An oscilloscope-like logic timing analyzer

Hewlett-Packard Journal, April, 1997 by Steven B. Warntjes

Market research indicated that some customers doing embedded development preferred to work with oscilloscopes instead of standard logic analyzers. The HP 54620 logic timing analyzer offers many oscilloscope features, including direct-access controls, a highly interactive display, computed measurements, delayed sweep, simplified triggering, and a trace labelmaker.

The principal contributions of the HP 54620 logic timing analyzer (Fig. 1) are not in its performance specifications but instead in its user interface, optimized feature set, and price. These are significant contributions, as evidenced by the HP 54620's receiving the Test & Measurement World Best in Test Award for 1995. Market research for the HP 54620 indicated that some customers doing embedded development work were frustrated with the cost and complexity of standard logic analyzers and preferred instead to work with oscilloscopes. The HP 54620 bridges the gap between standard logic analyzers and oscilloscopes by providing the functionality of a timing analyzer and the usability advantages of an analog oscilloscope.

[Figure 1 ILLUSTRATION OMITTED]

Product Description

The HP 54620 is best described as 16-channel timing analyzer. Each timing analyzer channel has one bit of vertical resolution indicating either a logic high level or a logic low level based on a user-selectable voltage threshold. The HP 54620 leverages HP second-generation logic-analyzer-on-a-chip technology to provide 500-MSa/s or 2-ns timing analysis on all 16 channels simultaneously. This acquisition engine and a custom display processor produce oscilloscope-like, high-throughput displays. Monochrome (HP 54620A) and color (HP 54620C) display versions are available. The HP 54620 leverages the CPU system and the mechanical design of the HP 54600 Series oscilloscopes for front-panel responsiveness and small form factor. The HP 54620 is positioned as a companion to the user's oscilloscope and therefore has the ability to be triggered from another instrument (trigger in) and to trigger another instrument (trigger out).

User Interface Oscilloscope Similarities

Most oscilloscope users will tell you that their oscilloscope is relatively easy to use. While some oscilloscopes are more user-friendly than others, the general feeling is that oscilloscopes are easier to use than logic analyzers. The primary reasons are twofold. First, oscilloscopes usually have direct-access controls, something not always true of logic analyzers. Second, oscilloscope displays are highly interactive because of their fast screen update rates. After all, they are primarily waveform viewing tools, and they feel very responsive. The HP 54620 leverages these two oscilloscope ease-of-use attributes and adds some standard oscilloscope features, previously unavailable in logic analyzers, that make the HP 54620 feel and run like an oscilloscope. The project design goal statement was, "Any user who can use an analog oscilloscope should quickly be able to use this new class of logic analyzer."

Direct-access control of instruments means that the most common controls are available on the front panel, through dedicated knobs or buttons. With an oscilloscope, the user often reaches up to expand or contract the time window, often without thinking. This intuitive control is beneficial because it allows the user to concentrate on the problem at hand and not worry about running the test instrument. The front panel of the HP 54620, shown in Fig. 1, has direct controls for time per division, waveform position, and time base delay. Instruments that contain several layers of menus under each front panel key can often confuse the user. The HP 54620 addresses this concern by keeping most menus to a single level under a given front panel key, allowing the user more direct control of the instrument.

The ability to display unexpected or undesired waveform conditions with a highly interactive display often leads the troubleshooter to a quicker solution of the circuit problem. Instrument displays with fast update rates seldom miss infrequent or random events. If the update rate is slow, say less than five updates per second, the instrument can fail to capture important waveform events because it is processing and not acquiring data when they occur. The HP 54620's multiprocessor architecture (Fig. 2) yields a highly interactive display that updates the screen about 16 times per second as a result of parallel processing of the waveform data.

[Figure 2 ILLUSTRATION OMITTED]

An important element of oscilloscope operation is how responsive it is to control changes. After a control such as the time per division is changed, the instrument should respond quickly to prevent user confusion. The multiprocessor architecture of the HP 54620 helps address instrument responsiveness.

To make the HP 54620 feel and run like a oscilloscope it was necessary to add a number of features that are standard in digital oscilloscopes. One of the advantages that digital oscilloscopes have over analog oscilloscopes and traditional logic analyzers is the ability to do automatic measurements such as signal frequency, period, duty cycle, and so on. Since the waveforms are stored in memory, the oscilloscope can postprocess the waveform to determine the frequency of the stored signal. Oscilloscope users have come to expect these measurements in digital oscilloscopes. Consequently, automatic measurements for frequency, period, duty cycle, positive width, and negative width are implemented in the HP 54620.