based on the considerable industry experience gained with the PMC/XMC standards, significant advances can be expected:
- Increased interoperability among vendors
- Improved connectors with signaling rates to 12 GHz
- More efficient space utilization by reducing connector sizes and keep-out areas
- Fewer power rails
- Greater power dissipation
- A wider range of sizes
The standardization of mezzanine specifications will continue to be challenging as SBC designers need to anticipate a multiplicity of mezzanine vendors plus future growth in capability and performance. The result can be overspecification or the provision of many unused resources to meet every eventuality. VITA 71 needs to address these issues to level the playing field, yet at the same time leaving scope for the continued innovation and evolution so essential to a standard’s long-term future.
To learn more, e-mail Duncan at [email protected].