Designing Embedded Hardware

Quick-and-Dirty Construction

It is possible to build very simple circuits by just soldering the components together in free space. For example, with the AVR design in this chapter, the leads of the watch crystal can be soldered directly onto the pins of the processor, with the crystal lying across the top of the processor. Wires are soldered onto the pins bringing in ground and power and connecting the processor’s I/O to the outside world. This technique is variously referred to as “a rat’s nest,” a “bird’s nest,” or “what the hell is that?”

This is a quick-and-dirty method, useful for rapid prototyping of extremely simple circuits. It’s not really recommended, but you can get away with it in a pinch. Don’t try it with anything that is even slightly complicated or running at any reasonable speed. If you do, you’ll spend more time debugging the construction than debugging the actual design or code!

Breadboarding

Breadboards are plastic blocks with arrays of holes. They are designed to hold DIP-packaged integrated circuits and discrete components. The term “breadboard” dates back to the olden days when valve radios were constructed on a base of solid wood (a cutting board for bread). The term has stuck, and the modern breadboard can still be found in electronics hobbyist stores and even the occasional university teaching lab.

While it is possible to build very low-speed microprocessor systems and general digital circuits on breadboards, try not to. There be dragons! As a general rule, breadboards are bad news and you should avoid using them at all costs. (Think of them as the hardware equivalent of COBOL.) Breadboards suffer from excessive capacitance, crosstalk, and noise susceptibility, which makes them completely inappropriate for microprocessor system construction. They can also suffer from mechanical failure (leading to short circuits) after extended use. Circuit interconnections on a breadboard are done with small sections of wire, which make great little antennas. They will pick up every scrap of stray electromagnetic radiation and channel it straight into your circuit! This is not the way to construct a robust and reliable system. If you really must, you could probably build the ATtiny15 or PIC12C805 computer on a breadboard, using their internal RC oscillators. But I’d advise against using breadboards for anything that uses a crystal or that has any fast-switching digital signals.

Wirewrapping

Once common as a construction technique, wirewrapping is now quite rare. It is intended for use with DIP-packaged integrated circuits, which are mounted on sockets with long pins (0.6”). Special tools (known as wrapping tools) allow you to quickly and efficiently wind thin wire around the pins. The pins are square in cross-section, and wrapping a wire around a pin forms a cold weld , a tight electrical connection with no soldering. Thus, a circuit is constructed by individually wiring point-to-point each connection within the system.

Wirewrapping is a very fast prototyping technique and is very robust and reliable. In the early days, NASA used to use wirewrapping for constructing spacecraft avionics, and many mainframe computers were built using the technique. Wirewrapping is good for prototyping (especially if you’re unclear as to the final form of the design and expect to make lots of changes to the hardware) or for building one-off designs. If you intend to make more than one computer based on your design (and you probably will), then skip wirewrapping and do it on a printed-circuit board.