Embedded Systems Design Europe - June/July 2008 - (Page 30)

debugging the timebase setting. However this does not mean it is better to use a slower timebase ranges to capture narrow glitches. Although the scope deﬁnitely has a better chance of capturing the narrow anomaly, assuming that the scope still samples at a sufﬁciently fast rate, it may not be possible to visually spot the narrow anomaly on these slower timebase ranges. Figure 3 shows an example of capturing the same meta% DT = 100 x (1- (125/s stable state shown previously, x 200ns)) but now with the scope’s time= 99.998% Fig 3: Although scaling the timebase to a slower range base set at 2 µs/div. The scope improves the probability that the MSO can capture the easily captures the 15-ns-wide With oscilloscope deadglitch, we are unable to visually “spot” the glitch onglitch, but we can’t see it at this time in this range, the statistithe-ﬂy while repetitively acquiring waveforms. timebase setting. cal probability of capturing Finding and debugging a glitch that occurs just 100 times per second is extremely low…… glitches/sec with an observation time of random and infrequent events is a key aspect to debugging embedded designs. 5 seconds was used for these measureMSO update rates directly determine P(5s) = 100 x (1 – [1-(100/s x ments and theoretical calculations. the probability of capturing and disNote that although the waveform 200ns)](125/s x 5s)) playing these types of random circuit update rates and dead-time percentage = 1.24% problems. appears to decrease on slower timebase With the addition of logic timing Table 1 summarizes dead-time per- ranges, absolute dead time (in seconds) channels and serial bus decoding, toincreases. Slower timebase ranges incentage and glitch capture probability day’s mixed signal oscilloscopes should of two MSOs tested using four different crease the display acquisition window, enhance the ability for rapid debug of which means that more digitized data timebase settings. embedded designs, but if waveform and must be processed, thereby increasing In all cases, two analog channels decode update rates are degraded when actual dead time. Slower update rates plus ﬁve logic channels were turned on using the additional functionality of and memory depth was either automat- on slower timebase ranges are primarthese scopes, the probability of capturily driven by longer display acquisition ically or manually optimized such that ing infrequent signal problems will also time. each scope sampled at its maximum be degraded. Also note that the probability of speciﬁed rate to provide 1 GHz realFor additional information about capturing a waveform anomaly also imtime bandwidth with the minimum the importance of waveform update proves on slower timebase ranges. This amount of acquisition memory to suprates, as well as documented waveform is primarily because the dead-time perport that sample rate. and serial decode update rates using centage is decreasing as you slow down A glitch occurrence rate of 100 three different vendor’s MSOs, download Agilent’s application MSO 1 (Agilent) MSO 2 note titled, “Evaluating Oscilloscopes for Best Signal Visibility” at Timebase Update Dead Glitch capture Update Dead Glitch capture http://cp.literature.agilent.com/litrate time probability rate time probability web/pdf/5989-7885EN.pdf vendor’s MSOs. Although 125 waveforms per second will produce a very responsive display, statistically speaking this update rate is much too slow to reliably capture infrequent anomalies such as a metastable state that occurs just 100 times per second on average. This is because the scope’s dead-time at 125 waveforms per second is extremely long: 2 ns/div 20 ns/div 200 ns/div 2µs/div 74,000 95,000 63,000 8,000 99.85% 98.1% 87.4% 84.0% 52.29% 99.993% 99.999..% 99.999..% 130 125 125 125 99.999% 99.998% 99.978% 99.780% 0.13% 1.24% 11.75% 71.39% Table 1: MSO dead-time and glitch capture probability using analog and digital channels. 30 JUNE – JULY 2008 | embedded systems design europe | www.embedded.com/europe Johnnie Hancock is a signal integrity applications engineer within Agilent Technologies Electronic Products Group. He is currently responsible for worldwide automotive applications supporting Agilent’s digitizing oscilloscopes. http://cp.literature.agilent.com/litweb/pdf/5989-7885EN.pdf http://www.embedded.com/europe

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Embedded Systems Design Europe - June 2008 Contents Work in Progress to Define Compact PCI Plus Power.org Demonstrates New Tools Project Supports Multi-core System Programming Altium Links Electronic to Mechanical Design PLDs Look to Cut Power Budget and Costs Project to Provide Coverage Analysis Tool Microsoft Details Windows Embedded Update Cover Feature: Leveraging Virtual Hardware Platforms for Software Allocating Memory in MATLAB-to-C Code MDD & IDEs: Making the Twain Meet in Embedded System Designs Debugging Mixed Signal Designs for Infrequent & Random Events Why Open Source is the Natural Choice for High-security Systems Bringing the Benefits of Low Power CPUs to Modular Design New Products Advertising Contacts

Embedded Systems Design Europe - June/July 2008

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