The Circuit Design

The electronic design is called a schematic or circuit diagram. It is constructed in CAD format. At AirBorn Electronics we use Protel CAD. Schematic diagrams are constructed according to a number of standards and rules, but the primary function is to explain the circuit design in an easily digestible form.

A circuit diagram is a strict document, and cannot be "mostly correct". A circuit diagram must reflect the actual construction of the printed circuit board which is made from it, exactly. To this end the printed circuit board CAD and schematic CAD are tied together through a Net-check, and modifications to one document must always be carried out in the other. The circuit diagram references each part on the printed circuit board with a designator (e.g. "IC5") and pin numbers for each connection.

A good circuit diagram will include extra information required to understand the circuit operation, have descriptive net and connector labels, and include all of the parts on the printed circuit board. At AirBorn Electronics we use "blank" components for hardware items such as IC sockets, heatsinks, submodules and the PCB laminate itself, so that these items appear on the circuit, parts list, and PCB in identical form. We have a tutorial on our design method if you wish to follow it.

A good circuit designer sees the circuit as a whole, and rarely just looks at each element of the design in isolation (the "keyhole" approach). I personally shudder at the term holistic approach - but it is probably the best label. While the keyhole approach is a useful simplification applied to the software industry, in the hardware field it will cause you grief. Circuitry affects other circuitry in unanticipated ways, looking at elements in isolation causes you to overlook such interactions. There is no substitute for being thorough.

Frankly, our whole design process works best as an integrated whole. We frequently get requests from clients who want just part of our design service. If the client wants just the realtime software - and provides working hardware that they have already used elsewhere - this can work well. If the client wants to design their own new PCB themselves, but have us prototype their design work, the job is often less successful than it should be - we really need to control the circuit design to start the project from a good base point.

Experience counts for a lot in circuit design. As an example, the small illustration is a circuit that we often use when an RS485 interface is required. Many other subcontract design companies will just have one part in their circuit, - they will use the same 75176 IC, but in surface mount, taking a tiny 0.5 square centimetres of PCB space. Our solution uses a DIP component in an IC socket, (so it's replaceable) and adds an indication LED (to aid diagnosis of cable and peer faults), supply pulling resistors (for better noise immunity), fusible series resistors (to protect against cable mis-wires), and overvoltage clamps (to protect against surges). Our solution provides a much better quality circuit.

I am not young enough to know everything. -- Oscar Wilde

The design of the circuitry also ties in with the programming for the product - especially when Programmable Logic Devices (PLD, CPLD, FPGA) are being used. Decisions are made as to which circuitry is inside which devices. In the case of microprocessors, some functions require special pins, such as interrupts - again, decisions are made as the circuit is designed that affect the coding performed later.

The package type selected for each component to some degree also affects the design of the circuit itself. A schematic for a surface mount board, for instance, will be subtly different from the schematic for a conventional board, as the SMT PCB will in general have parts which can dissipate less heat (e.g. a standard 0805 SMT resistor has a max dissipation of 0.1W, whereas a standard 0.4 inch axial conventional resistor has a max dissipation of 0.25W) Also the SMT circuit will have less parasitic capacitance and inductance, although unless the design is RF or really high speed you generally won't see any differences in component values. There are also some parts that are hard to get in conventional - and others that are hard to get in SMT.

The design of digital circuitry for small microprocessor projects used to be very much a building block affair - it was just a case of connecting the right bricks together to make your design. However, as devices get more complex and requirements more demanding, experience and attention to detail become paramount. We work with all types of circuitry - including low level analog, low power circuitry, robust interface circuitry and opto-isolated circuitry. By choosing AirBorn as your subcontractor, you get assurance that the circuit will be designed, reviewed and debugged thoroughly.

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Hardware, n.: The parts of a computer system that can be kicked. -- from "Fortunes"

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