How Does the FMC (FPGA Mezzanine Card) Standard Measure up ...

Page 1
Te c h n o l o g y W h i t e P a p e r
How Does the FMC (FPGA Mezzanine Card) Standard Measure up Against
Abstract
the PMC/XMC Format for Embedded Defense/Aerospace Applications?
Interest in reconfigurable embedded computing
in the defense and aerospace market has grown
significantly as new generations of FPGAs present
developers with a level of processing performance
and potential I/O bandwidth that cannot easily be
matched by conventional CPU configurations. There
are many COTS solutions that allow developers to
readily make use of FPGAs for processing, but the
real challenge to an application is often measured in
terms of I/O bandwidth, latency and connectivity. For
example, military Electronic Counter Measures (ECM)
applications require high bandwidth data input,
processing and data output with minimum latency.
FPGA Mezzanine Card (FMC) directly addresses
the challenges of FPGA I/O by solving the dual
problem of how to maximize I/O bandwidth, while
still being able to change the I/O functionality. The
elegance of the FMC solution is in its simplicity. On
FMC modules there are only I/O devices, such as
ADCs, DACs or transceivers. The modules have no
on-board processors or bus interfaces, such as PCI-X.
Instead, FMC 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 module’s host, while maintaining direct
connectivity between the FPGA and the I/O interface.
Introduction
Darwin’s theory of evolution doesn’t necessarily apply
to just the plant and animal world, as evidenced
in the embedded computing industry, where only
the fittest mezzanine card formats have survived. A
wide variety have come and gone, with only the best
formats gaining broad market appeal, with some
specializing and excelling in niche areas. Others have
been consigned to the drawing board of history. The
reasons for this are many.
Mezzanines for Rugged Computing
Perhaps the strongest mezzanine format for defense
embedded computing is PMC (IEEE 1386.1-2001[1]),
which uses the PCI and more recently PCI-X bus
(ANSI/VITA 39-2003[2]), and offers higher levels of
ruggedization defined in ANSI/VITA 20[3]. PMC has
succeeded because it has been able to evolve through
speed improvements and environmental specifications.
PMC has also been able to meet a wide range of
market needs including sufficient space to implement
useful functionality. PMC’s latest incarnation is XMC
(ANSI/VITA 42.0[4]) where the parallel PCI or PCI-X
bus has been replaced with a serial interface, of
which the most common protocol supported is PCI
Express ® (ANSI/VITA 42.3-2006[5]). Interfaces such
as PCI, PCI-X, PCI Express and Serial RapidIO ® have
evolved to address the needs of computer systems
dominated by conventional CPUs, and the need for
standard interfaces that abstract the specific details of
their hosts.
cwcembedded.com

However, as the performance barriers are pushed
back, some applications look toward FPGAs as
the only practical way of achieving the necessary
throughput, which could be beyond the capabilities of
PMC or XMC. FPGAs can also be used to implement
the necessary interfaces, so advantage can be made
out of the direct coupling of processing performance
and I/O bandwidth. This applies very well to
applications such as Electronic Counter Measures
(ECM), which can require latencies and bandwidth
exceeding the theoretical capabilities of PCI Express
(2Gbytes/sec using a x8 interface for PCI Express,
Generation 1). PCI Express or PCI-X latency can be in
the order of 1-2µS.
FPGA Mezzanine Cards (FMC)
FMC (ANSI/VITA 57.1-2008[6]) is aimed at solutions
that require the benefits of an FPGA. This standard is
showing a lot of promise in terms of longevity, as it
does not compete with PMC/XMC – the recognized
mezzanine leader for rugged computing – but rather
solves a problem for high-end applications (Figure
1 shows a typical FMC module). In fact, the FMC
specification leverages some of the goodness of the
PMC and XMC specifications. The alternative for using
FMC for high bandwidth, low latency data throughput
is not to use mezzanines at all, but a monolithic PCB,
a route that would lose the advantage of flexibility.
Page 2
Figure 2: Summary of FMC connectivity
Power Supply
Parallel or
High-speed Serial I/O
Device(s)
Analog, Digital, Fiber,
Video, FPGA/CPU,
Memory, etc.
Figure 1: Typical FMC Module (Curtiss-Wright
FMC-516 ADC [8])
Mezzanine cards for FPGA-based solutions are not
new, but they are invariably based on a proprietary
standard which means users are locked into a
particular vendor, and the evolution of the standard
is not subject to peer review. The FMC standard
has been ratified for three years now, so it is worth
reviewing how successful the FMC specification has
become. But first, it would be useful to review what
the FMC specification entails. The general connectivity
is outlined below in Figure 2 and includes a large
number of parallel and serial connections directly to a
host based FPGA.
FMC (VITA 57) Connector
Power (12V, 5V, Vadj),
JTAG, I2C, Clocks
High-speed Serial Connectivity
Up to 10 MGT pairs
Parallel Connectivity
HPC: Up to 160/80 (SE/Diff pairs)
or
LPC: Up to 68/34 (SE/Diff pairs)
Host FPGA Interface
cwcembedded.com

The purpose of the FMC specification is to allow
(usually) one FPGA on a host card to connect directly
with the I/O devices on the mezzanine module – just
as if the device were on the host board. Busses like
PCI-X are redundant and would get in the way of the
FPGA and its I/O devices. This intimacy means the
interface can be optimal and savings can be made in
real estate, cost and power, while boosting bandwidth
and reducing latency.
An FMC is similar in height and width to a PMC,
but around half the length. The reduced width,
compared with PMC or XMC, allows up to three
FMCs to be fitted to a 6U host. The FMC specification
has a default stacking height of 10mm, but permits
a stacking height down to 8.5mm for low profile
solutions.
The majority of FMC host/carriers use 3U/6UVPX,
VXS and AMC formats but there are also PCI Express
solutions such as the Xilinx ® ML605 Virtex ® -6
evaluation card[7].
The FMC specification provides for a large number
of differential connections – up to 80 pairs or 160
single-ended signals – to support high speed parallel
interfaces between the FPGA and I/O devices. There
are also a number of serial connections (up to ten
pairs) suitable for Multi-Gigabit Transceivers (MGTs)
operating up to 10Gb/s. FMC modules and hosts
support two connector options; a Low Pin Count (LPC)
160 pin connector or High Pin Count (HPC) 400 pin
connector. The majority of FMC solutions are likely to
use the HPC variant.
Although aimed at I/O, FMC can be used for any
function that might connect to an FPGA including
DSPs, memory or even another FPGA.
Connectivity
Connectivity for FMC modules is unusual in that only
the upper limit of active connections is defined, as
opposed to the number of active connections. This
means that host carriers need not provide the same
number of FPGA signals as another host. This matters
only in defining a given host’s ability to support
certain FMC modules. To fully populate an HPC
Page 3
solution may require a large FPGA, so reduced pin-out
offers cost sensitivity. This is something to be aware of,
but the specification defines the signals to populate the
LPC or HPC connector starting at a given position and
add to the connector in a given sequence, such that
if two hosts provide x signals, they will use the same
connector pins and be compatible.
Power Supply
For power supply requirements, the FMC specification
employs a useful trick, at least for the FMC: the
host detects what the FMC’s power should be on its
primary power rail and provides it. This is achieved
through the host interrogating the FMC’s E2PROM,
coupled with an adjustable power supply. The benefit
to the FMC is a simplified power requirement, thereby
freeing up valuable real estate for more I/O on the
FMC.
Usable Printed Wafer Board (PWB) Real
Estate
Although occupying around half the PWB area of an
XMC, the FMC can sometimes achieve greater I/O
functionality, most notably for rugged applications.
If the solution requires a large FPGA and if the XMC
module complies with the VITA 20 specification, there
are restrictions as to where the FPGA can be located.
In turn, this may limit the available area to fit the I/O
devices. Consider an actual example with a pair of
designs using the same I/O devices for a rugged
application; one using an XMC format card and
one using an FMC format card (Figure 3). Since the
rugged XMC specification requires an area across the
middle of the board to mate up with a host stiffening
bar (which doubles as a primary thermal interface
on conduction-cooled variants), a large FPGA (for
example 35x35mm) invariably needs to be fitted
to the area of the circuit board closest to the front
panel, just where the design would want to fit the
I/O devices. The useful space in which to fit the I/O
devices is perhaps a quarter of the overall real estate
of the XMC and not very efficient.
In comparison, the FMC, though around half the size
of the XMC, has a far greater real estate area for
the I/O devices. In this example, the FMC is able to
support two ADCs for two 3GS/s channels compared
cwcembedded.com

to the single channel of the XMC. Of course an XMC
using a smaller FPGA, or not restricted by the rugged
XMC specification, may not be affected to such an
extent, provided it still has a sufficient number of I/O
connections to the devices. An FMC may be smaller,
but it may still be able to support greater functionality
than its larger XMC equivalent.
Figure 3: Rugged XMC with large FPGA real estate
example comparison with an FMC
Page 4
Cooling
Cooling rugged FPGA-based XMC cards can be
a significant challenge as such boards can easily
exceed 20W power dissipation. Typical rugged aircooled
specifications for such cards define upper
air-inlet temperatures of 70°C and conduction-cooled
cold walls of 85°C. Making a mezzanine work within
this environment with much lower power levels is
already difficult. To compound this, XMC hosts often
have two XMC sites. Consider the size and orientation
of an XMC. When plugged onto a 3U host card,
such as 3U VPX, the XMC covers the majority of
host’s real estate which means, if there are any hot
devices on the host, they are under the XMC. This is
not ideal and can seriously affect cooling. The XMC
mezzanine’s devices face down onto the host, not the
outside, placing the heat generating devices opposite
those on the host and compound the cooling problem
further. To cool the XMC, the air needs to be squeezed
between the host and mezzanine which can be a
very small cross section, thus limiting the volume of air
available to cool the assembly. Conduction cooling is
less difficult but having all the heat generators in one
plane is still a problem as there may be hot spots.
A 6U solution is not really any better; some of the
host’s real estate is not covered by mezzanines, but
the thermal paths to either the cooling air inlet or cold
wall interface are longer.
Cooling is another advantage for FMC based designs.
An FMC, being smaller than an XMC, ensures that
a larger amount of space on the host carrier is not
covered by the mezzanine itself. Appropriate FMC
host design allows for suitable heat sinks to be
implemented in the areas not restricted by mezzanine
placement. Superior cooling with either greater airflow
or greater heat sink cross sectional areas may
be the factor that determines whether the solution is
viable. In addition, as the FMC has no FPGA, only
I/O devices, the FMC will be easier to cool as well –
especially if the devices are not above a host’s thermal
hot spot (see Figure 4). The FMC specification limits
the power dissipation of a single width module to
10W.
cwcembedded.com

Figure 4: 6U VPX FMC host with two FMC sites (and
4x FPGAs)
Key Benefits of FMC Compared to PMC/
XMC
Latency
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. In this case no
busses, not even between FPGAs, are required. The
only delay is the FPGA processing which can be
as low as a few nanoseconds. This type of solution
is ideal for an application like Electronic Counter
Measures (ECM) and Electronic Support Measures
(ESM) where processing response time is critical.
Page 5
Bandwidth
XMCs might get around the bandwidth problem,
compared with FMCs, through decimation or using
newer generation serial interfaces operating at ever
higher speeds. However, some applications may
not be able to tolerate this reduction in data off the
mezzanine. Beam-forming applications may fall
into this category where high bandwidth data, from
potentially a large number of channels, must be
shared between processing elements. Therefore, high
bandwidth beam-forming is another good application
area for FMC technology because it would not suffer
from data reduction problems.
Simplicity
FMCs by nature promise a faster development
turnaround 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 high resolution devices come
onto the market. This is an advantage for Software
Defined Radio (SDR) applications. There is a great
deal of flexibility afforded by implementing different
modulation and coding schemes which 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. FMCs and FPGA
processing make an ideal platform for a wide range
of digital receivers.
Flexibility
RADAR has a thirst for increasing resolution and
bandwidths, and benefits from direct RF digitization
for coherent sampling. Lower bandwidth solutions
require IF stages and I/Q sampling which introduces
noise. FMC provides the easiest format to upgrade
the platform as new faster ADCs become available.
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 quick and easy.
cwcembedded.com

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.
See Figure 5 for example applications mapping onto
different latency and bandwidth requirements.
Limitations
Finite Connectivity
No mezzanine specification is perfect because its
target markets are usually varied, and therefore a
compromise. If the compromises are few, then these
imperfections can be overlooked. Supporting up to 80
differential signals, it is easy to conceive of parallel
interfaces providing data throughputs in excess of
10GB/s and largely limited only by the host FPGA’s
capabilities. However, although 80 differential
signals represent significant connectivity, it is finite.
A monolithic design, not using a mezzanine, could
provide more than 80 FPGA pairs to connect to the
devices on the FMC. The practicality of FMCs is down
to the host FPGA and I/O devices. An example is a
3.6GS/s 12bit ADC, which exists today and might
use 1:4 multiplexer to allow it to be interfaced to an
FPGA. Such a device would require 48 LVDS pairs,
probably clocked at 450MHz DDR. This would use
more than half of the FMC’s connectivity for parallel
connections for the data path alone, on top of which
control signals would be needed. In this example only
a single ADC device could be implemented on an
FMC even though there may be sufficient space and
power budget to fit on a second part. However, by
comparison, the performance bandwidth for FMCs
over XMCs is considerable.
Page 6
Front Panel I/O
FMC is a front panel only format, unlike XMC and
PMC which define user I/O signals that might be
routed to a backplane for convenience. However, this
is rarely used for analog signals where noise might
be introduced through such routing. Mechanically,
the FMC front panel width is slightly narrower than its
XMC counterpart, leaving less room for connectors.
FPGA Centric
By its very definition, FMC requires an FPGA. This
means the FMC will never achieve the universal
appeal of PMC or XMC, but for applications that
require such levels of performance as provided by an
FPGA, this is of minor consideration. Today, there are
relative few FMCs and FMC host carriers, where PMC
and XMC have built up an extensive portfolio number
in the 100s or even 1000s with proven track records.
Cross-Vendor Compatibility
By definition, software and HDL for FMCs is defined
by the host. And because the FMC is controlled by
an FPGA, there is no real concept of a driver. So
although FMCs from different 3 rd parties will fit together
electronically and mechanically, there will be differences
in the host’s environment leading to incompatibility.
However, within the FPGA world, electrical and
mechanical compatibility are of primary performance.
HDL is often user specific and often not an issue.
FMC Market Traction
The FMC format is gaining momentum. There are
already more than fifteen vendors providing FMC
modules, host or complete combinations. The majority
of FMC solutions on the market today are high speed
analog input or output with good coverage of channel
density, resolution and ruggedization levels. This is not
surprising because this is an area that requires raw
bandwidth and FPGA processing. In addition, there
are now approaching a hundred products covering
functionality such as tuners, digital I/O, fiber-optics,
10GEthernet and even FPGA co-processors. There
is even product supporting system busses such as
1553. Analog input products include solutions up to
250MS/s 16-bit for high resolution, and 5GS/s (10b)
for high speed capability.
cwcembedded.com

© Copyright 2011, Curtiss-Wright Controls
All Rights Reserved. MKT-WP-FMC-052611v1
Summary
The choice of which mezzanine format is best
for rugged embedded computing solutions will
ultimately be down to issues such as application
details, perception of risk, development timeline and
personal preference. The baseline for choosing which
mezzanine is most suitable for certain applications is
how it compares with a monolithic board (i.e. a single
PWB with all functionality onboard). A monolithic card
usually provides the best technical solution because it
does not have the restrictions imposed by segmenting
the design, such as number of connector I/O pins to
the mezzanine.
While not absolute, Table 1 below provides a first
cut assessment as to which technology scores best
against different parameters. However, differences in
the requirement from one application to another may
affect these conclusions.
Table 1: Relative comparison of mezzanine
capabilities
PMC XMC FMC Monolithic
Bandwidth a aa aaa aaa
Latency a a aaa aaa
Installed base of
users
Time to design new
board
Power dissipation,
ease of cooling
Note: (aaa best)
Page 7
aaa aa a aa
aa aa aaa a
a a aa aaa
Figure 5: How typical applications map onto
bandwidth and latency requirements
FMC Continues to Evolve
Although FMC is now a published standard, there are
some parallel specifications in development within the
VME Industrial Trade Association (VITA). These deal with
abstractions to simplify integration into systems such as
FPGA driver concepts. When ready, these additional
standards will strengthen some of the perceived weakness
with FMC. FMC is ready now as an electro-mechanical
solution that will serve a wide range of applications now.
Conclusion
FMCs promise to do for FPGA based solutions what PMC
and XMC did for embedded CPU based systems. However,
FMC is not really competing with PMC or XMC, but actually
complements it, particularly for high bandwidth, low latency
applications.
References
[1] IEEE 1386.1-2001 Standard Physical and Environmental Layers for PCI
Mezzanine Cards
[2] ANSI/VITA 39-2003: American National Standard for PCI-X Auxiliary
Standard for PMCs and Processor PMCs.
[3] ANSI/VITA 20 (R2005): American National Standard for Conduction
Cooled PMC
[4] ANSI/VITA 42.0-2008: STANDARD FOR VITA 42.0 XMC
[5] ANSI/VITA 42.3-2006: American National Standard for XMC PCI
Express Protocol Layer Standard
[6] ANSI/VITA 57.1:-2008: American National Standard for FPGA
Mezzanine Card (FMC) Standard
[7] Xilinx ML605 Virtex-6 evaluation card: www.xilinx.com
[8] Curtiss-Wright Controls Embedded Computing web site
www.cwcembedded.com
All other brands and names are property of their respective owners.
cwcembedded.com