USB, SPI, SMB, and LPC also must be
considered with PCIe. They target lowerend,
peripheral complements that may or
may not contain PCIe devices. This actually makes PCIe a better
replacement for PC/104. Moreover, the PCIe/USB combination
permits support for ExpressCard devices.
The standard also allows processor boards to implement a
subset of the interfaces, making the standard very interesting
from a microcontroller perspective. That’s because many microcontrollers
don’t have a PCIe interface, but they do support USB,
SPI, SMB, or LPC.
Express104 is an interesting combination that mixes bus interfaces
(SMB and LPC) with point-to-point interfaces (PCIe and
USB). The signal shifting on a board for USB depends on the
interfaces required by the board. Express104 also has different
board sizes, though a PC/104-size board is in the mix. The smaller
boards enable the stacking architecture to go into places where
PC/104 will not fit.
The Small Form Factor SIG and the PC/104 Consortium standards
overlap in their target audience, but they also address different
arenas. Express104 is I/O-oriented with a nod to the mobile end
of the spectrum. It can handle almost any peripheral used for data
acquisition except for high-end video peripherals, though it can easily
handle video with its PCIe links.
The PC/104 Consortium is looking toward the high end
where high-performance video needs the x16 bandwidth. In the
not too distant future, don’t be surprised to see a combination of
the two in a single system.
Both PCIe standards will require new processor boards. The
mix of peripheral boards will change as well. In fact, it will even
be possible to build a board that mixes the two standards. Still,
PC/104-Plus single-board computers like VersaLogic’s Cheetah
will make up the bulk of shipments in this space for the next
couple of years (Fig. 2).
Don’t expect to see InfiniBand and RapidIO in this space, but
Ethernet, including Gigabit Ethernet, is quite common. The big
difference is that Ethernet is used to link an end node to a network
versus the backplane fabric found in new rack-mount systems.
RACKING STANDARDS
Based on Motorola’s VERSAbus back in the 1970s, VME has
one of the longest track records in the business. Since then, the
system has grown to the 64-bit VME64. The 2eSST protocol
found on VME320 systems boosted performance—though the
320-Mbyte/s throughput is high, high-speed serial links deliver
better performance.
CompactPCI’s history isn’t as long as VME, but it has a similar
3U and 6U form factor. It showed up in 1995 as PIGMG’s PICMG
2.0 standard, based on the parallel-bus PCI architecture.
Both CompactPCI and VME are now found in rugged and
military applications. They’ve also served as the backbone for a
range of industrial applications. Both have complements in the
test arena with board standards such as VXI and PXI. Likewise,
each has moved into the high-speed serial space.
AdvancedTCA and its little brother MicroTCA are more recent
entrants in the board space. Designed for carrier-grade communications
applications, AdvancedTCA is a larger form factor that’s
slowly emerging into other embedded application areas. Elma
Bustronic offers an AdvancedTCA backplane that implements a
dual-star serial fabric for redundancy not typically found in PCbased
systems (Fig. 3).
MicroTCA is based on the Advanced Mezzanine Card (AMC)
standard. AMC cards come in a range of sizes and can be found
in AdvancedTCA carrier boards as well as MicroTCA racks.
PICMG 3.x standards address the AdvancedTCA, MicroTCA,
and AMC architectures.
The big difference between AMC and the VME/CompactPCI
systems is the backplane. VME and CompactPCI use a parallel
bus. Of course, neither of the standards organizations for these
platforms has remained idle. For instance, PICMG 2.16 incorporates incorporates
Ethernet into the backplane, while the newer CompactPCI
Express standard blends PCI Express with the CompactPCI form
factor. It’s even possible to use the Ethernet support without the
PCI or PCI Express side, depending on the application.
In a similar vein, VXS and VPX build on the VME tradition with
a range of serial fabrics. The VME International Trade Association
(VITA) defines the VME, VXS, and VPX standards. VPX comes in
3U and 6U form factors and uses only serial interfaces for backplane
communication. VXS (VME Switched Serial) is essentially a blend
between VME’s parallel interface and VPX’s serial interface.
VPX was adopted for a growing number
of applications using boards such
as Curtiss Wright Controls Embedded
Computing’s CHAMP-AV6 (Fig. 4). Like
AdvancedTCA and MicroTCA, the VPX
and VXS standards define a range of serial
fabric support.
Mezzanine cards for the VME and
CompactPCI factions haven’t turned into
another backplane standard like AMC
and MicroTCA, though they do tend to
overlap. PMC (PCI Mezzanine Card) and
XMC (Express Mezzanine Card) sockets
can be found on a range of VME and CompactPCI families of boards. XMC
supports PCIe, but, as with the other serial
board standards, it also can handle interfaces
such as Serial RapidIO.
Given the SERDES standardization, it’s
not surprising that this is true. Of course,
moving between standards and off-the-shelf
products is another matter since demand
usually drives availability. Something like
an InfiniBand XMC board is likely to be a
rare commodity. On the other hand, Infini-
Band boards with a PCIe link through the
XMC connection do exist.
KNOW YOUR OPTIONS
Surprisingly few choices exist when it
comes to form factors. 3U and 6U boards
are the most common in the board space,
and CompactPCI, VME, and MicroTCA
offer designers a large number of options
that’s only exceeded by what’s available in
the PC/104 and PC space.
Adding in the serial fabrics quickly
raises the number of combinations significantly,
but in practice many solutions
wind up targeted at specific markets.
InfiniBand settled into a supercomputer
niche. Meanwhile, Serial RapidIO garnered
much of the interest for designers
in the high-performance military space
as well as in midrange communications.
AdvancedTCA remains the carrier-grade
communications alternative.
PCI Express and Ethernet persist as the
mainstays for low- to mid-range solutions
from a backplane perspective. Both have
significant limitations, as well as significant
advantages. For example, legacy support
is key to the success of PCI Express,
yet the rooted-tree architecture makes
peer-to-peer communication an interesting
exercise that isn’t as easily expandable
as RapidIO or InfiniBand.
Likewise, Ethernet offers the advantages
of compatibility across its range and
familiarity to developers because of its
extensive use in networks. Ethernet interfaces
are also integrated onto most higherend
microcontrollers or the interface support
chips of high-end microprocessors.
Downsides include power consumption,
latency, and overhead on the network
and the processor. TCP offload
engines (TOEs) help, but they usually
aren’t found in the low-cost Ethernet
interfaces in most microcontrollers.