What To Look For In A Logic Analyser


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Selecting the right logic analyser at first may seem like a thankless task. But it doesn't have to be that painful if you concentrate on three key requirements: speed, width and depth. Simply put, you'll need a logic analyser that can capture the fastest events in a design, supply enough channels and give you plenty of memory. Your current design might only demand a logic analyser that excels in one or two of these areas. But as clock rates, edge rates and complexity increase, there is a very good chance that all three requirements will become crucial for successfully evaluating and testing tomorrow's designs.

Raising the speed limit

It is absolutely essential that a logic analyser be able to keep pace with the fastest circuitry in your design. To ensure clear visibility into the inner workings of your design, a logic analyser must be able to capture and analyse its fastest events. That means for today's high-speed processors, the logic analyser should deliver timing resolution in the subnanosecond region. In addition, precise setup and hold windows are needed to support high-speed state acquisition. Although most logic analysers today are equipped to handle 100MHz state speeds, they don't have the horsepower to keep up with next wave of high-speed general-purpose processors.

Speed alone, however, does not make for success. A logic analyser must also intelligently sift through millions of pieces of data to quickly find the information you need. That's why triggering - how a logic analyser determines when to capture data - is so important. When searching for a problem in a sophisticated design, the visible symptoms are often an intricate web of interrelated hardware and software events. As a result, a logic analyser must be able to monitor a multitude of signals and search for any number of possible combinations of software instructions, data access, timing violations, glitches, or setup and hold violations. Traditional triggering, that only makes one if-then-else decision per clock, often fails to locate the subtle, asynchronous events that typically threaten the integrity of advanced designs.

The wider the better

Applications today are not only getting faster, they are growing in size. High end processor designs already require 100 to 150 channels to cover all the inputs and outputs of the device. In complex applications, such as a multiprocessor design, the channel count soars into the hundreds. There are even designs that exist today that require monitoring over 1000 signals to fully analyse the application. So even if your design does not require hundreds of channels now, you'll want a logic analyser that can easily expand to accommodate future design demands.

It is equally important that each and every one of those channels be capable of sustaining the logic analyser's top acquisition speed. Unfortunately, many logic analysers only provide a limited number of high-speed channels even when they support hundreds of connections. But designs no longer just have a handful of high-speed signals. It is not uncommon to need to view every input and output with sub-nanosecond timing resolution.

A logic analyser's probe adaptors and disassembly software is another crucial factor in how effectively you can analyse a complex processor with hundreds of connections. With so many processor families on the market, a logic analyser must support literally scores of probe adaptors. And the disassembly software should give a clear view of which instructions are actually executed and which conditional branches are actually taken.

Deep memory

Untangling the relationship between tens or even hundreds of signals over many cycles requires that the logic analyser extract and store an extraordinary amount of data. Although most logic analysers offer deeper memory capabilities, it usually comes at the expense of some other function. Only by shutting down an important feature, such as time stamping, can some logic analysers deliver the memory depth required for advanced debugging. Such compromises in acquisition capabilities heavily dilute the benefits of longer memory.

Keep it simple

If you keep focused on the three basic requirements of speed, width and depth, you'll quickly zero-in on the right logic analyser for your design requirements. Although it is important to determine up front which of those criteria is the most important, remember that there is a good chance your next design will need a logic analyser that gives you exceptional performance in all three areas.

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Last revised: Sunday, 18 May 1997