It’s likely that your current designs
have you pushing the proverbial
envelope. If so, then high-speed serial
interfaces are the way to go. Their overall
bandwidth beats their parallel counterparts.
Also, the newer technologies
offer plenty of other benefits, such as
lower pin counts and hot-swap support.
The clear leader is PCI Express
(PCIe), followed by Ethernet, Serial
RapidIO, and InfiniBand. HyperTransport
remains a chip-to-chip link, even
though board standards are defined for
it. PCI Express has edged out AGP, and
PCI is quickly disappearing on the PC motherboard, just as
PCI pushed ISA into near oblivion. Still, the compatibility
between PCI and PCIe makes PCIe much easier to support.
Serial links also have significant advantages outside the box.
External PCI Express (ePCIE) is finding homes in a host of
applications, from box-to-box links to external peripherals.
Likewise, eSATA is an alternative to USB and IEEE 1394
when it comes to storage. In terms of backplane standards, only
USB has found a niche at the low end. Storage connections
tend to wind up in specialized environments.
GOING ALL SERIAL
Established parallel bus standards remain the mainstay, but
serial alternatives exist with the same form factors (see “Serial-
Parallel Alternatives” at www.electronicdesign.com, Drill Deeper
18798). Only the connectors are different, giving designers a significant advantage since it’s often
possible to mix boards with the proper
backplane. This is typical on the PC side,
where motherboards have a collection of
PCIe, PCI, and, sometimes, ISA slots.
These often show up in motherboards
targeted at embedded environments
where legacy boards abound.
The high-speed serial interfaces
each have their niche with minor overlap,
typically involving PCIe and Ethernet.
Interestingly, from a backplane
point of view, the wiring and connector
requirements for PCIe, Ethernet, Serial
RapidIO, and InfiniBand are essentially identical, as are the
serializers/deserializers (SERDES) used to implement them.
The SERDES are found in FPGAs, which is why many serial
backplane standards support a range of serial interfaces and
why a single FPGA board can support any of these standards.
The convergence of board and connector form factors contrasts
with the partitioning of products based on the backplane
interface. The rooted-tree nature of PCIe is great from a compatibility
standpoint, but it means alternatives like Ethernet,
Serial RapidIO, and InfiniBand are needed for more networkstyle
connectivity. PCIe can support multiple peers, though
architecture, overhead, and legacy support tend to get in the
way of turning it into a fabric backplane.
The switch to serial has also made a difference in the move
from 6U to 3U form factors. Of course, increased integration
and higher-performance chips have played a part as well. But
to take advantage of these advances, the smaller boards need off-board throughput more than
they did than in the past.
Developers continue to innovate,
but the new high-speed serial interfaces
are up to the challenge. Tom Cox, executive
director of the RapidIO Trade Association,
notes that members are quite comfortable with the
performance of the RapidIO Specification
1.3, even though 2.0 has been approved and
3.0 sits on the drawing board.
Performance remains an issue as evidenced
by the x16 PCIe video interface. But
the range of requirements in the embedded
space often makes even x1 PCIe overkill. This does illuminate
another key advantage of serial interfaces—they’re scalable, allowing
designers to jump from x1 to x2 to x4 and so on without needing
to move up to higher link transfer rates.
The combination of interfaces and board form factors leads to a
large number of options, though in practice, only a few areas compete
directly. For example, the 3U and 6U CompactPCI/Compact-
PCI Express form factors match up with the VME/VPX/VXS standards,
but established use of the parallel versions often dictates the
choice of the serial versions. Things are getting a little more interesting
with serial interfaces when it comes to stacking standards.
STACKING UP STANDARDS
The 2008 Embedded System Conference last month in San Jose
was the site of two key announcements from the PC/104 Embedded
Consortium and the Small Form Factor SIG. Both look to
bring PCI Express to the stacking, small-form-factor arena built by
PC/104 (see the table). They both use a high-performance connector
like Samtec’s SUMIT (Stackable Unified Module Interconnect
Technology) with a 0.6-in. height that matches the PC/104 standard.
The PC/104 Embedded Consortium defines two standards:
PCIe/104 and PCI/104-Express. Like PC/104 and PC/104-Plus,
the new standards designate a single- and double-connector implementation.
PCI/104-Express maintains the PC/104-Plus PCI connector,
replacing the ISA connector with a PCI Express connection.
As a result, the new boards can be combined with new and existing
PCI-104, PC/104 boards that have only the PCI connector.
The PCI/104-Express stacking system (Fig. 1) resembles the system
originally defined for EPIC Express, the precursor to this standard
(see “EPIC Express Rides The Rails” at www.electronicdesign.com, ED Online 14190). This allows point-to-point PCI Express
connections to be routed to boards in a stack.
The PCI-104 and PC/104 boards don’t have this problem because
they utilize a bus. With PCI Express, each board uses the first PCI
Express connection and shifts the connections so the next board in
the stack sees the next unused connection as its first connection.
The shift only works if there are enough connections at the outset.
The number depends on the types of connectors that are used.
A full-featured system can support an x16 link or a pair of x4 links
and four x1 links. The approach essentially puts PC functionality
in a PC/104 form factor. This is comparable to what the original
PC/104 standard did, followed by PC/104-Plus.
The Small Form Factor SIG
Express104 employs a similar
stacking approach, but the connections
aren’t limited to PCIe. It
uses the SUMIT standard connector, even
though SUMIT is the connector standard while
Express104 is an instance of a standard that uses SUMIT.
Expect more standards from the Small
Form Factor SIG based around SUMIT in
the near future.
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