- an overview of the logic analyzer used for monitoring digital signals.
Logic analyzers or logic analysers are widely used for
testing 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. TH 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.
What makes a logic analyzer
Logic analyzers are designed to monitor a large number of digital lines.
They possess a horizontal time axis and a vertical axis to indicate a logic
high or low states. As logic analyzers are optimised for monitoring a large
number of digital circuits, typically they may have anywhere between about
32 and 132 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.
One of the main points to note about a logic analyzer is
that it does not give a full analogue display of the waveform. Although it
shows the logical high and low states as a waveform on the display, it only
looks for whether the state of a line is high, i.e. above a certain trigger
voltage, or low, i.e. below the trigger voltage. Having decided whether a
line is high or low, the logic analyzer then displays the relevant level.
This means that it is not possible to see small amplitude variations such as
ringing on the signal, however the state of the lines and their timings are
displayed.
To achieve this the logic analyzer will sample the
waveforms states and store the level as either high or low at each sample
time. The displayed waveform will then look like a timing diagram from a
simulator or data-book. It will display the state of the lines, and the
timings of any transitions.
In this way it is possible to analyze the waveforms
produced by the circuit and ensure they match those that are expected. Any
differences can then be used to trace any problems in the circuit design
Probes
With the large number of signals required to be monitored, often from a
small area on a board and possibly even from one integrated circuit, the
design of the probes can be a critical issue.
The logic analyzer probes contain an internal comparator
where the voltage of the waveform on the board is compared against the
threshold voltage. This can be set using the main instrument to a variety of
levels so that signals from a variety of logic families can be monitored.
Logic analyzer probes can take a variety of physical
forms, but generally fall into one of three categories:
- Multichannel probes that use a dedicated connector on the circuit
board. These probes enable a large number of points to be accessed using
a high density connector. This facility has to be designed on to the
board and may only be retained for the development phase where access
for a logic analyzer is required.
- High density compression probes. These probes use a compression
contact that does not have a dedicated connector. Contacts on the board
are required for this type of connection.
- Flying lead probes. As the name implies, these probes are on a
flying lead that is connected to a small electronic unit that contains
the electronics for detecting the high and low levels. However these
flying lead probes are used to monitor points that may not be included
on any of the other access points.
Triggering
One of the key features of a logic analyzer is its triggering capability.
When investigating and debugging complex software driven digital circuits it
is necessary to be able to see the response of the system after a particular
occurrence. As this may involve a number of lines to be in a given state, it
is necessary for the logic analyzer to be able to trigger after this
combination occurs. This facility is one of the key advantages of logic
analyzers and enables them to be used to quickly home in on problems that
may only occur under a particular set of circumstances.
Logic analyser operation
Although the operation of a logic analyzer may appear to be fairly
complicated at first sight, a methodical approach to the use of one enables
it to be set up correctly and to be used effectively. Once the probes are
connected, the logic analyzer is programmed with the names of each signal.
The analyzer can also associate several signals into groups so that they can
be manipulated more easily.
Wit the basic set-up of the logic anlyzer complete the
capture mode for the data needs to be chosen. This can be set to one of two
modes:
- Timing mode Using this mode signals are sampled at
regular intervals based on an internal or external clock.
- State mode Here one or more of the signals are defined as
clocks, and data is smapled on the edges of these clocks.
Once the logic analyzer mode is chosen then the trigger
condition can be set. The analyzer trigger condition may vary from a very
simple signal edge to a set of conditions that must be met across a variety
of lines. The complex trigger conditions aid in locating problems that occur
when a particular set of conditions occur.
With the trigger condition set, the logic analyzer can be
set to run, triggering once only, or repeatedly. The data that is captured
can then be displayed and analysed.
Summary
Logic analyzers are an essential tool for many applications where digital
circuits employing a large number of lines that need to be monitored. They
are used to assist in the development of many of the circuits involving
digital hardware and software. By using a logic analyzer is it possible to
be able to look at these lines in a practicable fashion and be able to
trigger on a preset pattern of a given number of lines. In this way the
events that happen after a predetermined occurrence can be viewed for
investigation. This is invaluable in enabling fault finding of complex
software driven circuits.
Although with the enormous increase in the complexity of
circuits, other techniques are often applicable, logic analysers are still
used for many applications
