Logic Analyzer Tutorial

- what is a logic analyzer, how it works; how to use it and get the best from it for testing complex logic / digital circuits and boards.

Logic analyzers or logic analysers are widely used for testing complex digital or logic circuits. They appeared shortly after the first microprocessors were used because to fault find these circuits required the instrument to have access to a large number of lines, more than could be seen using a conventional oscilloscope. Since then the need for logic analyzers has grown, especially as the complexity of circuits has continued to grow.

Although oscilloscopes can perform many of the functions of a logic analyser, the analyzer is more suited to operating in a digital environment because it is able to display relative timing of a large number of signals. Essentially a logic analyser enables traces of logic signals to be seen in such a way that the operation of several lines in a digital circuit can be monitored and investigated.

Logic analysers come in a variety of formats. One of the most popular is a typical test instrument case. However it is also possible to utilise the processing power of a computer and PC based logic analysers are available. The actual choice of logic analyser will depend upon the cost budget and the actual requirements. The PC logic analyzers are a particularly cost effective method of creating an analyzer. However the main drawback of the PC logic analyzers is that their functionality is not as great as the dedicated logic analyzers, which is only to be expected in view of the cost differential.

Logic analyser development

The first logic analysers were developed out of the need to be able de-bug and undertake fault-finding on microprocessor based systems.

In the early 1980s when these chips first started to become widely used, the urgent need arose to develop techniques that would enable the many lines and test points to be monitored simultaneously. Existing oscilloscopes were not able to provide the required levels of functionality.

After the early introduction of the first analysers, their complexity increased in line with the complexity of the circuits being tested. The number of channels grew, their speeds rose and the functionality in areas such as triggering improved greatly.

Logic analyser key characteristics

There are several key characteristics of a logic analyser that separate it from multi-channel oscilloscopes and other test instruments:

  • Provide a time display of logic states:   Logic analysers possess a horizontal time axis and a vertical axis to indicate a logic high or low states. In this way a picture of the digital lines can be easily displayed.
  • Multiple channels:   Logic analyzers are designed to monitor a large number of digital lines. As logic analyzers are optimised for monitoring a large number of digital circuits, typically they may have anywhere between about 32 and 200+ channels they can monitor, each channel monitoring one digital line. However some specialised logic analyzers are suitably scaled to be able to handle many more lines, and in this way enable tracking and fault finding on much more complex systems.
  • Displays logic states:   The vertical display on the analyser displays the logic state as a high of low state. The signals enter the various channels and are converted into a high or low state for further processing within the analyser. It provides a logic timing diagram of the various lines being monitored.
  • Does NOT display analogue information :   These test instruments do not present any analogue information, and in this way they differ from an oscilloscope. They are purely aimed at monitoring the logic operation of the system. If any analogue information is required, then an oscilloscope must be used in addition.

Oscilloscope vs Logic Analyzer

Oscilloscopes and logic analysers are very different test instruments. While both have a very similar form of display, i.e. displaying waveforms, they use fundamentally different operational concepts.

Comparison of Oscilloscope and Logic Analyzer
Oscilloscope Logic Analyzer

Oscilloscope is used for measuring analogue waveforms: amplitude, phase values, or edge measurements such as rise times, etc.

Typical oscilloscope applications:

  • Investigating waveform shapes, ringing, rise time, etc.
  • Characterise aspects like waveform jitter and stability.
  • Measure signal amplitudes.
  • Detect transients, and unwanted pulses.

A logic analyser is used for verifying and debugging the operation of digital designs looking a logic states and timings.

Typical logic analyser applications:

  • Investigate the system operation.
  • Correlate a large number of digital signals
  • Detect timing violations
  • Trace embedded software operation.

Logic analyser types

Although development of these test instruments is on-going and new variants are constantly being launched and many technology innovations are being achieved, there are some main categories into which most logic analyzers can be split:

  • Modular logic analyzers :   This type of logic analyser is probably what may be thought of as the most typical form of test instrument, although it is the highest cost option providing the highest level of functionality. It comprises a chassis and the various modules - including channel modules. The number of modules being larger for the higher channel counts.
  • Portable logic analyzers :   In a number of instances there may be a need for a smaller analyser, possibly for restricted budgets or for field service. These test instruments incorporate all elements of the analyser into a single box for ease of transportation.
  • PC based logic analyzers:   There is a growing number of PC based logic analysers. These consist of an analyser unit that is connected to a PC. USB is an obvious option for this, but Ethernet is also widely used because of its high speed. This form of PC based instrument uses the processing power of the PC combined with its display to reduce the cost of the overall system.

Logic analyzers are an essential tool in the kit for electronics development engineers. Typically used in association with processors and embedded systems, they enable a view of the operation of the circuit or system to be gained that could not be ascertained by any other method.

By Ian Poole

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