Digital oscilloscope persistence - autostore in the HP 54600 oscilloscope - Technical

Hewlett-Packard Journal, Feb, 1992 by James A. Kahkoska

[NOTE: SOME FORMULA'S HAVE BEEN OMITTED]

Autostore, a storage technique for monochrome digital storage oscilloscopes, displays historical traces at half Intensity and the most recent, or live, trace at full intensity. The technique allows new ways of viewing signals.

The advent of digital oscilloscopes brought new techniques of simulating analog persistence and storage using raster CRT technology. Digital raster persistence in digital storage oscilloscopes simulates the screen persistence found on analog storage oscilloscopes. To date, various forms of digital raster persistence have existed. The latest form for monochrome digital storage oscilloscopes allows the user to view both the current signal and all worst-case signal excursions. This storage technique can be applied to a wide variety of real-world problems, ranging from its use as a simple "electronic grease pencil" to complex CPU system analysis.

To simulate analog persistence, digital variable persistence is used. Each pixel that is excited as the signal is displayed is time-tagged. The time tag allows the pixel erasure time to be determined based on the user-selected persistence time.

The most powerful use of digital persistence is infinite persistence. Pixels are never erased, so all signal excursions are accumulated indefinitely without the fading or blooming commonly associated with analog storage oscilloscopes. Infinite persistence is invaluable for catching infrequent events or making worst-case measurements. One drawback of infinite persistence is that the live trace information is lost in the historical signal excursions, making it difficult to see the current signal.

Autostore

To overcome this problem, Hewlett-Packard has introduced a new form of infinite persistence, called autostore. In autostore mode, half intensity is used to display all historical signal excursions while the current signal is shown at full intensity. Simple one-key operation allows the user to move into and out of autostore mode quickly.

A common troubleshooting technique used by technicians is comparison of a good unit with a bad unit. Either operator memory or a grease pencil is used to remember the signal characteristics of the good unit. Autostore allows fast, accurate signal comparison with the minimum number of keystrokes. The user simply enables autostore briefly to create a half-intensity image of the good signal, then probes the bad unit for comparison. Small signal details that might otherwise go unnoticed are easily identified. To remove the electronic grease pencil, the user simply presses Erase. With three keystrokes, the user can record, compare, then erase the image. Fig. I shows the comparison of an autostored good unit signal with the bad unit signal at full intensity. The slight variation in pulse width might go unnoticed if the two units were compared visually.

The main strength of autostore is the ability to view all signal excursions while distinguishing the current live trace. This allows the user to adjust the unit under test while capturing the entire range of the adjustment. For making adjustments to minimize overshoot, jitter, or noise, autostore provides visual feedback without which it would be difficult to see the effects of adjustments when the signal changes become small. For example, for a fine adjustment to minimize overshoot, adjustments are made so that the current full-intensity trace is always bounded by the historical trace information. Autostore provides details down to pixel resolution to verify that the overshoot is truly minimized (see Fig. 2).

Autostore is also useful for building a set of characteristic curves. Coupled with the digital oscilloscope's ability to capture single-shot events and view negative time, autostore allows a family of single-shot events to be captured on the screen, the most recent at full intensity. This technique can be applied when characterizing single-shot events over a range of specified conditions. For example, an important criterion for power supply testing is the power-up characteristics of the supply over a wide range of loads. Fig. 3 shows the power-up characteristics of a power supply over the specified range of current capacity, with the most recent power-up cycle at full intensity.

Worst-case signal variations, such as jitter or noise, are easy to analyze using conventional infinite persistence, but the current state of the system becomes lost as the traces accumulate. This makes it difficult to analyze the effect of any change to the system, such as adjustments, component changes, or temperature, relative to the worst case. Autostore solves this by displaying all but the current trace at half intensity. Fig. 4 shows a jittery signal with the range of jitter at half intensity and the current signal at full intensity.

Digital System Trouble-shooting

For digital system troubleshooting, the ability of autostore to capture a full spectrum of valid and invalid transitions is invaluable. For example, the normal read cycle of a CPU system results in exactly one device at a time being enabled to drive the data bus. If the CPU occasionally reads bad data, two likely causes are either a bus conflict or a floating bus. Bus conflict results when two different devices attempt to drive a data line at the same time. This conflict may be caused by either a bad bus device or bad decoding of the chip select signals so that two bus drivers are enabled at once. The result is an invalid voltage level between logic levels. This conflict may only occur when different output levels are present on each bus driver, resulting in a "soft error" that may be difficult to diagnose because of its context dependence. The upper trace in Fig. 5 represents the read enable signal and the lower trace represents the data line. Normal bus operation results in the data line's being valid-either a one or zero as indicated by the markers-on the positive edge of the read enable signal. Exercising the system, perhaps with a memory test, shows that one of the read cycles generated an invalid level because of a bus conflict. With autostore, not only can the anomaly be captured with half-intensity infinite persistence, but also the frequency of occurrence can be seen as full-intensity variations. Normal infinite persistence makes it impossible to see the frequency of occurrence once the anomaly occurs because there is no intensity variation between the infinite persistence traces and the current trace. Seeing the current trace at full intensity allows the user to attempt to localize the problem, perhaps by using a software test to stimulate different CPU addresses and data values.