TDS measurement for Hi-speed Processor

IntroductionWith rapidly ascending pro-cessor clock rates and tighter timing considerations comes a more pressing need to pre-cisely determine timing mar-gins. Traditional timing mea-surements, which use cursors or histograms to make statisti-cal determinations of timing, have been limited by their inability to directly correlate a particular clock edge to the corresponding data edge. Inaddition, these techniqueshave not been able to charac-terize specific data transi-tions.Today’s high bandwidthacquisition circuits, extremelylong record lengths, and cus-tom SW applications are mak-ing possible a new level ofprecision in timing measure-ments. This application notewill use the most commontiming measurement, setup-and-hold timing, to illustratethis new capability. We willstart with a traditional setuptime measurement – whichuses multiple triggered acqui-sitions to acquire timing data– then compare it with thenew timing capability – whichcaptures all timing data in asingle acquisition. Then we’lldiscuss the advantages of thenew capability.Copyright © 1999 Tektronix, Inc. All rights reserved.New Timing Measurement Techniques for High SpeedProcessors using TDS OscilloscopesTraditional Setup Time MeasurementFigure 1 shows the output of a Tektronix oscilloscope per-forming a typical processor setup time measurement. For this measurement, Channel 1 is clock and Channel 2 is data. The oscilloscope’s infi-nite persistence mode was used to capture the data, and cursors are used to determine the minimal setup time. From this illustration, you can see how difficult it is to precisely measure setup tim-ing with histograms and cur-sors. Parameters such as trig-ger jitter and vertical noise can cause both the clock anddata waveforms to shift hori-zontally in time. As a result,the right-most data transitionis probably not acquired atthe same time as the left-mostclock transition. The mini-mum setup time of 400psmeasured with the cursorsdoes not truly show the mar-gin available. Note:•The scattering of points onthe data channel•The positive and negativedata transitions have differ-ent setup time margins•The oscilloscope is inequivalent time mode (it’scapturing data over multi-ple trigger acquisitions)A New Way To Make TimingMeasurementsIn Figure 2, the TektronixTDS694C oscilloscopeequipped with the newTektronix TDSPSM1 Proces-sor Specifications Measure-ments package performs asetup time measurement. Themeasurement summaryshows that 119 valid data-to-clock timing measurementswere performed on a single250,000 point acquisition.The setup time measure-ments have a minimum mar-gin of 7.2966 ns with a stan-dard deviation of 667.39 ps.page 2How Is This New Technique Different?The traditional cursor and histogram measurement tech-niques are performed over multiple triggered acquisi-tions as shown in Figure 3. With traditional timing mea-surements, information about specific margins is con-founded by the nature of the acquisition. Trigger jitter of the scope influences the acquisitions and can appearto reduce the true margin.The new timing measurementtechnique, performed by theTektronix TDSPSM1 applica-tion package, measures tim-ing on each and every validedge in an acquisition wave-form whose length can rangefrom 500 to 8 million points(Figure 4). In addition, the10GS/s sample rate and3GHz bandwidth of theTDS694C allows single-shotviewing of high speed digitalsignals at high resolution.This allows true one-to-onecorrelation between a specificclock edge and its corre-sponding data transition.Another advantage of thisnew capability is that theuser can collect informationabout specific edges. In Fig-ure 1, timing information iscollected on both data edgetransitions. As can be seen,page 3the setup time budget varies depending upon whether the data is going positive or nega-tive. Figure 5 shows the TDSPSM1 measurement of set-up timing information for 46 valid edges that transition from positive to negative. The TDSPSM1 application pack-age is designed to be versatile so that it can collect statistics from user-defined clock edges to user-defined data edges.The setup and hold timing measurements described above have many compo-nents that comprise the datavalid window. These compo-nents include rise, fall, low,and high times. Figure 6shows these critical measure-ments made on approxi-mately 2,300 valid cycles.Again, what is significant about these measurements is that they were made on each and every valid cycle in a sin-gle-shot acquisition. For example, the max rise-time parameter tells you the worse case rise-time out of the 2,340rise-times that were captured with single-shot acquisitions.ConclusionWith the Tektronix TDS Digi-tal Oscilloscopes and the new TDSPSM1 application pack-age, Tektronix introduces a way to make more precise timing measurements. By combining the newTDSPSM1 measurement tech-nique with TDS500D/600C/-700D Digital Oscilloscopes that provide sample rates to 10 GS/s, bandwidths to3GHz, and record lengths to 8 MB, customers can make timing measurements ofunprecedented accuracy from single-triggered acquisitions.7/99TD/XBS55W–13158–0Copyright © 1999, Tektronix, Inc. All rights reserved. Tektronix products are covered by U.S. and foreign patents, issued and pending. Information in this publication supersedes that in all previously published material. Specification and price change privileges reserved. TEKTRONIX and TEK are registeredtrademarks of Tektronix, Inc. All other trade names referenced are the service marks, trademarks, or registered trademarks of their respective companies.。