Digital circuit tips

- guidance and hints and tips on using digital logic circuits

Digital logic circuits are widely used in today's' electronics. These circuits are used for a very wide variety of applications. From simple logic circuits consisting of a few logic gates, through to complicated microprocessor based systems.

Whatever the form of digital logic circuit, there are a number of precautions that should be observed when designing, and also when undertaking the circuit board layout. If the circuit is correctly designed and constructed then problems in the performance can be avoided.

Decoupling

One of the main points to ensure is that the power rails are adequately decoupled. As the logic circuits switch very fast, switching spikes appear on the rails and these can in turn appear on the outputs of other circuits. In turn this can cause other circuits to "fire" when they would not normally be intended to do so.

To prevent this happening all chips should be decoupled. In the first instance there should be a large capacitor at the input to the board, and then each chip should be individually decoupled using a smaller capacitor. The value of the capacitor will depend upon the type of logic being used. The speed, and current consumption will govern the size of capacitor required, but typically a 22nF may be used. For chips running with very low values of current a smaller capacitor may be acceptable, but be aware that even low current logic families tend to switch very fast these days and this can place large voltage spikes onto the rails.

Some manufacturing companies suggest in their codes of practice that a proportion of the chips should be decoupled. While this may be perfectly acceptable, the safest route is to decouple each chip.

Earthing

The ground lines in a logic circuit of great importance. By providing an effective ground line, problems such as ringing, spikes and noise can be reduced. In many printed circuit boards a ground plane is used. This may be the second side of a double sided board, or in some cases an internal layer in a multilayer board. By having a complete, or nearly complete layer in the board, it is possible to take any decoupling or earth points to the plane using the shortest possible leads. This reduces the inductance and makes the connection more effective. With the sharp edges, and the inherent high frequencies that are present, these techniques are important and can improve the performance. For the more simple circuits that may be made using pin and wire techniques, good practise is still as important, if not more so. Earth loops should be avoided, and earth wires should be as thick as reasonably possible. A little planning prior to constructing the circuit can enable the leads to be kept as short as possible.

General layout

The layout of a digital logic board can have a significant affect on its performance. With edges of waveforms being very fast, the frequencies that are contained within the waveforms are particularly high. Accordingly leads must be kept as short as reasonably possible if the circuit is to be able to perform correctly. Indeed many high end printed circuit board layout packages contain software that simulates the effects of the leads in the layout. These software packages can be particularly helpful when board or system complexity dictates that lead lengths greater than those that would normally be needed are required to enable the overall system to be realised. However for many instances this level of simulation is not required, and lead lengths can be kept short.

Unused inputs

In many circuits using logic ICs, inputs may be left open. This can cause problems. Even though they normally float high, i.e. go to the "1" state, it is wise not to leave them open. Ideally inputs to gates should be taken to ground, or if they need a logical "1" at the input they should be taken to the rail, preferably though a resistor.

In many designs, spare gates may be available on the board. The input gates to these circuits should not be left floating as they have been known to switch and cause additional spikes on the rails, etc. It is best practice to take the inputs of these gates to ground. In this way any possibility of them switching in a spurious manner will be removed.

Summary

At first sight digital logic circuits may not appear to need all the care and attention given to a radio frequency (RF) circuit, but the speed of some of the edges on the waveform transitions means that very high frequencies are contained within them. To ensure that the optimum performance is obtained, good layout is essential. Obeying a few simple rules can often ensure that the circuit operates correctly