Packaging competes with performance, cost, modularity as a top design consideration

While these segments also have increased need for I/O, higher processing speeds, and increased system bandwidth as well as lower cost, the upgrade solutions that meet thse conflicting requirements, also pose new challenges for system packaging. Issues such as increased cooling demands, cabling-particularly fiber-and increased power utilization all pose challenges for the space-limited packaging requirements of embedded controllers.

Being embedded within a larger piece of equipment, controller packaging must frequently be adapted to unusual mounting schemes and size constraints.And many control board applications require the addition of cooling for the controller chassis. Given the constraints on airflow, particularly intake/exhaust options, this can often be one of the most challenging design tasks.

Increased system performance usually means increased power density. A typical VME64x or compactPCI system can dissipate about 30-50W per board. Some of the new CPUs are targeted at 120W per chip. These CPUs will cause to the power density to go beyond what can be cooled with forced air. Spray cooling or conduction cooling is a reasonable alternative for very high power systems.

These cooling methods are also appropriate when systems are located in harsh environments and the air is contaminated with particulate material or corrosive elements. Liquid cooled VME64x or compactPCI systems could exceed 150W per board. The VITA-34 Draft Standard group is working on a replacement for VME64x that will support liquid cooling through the backplane with detachable connectors.

In general, embedded controller boards are powered from the main power source in the equipment into which the controller is embedded. This eliminates the need for a separate controller power supply but complicates the power wiring. Provisions for power cabling must be incorporated into the packaging design.

When considering alternative bus and board configurations, whether shared bus such VME, PCI, compact PCI or one of the modular mezzaine buses or some of the newer switched fabric alternatives, such packaging issues need to be addressed by the developer.

With more than 20 years of successful design-ins, VME has long been a dominant factor in the embedded controller market. Although it has been more than a decade since the introduction of VME64, VME still continues to be a strong force in the embedded controller market. Long a favorite in the industrial control and military/security markets, the stability of VME component designs (unlike the constant change in the PC world), has made VME the workhorse and the de facto standard. But, new options are emerging to support next generation designs, including VME64 extensions and 2eSST (2 Edge source synchronous transfer).

VME64 considerations
Introduced in 1997, VME64 extensions adds features such as 3.3V power, geographic address, shielded front panels, ESD control, and RTMs, while maintaining the same physical form factor (6U x 160 mm board). VME64 Extensions has been widely used in the military market, particularly for upgrades from VME64.

The demand for commercial- off-the-shelf (COTS) equipment continues to increase and VME64 extensions offers the ability to increase performance, often with only limited software upgrades required. Maintaining the same form factor also facilitates field upgrades in existing equipment, an important consideration when considering an upgrade or a switch to VME64.

The injection force of a VME64x board is approximately 60% more than a VME64 board. A new injection/ejection handle was designed to address the higher forces. The new handle requires modifications to the subrack to the support for injection/ejection capability. The injection/ejection handles provide not only injection capability, but also support locking, and keying-important features for many military applications.

Adoption of the 2eSST protocol within VME adds new upgrade options for VME users. Motorola is supporting this protocol with the development of new bridge chips that must be combined with new transceiver chips from Texas Instruments. Anticipated improvements in system performance resulting from the adoption of this approach are approximately a two times increase in bandwidth-all without having to change the system backplane.

CompactPCI offers designers a number of packaging advantages over VME, particularly when pin density is an issue. With the increasing demands for I/O and customer-specific bussing, compactPCI's high pin density (565 pins in a typical 6U configuration vs. 415 for VME) provides opportunities for increased performance without increasing the size of the packaging. Like VME, compactPCI boards are typically 3U or 6U high and 160 mm deep. However, there are 145 more user-defined pins in compactPCI than in VME.

Another important advantage to compactPCI is the availability of software options from the PC world. Long a mainstay in PCs, PCI bus and the associated silicon and software offers developers the opportunity to leverage existing software from a PC-based solution. Companies such as Analogic and National Instruments have extensive ranges of I/O, data acquisition boards, etc. for both PCs and compactPCI.

Applications such as medical imaging - another strong growth area for embedded controllers - require increased processing performance for analysis, as well as instrumentation control. For applications such as CT and PET scanners, not only are there greater and greater amounts of imaging data to process, but the demands for enhanced control of the tables and imaging devices is also key. compactPCI's processing performance can provide the solution - with future opportunities for performance upgrades.

One of the challenges in compactPCI applications is handling of cabling. Often, the cabling is attached to the front of the compactPCI boards and must be carefully routed from the boards to the associated equipment. Cable routing has become increasingly challenging-especially with the incorporation of fiber cables and with the limitations on bend radii on these cables.

VME and compactPCI are widely used platforms for embedded controller applications.

In addition to a well-defined mechanical and electrical interface, both platforms offer flexibility in configuration, scalability, and a strong software support base. The wide range of available third party products is also key to implementation of solutions in a cost effective manner. The challenge is to address the unique packaging constraints posed by an embedded application.