Developing a platform for space-constrained applications that meets rigorous demands for environmental compliance and rugged performance presents a challenge, especially when faced with a short development window. Using a three-board stack approach, designers in this case study demonstrate how a Pentium M PC104 module with a custom-designed carrier board fits the bill for this type of system.

The approval certifications needed in the equipment supplier's particular market segment call for strict adherence to high standards in areas such as vibration, temperature, humidity, and EMC. Table 1 shows a list of the various system requirements. Given these constraints, designers had to develop a compact, high-reliability system with scalable performance and a number of special interfaces that complied with international market specific directives as well as RoHS, UL, and other similar regulations.

Other demands included service-free lifetime, passive cooling, and low-priced entry-level models. The product also had to be tested and launched within a short timeframe at competitive cost levels.

Normally, high reliability and low cost demands result in a full custom design. But in this case, the wide range of performance requirements for different applications, from the lowest-possible 200 MHz control function unit up to a high-performance 1.4 GHz Pentium M LV, could not be handled by simply changing the CPU speed and memory sizes. Therefore, designers opted for a separate CPU module and carrier board design; even though interconnect costs added to the entry-level unit. Subsequent volume increases for each model could provide incentive for conversions to full custom designs of certain models in the future.

Basic demands such as passive cooling and a fully EMC sealed enclosure for wall or cabinet mounting resulted in a design approach with a three-board stack solution (Figure 1). In this design, the CPU module is mounted at the bottom of the stack with passive heat transport through the bottom plate, utilizing the specified mounting requirements on cabinet walls.

The top board is equipped with external interface connectors, EMC ground plane and filters, overvoltage protection, and isolation components providing fully galvanic isolation and protection for external serial and fieldbus interfaces. Changing the top board and enclosure faceplate added design capabilities for other models with different interface demands, such as heavy-duty connectors or fiberoptic interfaces.

Because COM Express boards with entry-level to mid-range alternatives were not available. Designers determined that ETX and PC104 computer boards were the best CPU module options. These alternatives provided the same performance range and price levels. The five-year life-cycle requirement was not an issue because these board types have full BIOS and life-cycle control capabilities.

Though PC104 does not typically support docking to carrier boards, mounting all I/O interface connectors in a way that enables docking to carrier board and using a thermal distribution plate alleviated this problem.

Two different PC104 computer boards with board-to-board connectors achieved the required 300-1,400 MHz performance range. A 300 MHz Geode board served as the low price entry model, a Celeron M 1.0 GHz ULV served as the mid-range performance model and a Pentium M 1.4 GHz served as the high-end model. Tests were also performed with a Pentium M 1.8 GHz CPU, but its higher power dissipation reduced the total system operating temperature range to an impractical level. Because the difference between the 1.4 GHz CPU's performance and that of the 1.8 GHz model is minimal, the latter model was not released.

Different tests performed on the heat spreader and rear-plate materials resulted in a selected Al-alloy with high thermal conductivity and a thin, high-quality heat grease. The larger rear plate was mounted toward a wall or cabinet that provides good heat transportation.

All environmental and mechanical tests proved successful the first time, though to pass the EMC qualification tests, designers had to make some minor adjustments to sensitive market specific RF bands. The high-speed 1.4GHz CPU did not cause any EMC problems because the PC/104 CPU module's PCB design has ground layers on the top and bottom of the board that reduced emission and minimized the need for extra design work.

Because the described hardware platform was intended to different control applications with fieldbus communication using small real-time kernel and larger applications with TCP/IP communication and Human-Machine Interface (HMI) graphics, designers had to adopt Linux distribution. With power-supply interrupts shutdowns, the application environment did not suit the standard Linux file system, so designers used the Helix toolchain kit for downscaling and ROM image generation.

Designers fine-tuned Linux drivers for all different system interfaces to work with different CPU module alternatives and provided training on handling the Helix toolkit for application software development and support. Abstraction layers were added on top of the operating system, and the total system enabled developers to handle application software without having to know different hardware models or labouring with platform software.

Read more about Helix, toolkit for Linux configuration »

For embedded systems in demanding applications, many small decisions can either make a project successful or cause technical problems. The most important things to consider when planning a complex system design are the power dissipation and heat transportation method and the EMC shielding and filtering method. In many projects, these aspects are difficult to combine but must be solved in a satisfactory way before the rest of the requirements can be fulfilled.

One commonly occurring problem in a system design with many serial interfaces is earth currents for nongalvanically isolated ports. Multiple internal DC/DC converters and optical isolators add to the heat loss; however, magnetic isolators with lower power dissipation can remedy this.

Unit cost can be kept low using enclosures for effective EMC countermeasures and heat transport. Instead of using several interface filters with other types of expensive cooling solutions like heat pipes or fans, the described PC/104 board-to-board system enclosure design solved these issues at a low cost. Periodic service was not needed, and without any moving parts, the system could maintain the low operating temperature through the course of its life cycle.