Chiprack modular electronic stacking system

Embedding the PC

Over the years, embedded computers have become a universal feature of everyday life.

We have been working on the development of design techniques to facilitate the development of new embedded computing systems. A key feature of this work has been the invention of a novel connector technology - known as Chiprack - which allows the designer to create embedded systems built up from reusable electronic modules - a form of electronic 'Lego'. This effectively circumvents the hardware design problem, because rather than starting from basic principles with each new product design, the designer can select from a library of proven modules - microprocessors, memories, input and output devices - to match the exact computing requirements of the product.

The Chiprack design style lends itself particularly well to embedded computing systems, which can readily be partitioned into functional modules - for example, processor, memory, communications, and so on. The modular approach allows the designer to divide product design problems into smaller pieces and focus on these individual pieces rather than on attempting to refine the entire system all at once. Clearly, the success of this scheme depends on the availability of a variety of compatible carriers which can be combined using a 'mix and match' approach. For example, one kind of memory carrier might provide a bank of read-only memory (ROM); others could provide random access memory (RAM) in varying quantities and speeds, to suit a range of needs. A fair amount of design effort is required to develop a library of carriers in this way, but the enormous benefits that accrue completely outweigh this. Apart from allowing the designer to tailor the embedded computer system to virtually any requirement, it facilitates upgrades or modifications 'in the field', and makes for easy test and maintenance. As new technologies provide memories of higher speed or capacity, or lower power consumption, the designer can incorporate these in a new carrier design, but need not redesign the entire system (as would be necessary in a conventional board-based system).

In order to establish the effectiveness of this new modular approach to embedded systems, we have developed a computer based on the PC architecture which has been applied to a number of different applications. This phase of the development has been supported by the Department of Trade and Industry through a grant awarded under the SMART scheme. The prototype Chiprack PC has aroused considerable interest among manufacturers of embedded systems, some of whom now plan to use the technology to help reduce development costs and speed their new products to the market.


There are two parts to Chiprack: leadless chip carriers, which hold the electronic devices (integrated circuits and discrete components), and the connectors themselves. The carriers resemble small rectangular printed circuit boards, and have contact pads at each edge on both the upper and the lower surface.

A Chiprack layer A Chiprack connector

The Chiprack connectors hold the carriers in such a way that a number of them can be stacked one above the other in a three-dimensional structure, with the upper surface of one coupled electrically to the lower surface of the next carrier.

Ways of interconnecting layersChiprack offers considerable flexibility in terms of freedom of interconnections between layers, some of which possibilities are shown in the illustration. In addition to these, it is possible to bring signals in and out of the stack by access to the sides, allowing true 3D connectivity.

Once the necessary layers have been designed and fabricated, a simple compression operation unites the connectors. The result is a compact, robust block of electronic circuitry (see below) which can easily be integrated into a wide range of products. The perfection of the mechanical, thermal and electrical characteristics of this connector system has taken some years, and has led to a number of patents being granted.   A company has been formed to pursue commercial development and market exploitation of the Chiprack concept - Chiprack Electronic Systems Ltd - and you can find its web page - currently under construction - right here.

The Prototype Chiprack PC

The Chiprack Embedded PC consists of a number of Chiprack carriers and connectors which, when stacked together, form a very compact microprocessor system - under 60 cm3 in this case.

Dynamic RAM layer CPU layer - Chips & Technologies PC/Chip

The version illustrated is based on the Chips & Technologies 'PC on a Chip' F8680 product. A special version with the PC stack mounted on a conventional circuit board has been developed. This can be substituted in place of most PC motherboards for the purposes of evaluation. The experimental PC system behaves exactly like a conventional PC; it can be connected to a full range of PC-compatible hardware, and is capable of running standard PC software.

The Chiprack Embedded PC stack Chiprack Embedded PC Evaluation Board

Embedded Computer

There is no comprehensive definition that meets everyone's intuitive notion of an embedded computer, for different applications place primary importance on different factors like manufacturing cost, design time, reliability, etc. However, most systems that use them share a belief on the part of the designers that implementing some of the system's functions on microprocessors (rather than using complex logic circuits) will make one or more of these objectives easier to achieve. Electronic products, from microwave ovens and cellular phones to laser printers and autopilots, rely on these built-in computers to execute their basic functions.

The earliest embedded systems made use of microprocessors developed specifically for these kinds of applications - often known as microcontrollers. Each new product required a costly and labour-intensive development phase. First, it was necessary to develop the necessary circuit boards containing the microcontroller and its supporting circuits. Typically, different products would have different requirements, so no two designs would be identical. Next, each new product would require the development of custom software to control the operation of the system. Often it was necessary to program these devices using low-level machine code - a notoriously tedious and error-prone activity. As a result many of the potential advantages of the embedded approach were sacrificed.

Embedded computing is unique because the hardware and the operating software must be designed together to make sure that the implementation not only functions as expected, but also meets performance, cost and reliability goals. There has been considerable interest recently in the use of conventional microprocessors (rather than microcontrollers) in embedded systems - for example the x86 family of processors from Intel, which form the basis of the ubiquitous PC, seen on many a desktop. The reason for this is one of economics. Since IBM first introduced the original PC design in 1982, over 110 million PC-compatible computers have been sold world-wide. Market forces have brought about a steady reduction in the cost of the electronic parts in the PC. In addition, there is a vast body of knowledge about the use of these microprocessors in a wide range of configurations. Just as important, the proliferation of the desktop PC has led to the establishment of a multi-billion pound market in PC software. Although embedded computers usually have little need for spreadsheet or word processor software, there is a plethora of development tools - compilers, editors, interpreters - and millions of lines of high-quality code, much of which is directly applicable to embedded systems.

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This page was prepared by David Holburn, and comes to you courtesy of Cambridge University Engineering Department. Last updated on 11th September 1997.

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