04-1 TriggerScan Application Notes
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TriggerScan
Peter J. Pupalaikis Principal Technologist June 21, 2008 Summary This paper contrasts multiple methods of anomaly capture when debugging circuits and searching for rare events. A new method called TriggerScan is shown to be superior in some cases to traditional methods of error capture. iesPerSecond captured min( MaxUpdateRate, EdgeRate) AnomaliesPerSecond EdgeRate
Equation 1
1
min(a, b) is simply the lesser of a and b
TriggerScan page | 1 of 5
TECHNICAL BRIEF
One important use of the digital oscilloscope is as a debug tool. As a debug tool, it is important for the oscilloscope to find anomalous events quickly. This is generally for two purposes. One purpose is to show evidence of a suspected problem. Another is to show enough about a problem to enable the setting up of the trigger to isolate the problem. Isolation of the problem is accomplished by repeatedly triggering on a problem and viewing the behavior at various circuit locations to identify cause-and-effect relationships. A good way to show evidence of problems is to trigger repeatedly on an edge of a waveform. With digital oscilloscopes, it is possible to build up a persistence view showing the multiple triggers overlapping. In this way, presumably an infrequent behavior occurring on the edge is captured. The Traditional View of Anomaly Detection The metric of usefulness of the digital scope in capturing anomalies is the average number of anomalies per second that the oscilloscope can accumulate on the screen. The reciprocal of this would be the average time to see the anomaly on the screen. Traditionally, the method has been to trigger the oscilloscope in an edge trigger mode and view the persistence (analog-like) traces that overlap on each acquisition. After waiting for some period of time, hopefully one of these traces would show the anomaly. When this type of method is used, the rate of anomaly capture can be calculated based on the edge rate of the waveform, the update rate of the oscilloscope, and the statistical frequency of anomalies occurring and is given by the following equation: 1
LeCroy Corporation
This means that if the edge rate of the waveform anomalies per edge is one in a billion. Using does not exceed the update rate of the oscilloscope, Equation 1, the number of seconds on average to then the scope captures every single edge (and detect an anomaly in this type of situation with an therefore every single anomaly). When the edge oscilloscope that has a fast update mode that rate of the waveform exceeds the update rate of the updates at let’s say 100,000 updates per second oscilloscope, the oscilloscope would be 2.8 hours! Fast Waveform FE ADC Update does not capture every edge Picture HW and the number of anomalies Again, this means that per second captured is equal to for an anomaly Trigger TimeBase the rate of anomalies occurring occurring once every System divided by the ratio of the edge five seconds, the scope rate and the scope update rate. would capture it after 2.8 hours of Fast Update Mode waiting. In this traditional view, when the edge rates are The problem here is that while in the traditional lower than the oscilloscope update rate everything works well, but when the edge rates increase method of anomaly detection the capability of the beyond the oscilloscope update rate, things oscilloscope is directly proportional to the update degrade. This is why many oscilloscope rate, the fast update modes provided are not really manufacturers have created very fast update rate that fast – or certainly they are not fast enough to modes of operation. Usually, these modes of accomplish the real problem. The user really wants operation are limited in a variety of ways such that an update rate that is much faster than provided to they are useful only for the purpose of providing the use the conventional approach. traditional view of overlapping traces on edge The Trigger System triggered waveforms. In other words, they usually The problem with edge triggering to search for provide a picture or pixilated view only. anomalies is the fact that each time the scope triggers There is a relationship on the edge and captures a between anomaly waveform, some time is frequency of spent where the scope does occurrence in time not look at the waveform. and frequency of This is called dead time. occurrence per edge Many oscilloscope users are in a signal. This surprised at how large this relationship is dead time is relative to the interesting because it reveals that time that the scope is live. In the TriggerScan Operation anomalies that occur very rarely per example, the reason it takes the scope edge might occur quite frequently in almost three hours to see an anomaly occurring time. Let’s take a situation where anomalies are every five seconds is because the oscilloscope sees occurring at a rate of once every five seconds on only 0.2% of the edges occurring; it is dead 99.8% of 200 MHz clock; a glitch for example. This means the time even with this fast update mode. The that the anomalies per edge would be calculated as: scope sees only one out of every 500 edges of the waveform. AnomaliesPerSecond
Peter J. Pupalaikis Principal Technologist June 21, 2008 Summary This paper contrasts multiple methods of anomaly capture when debugging circuits and searching for rare events. A new method called TriggerScan is shown to be superior in some cases to traditional methods of error capture. iesPerSecond captured min( MaxUpdateRate, EdgeRate) AnomaliesPerSecond EdgeRate
Equation 1
1
min(a, b) is simply the lesser of a and b
TriggerScan page | 1 of 5
TECHNICAL BRIEF
One important use of the digital oscilloscope is as a debug tool. As a debug tool, it is important for the oscilloscope to find anomalous events quickly. This is generally for two purposes. One purpose is to show evidence of a suspected problem. Another is to show enough about a problem to enable the setting up of the trigger to isolate the problem. Isolation of the problem is accomplished by repeatedly triggering on a problem and viewing the behavior at various circuit locations to identify cause-and-effect relationships. A good way to show evidence of problems is to trigger repeatedly on an edge of a waveform. With digital oscilloscopes, it is possible to build up a persistence view showing the multiple triggers overlapping. In this way, presumably an infrequent behavior occurring on the edge is captured. The Traditional View of Anomaly Detection The metric of usefulness of the digital scope in capturing anomalies is the average number of anomalies per second that the oscilloscope can accumulate on the screen. The reciprocal of this would be the average time to see the anomaly on the screen. Traditionally, the method has been to trigger the oscilloscope in an edge trigger mode and view the persistence (analog-like) traces that overlap on each acquisition. After waiting for some period of time, hopefully one of these traces would show the anomaly. When this type of method is used, the rate of anomaly capture can be calculated based on the edge rate of the waveform, the update rate of the oscilloscope, and the statistical frequency of anomalies occurring and is given by the following equation: 1
LeCroy Corporation
This means that if the edge rate of the waveform anomalies per edge is one in a billion. Using does not exceed the update rate of the oscilloscope, Equation 1, the number of seconds on average to then the scope captures every single edge (and detect an anomaly in this type of situation with an therefore every single anomaly). When the edge oscilloscope that has a fast update mode that rate of the waveform exceeds the update rate of the updates at let’s say 100,000 updates per second oscilloscope, the oscilloscope would be 2.8 hours! Fast Waveform FE ADC Update does not capture every edge Picture HW and the number of anomalies Again, this means that per second captured is equal to for an anomaly Trigger TimeBase the rate of anomalies occurring occurring once every System divided by the ratio of the edge five seconds, the scope rate and the scope update rate. would capture it after 2.8 hours of Fast Update Mode waiting. In this traditional view, when the edge rates are The problem here is that while in the traditional lower than the oscilloscope update rate everything works well, but when the edge rates increase method of anomaly detection the capability of the beyond the oscilloscope update rate, things oscilloscope is directly proportional to the update degrade. This is why many oscilloscope rate, the fast update modes provided are not really manufacturers have created very fast update rate that fast – or certainly they are not fast enough to modes of operation. Usually, these modes of accomplish the real problem. The user really wants operation are limited in a variety of ways such that an update rate that is much faster than provided to they are useful only for the purpose of providing the use the conventional approach. traditional view of overlapping traces on edge The Trigger System triggered waveforms. In other words, they usually The problem with edge triggering to search for provide a picture or pixilated view only. anomalies is the fact that each time the scope triggers There is a relationship on the edge and captures a between anomaly waveform, some time is frequency of spent where the scope does occurrence in time not look at the waveform. and frequency of This is called dead time. occurrence per edge Many oscilloscope users are in a signal. This surprised at how large this relationship is dead time is relative to the interesting because it reveals that time that the scope is live. In the TriggerScan Operation anomalies that occur very rarely per example, the reason it takes the scope edge might occur quite frequently in almost three hours to see an anomaly occurring time. Let’s take a situation where anomalies are every five seconds is because the oscilloscope sees occurring at a rate of once every five seconds on only 0.2% of the edges occurring; it is dead 99.8% of 200 MHz clock; a glitch for example. This means the time even with this fast update mode. The that the anomalies per edge would be calculated as: scope sees only one out of every 500 edges of the waveform. AnomaliesPerSecond
