Logic Analyzer

Logic analyzer basics

How to use a logic analyzer

Triggering

Logic analyzer specifications

Probes & connectors

Logic analyzers or logic analyzers 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 analyzer, 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 analyzer 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 analyzers come in a variety of formats. One of the most popular is a typical test instrument case. However it is also possible to utilize the processing power of a computer and PC based logic analyzers are available. The actual choice of logic analyzer 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 analyzer key characteristics

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

Provide a time display of logic states:   Logic analyzers 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 optimized 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.

Displays logic states:   The vertical display on the analyzer 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 analyzer. 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.

logic analyzer types:

Modular logic analyzers :   This type of logic analyzer 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 analyzer, possibly for restricted budgets or for field service. These test instruments incorporate all elements of the analyzer into a single box for ease of transportation.

PC based logic analyzers:   There is a growing number of PC based logic analyzers. These consist of an analyzer unit that is connected to a PC. 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.

A logic analyzer is like an oscilloscope that is used to view multiple digital binary waveforms. We use logic analyzer for precise hardware troubleshooting, especially for timing issues. While we focus on microcontrollers here, logic analyzers can be used for any type of digital circuit that has a binary output. We can set the threshold values for the logic highs and lows in the logic analyzer software. In the LA2124 Logic Analyzer we use in this demo and the EE3176 labs, the threshold is set at 1.40 V, so a signal voltage > 1.40 volts is a logical one, while a voltage less than 1.40 V is a logical 0. This is a 24 channel analyzer that can hold 128Kbytes of data in the buffer, and samples at rates up to 100 M samples/sec (50 MHz bandwidth). It hooks up to the parallel port of your PC, and displays the data on the CRT. The 24 channels are displayed simultaneously, so we can easily see timing relations between the different digital sources. We can also set up complicated trigger events with the logic analyzer that are not possible with the oscilloscope.

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.With the basic set-up of the logic analyzer 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.

The output of a Logic Analyzer is usually in one of two forms. Most commonly, we use a time-domain display, where the binary waveforms from one or more digital sources is viewed as a function of time. An example screenshot from the LA-2124 Timing View is shown below:

Note that the time division of the grid (dashed vertical lines) above is 10 useconds ("u" for micro) per division. So, for the Channel 0 waveform, the first pulse train starts at 11 usec, with the first few pulses having a 2 usec period .

State mode   :  Here one or more of the signals are defined as clocks, and data is sampled on the edges of these clocks.

We can also view the logic analyzer output in the "state" domain. This gives us the binary output in terms of 0's and 1's.:

we can group channels into logical groups that make sense for the problem at hand. In the view above, Group 1 is formed from Channels 0-7, with Channel 0 being the leftmost bit and Channel 7 being the rightmost bit in that group. So, at 1 usec., Channel 0 has binary value "0", Channel 1 = 0, ..., Channel 7 = 0. At 11 usec., Channels 0,1 and 3 turn "on" and are binary 1s, while the others are binary 0s. This state view corresponds to the time domain view above, so we can directly compare them.

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.

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.

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.

we can set the "trigger word" to any binary pattern you wish. When the input channels have this pattern, a trigger will occur. A "1" is a logic high, a "0" a logic low, and an "X" a don't care. For instance, if you set the trigger word to be "XXXXXX11" in Channels 0-7, then you will get a trigger when both channels 0 and 1 are high, regardless of the other channels. we can set the binary pattern for the trigger for all 24 channels as desired, so you can trigger on very specific events.

Specifications:

The specification for a logic analyzer covers many areas of its performance, but there are a number of parameters that are key to ensuring that it will meet the majority of its operational requirements and be fit for the purpose for which it was intended.

Logic analyzer speed

One of the major requirements for any logic analyzer is the speed of the instrument. With today's high speed circuits it will be necessary to the logic analyzer to typically deliver sub nano-second resolution.

The speed of the logic analyser is chiefly governed by the timing resolution. This is the smallest time element that the analyser can see. If the resolution is too coarse then it will not be possible to see many of the fast occurrences happening in the circuits.

The speed of the analyzer must be such that it is able to capture and display a variety of scenarios ranging from transient glitches, any variety of software instructions which may lead to problems occurring, timing violations, or set-up conditions. It is often within these areas that the difficult problems can be found, and it is here that the capability of the logic analyzer is needed. Without sufficient speed many of these elements will not be seen.

Logic analyzer channels

Today's digital circuits are becoming more complicated, and they are normally software driven. This means that it is important to ensure that sufficient channels are available within the logical analyzer. Often high end processor designs require between 50 and 150 channels to cover all the lines that are relevant to the testing. If a logic analyzer with insufficient number of inputs is purchased, then this will considerably hamper testing.

Unfortunately, increasing the number of channels in a logic analyzer considerably increases the complexity of the instrument..

To help reduce this problem, most logic analyzers have only a proportion of their channels that support the full specification in terms of speed in resolution. As it is un-necessary for all the channels to be able operate at the maximum spec, reducing the performance of some simplifies the circuitry and reduces the requirements for memory and processing.

Logic analyzer memory

In order that the logic analyzer can display the information it retrieves, it must store it in memory. If only a small amount of memory is available then it will only be able to store short sequences, and this may be insufficient to analyze all the events occurring. Additionally a greater the number if inputs, longer sequences that need to be stored, and greater levels of resolution increase the requirement for memory. As memory can be expensive, it is necessary to gain a sensible balance between memory requirements and cost for the logic analyzer.often memory requirements may be reduced by turning off some functions such as time stamping, etc that may be essential for some debugging.