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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