Alfred, It is true that the longevity of impulse response measured in bit intervals determines the problem complexity. Good design is to prevent reflections and sharp resonances thus making ISI shorter and smaller. However, in presence of low frequency resonances (much lower than the bit rate) we may have problems. I'm not going to analyze the causes of such resonances; these could be signal paths, or PDN or something else. Just let us say that a priori we do not exclude a possibility for a channel to have a relatively long impulse response. If it is short, that's OK: peak distortion or another worst case analysis will show that there are no serious problems. But if it is longer than we think it is, that's not good. Sometimes, it is difficult to decide where we have to truncate the response. Here, worst case solution also helps. If we perform such analysis twice and don't see further eye degradation with longer response, this gives us some confidence. If we look at the spectrum density of non-cyclic PRBS, most of its energy is concentrated below 0.5...0.7 of its base frequency. For 8b10b constrained pattern, spectral density has a cut-out around zero frequency and most of its energy is between 0.01 and 0.5 of the base frequency. Hence, much depends on whether this portion of the spectrum coincides with some peculiarities of the channel's transfer function. Yes, it is true that there are many other factors affecting the eye (crosstalk, noise and jitter). In the worst case analysis we can try to account for some phenomena that can be considered in a relatively simple deterministic way (ISI, some types of SSO and crosstalk). But if we cannot (algorithms become too complicated, or the effect is not deterministic) then yes, we perform other types of analysis, such as time domain and statistical, as we always did. Still, we may want to see how these additional factors will affect the eye already stressed by the 'worst case' input pattern. My opinion about 'gazillions' of bits is the same: we don't have to, because just the effect of ISI, mode conversions or crosstalk can be estimated much faster with worst case analysis. However, another interesting side of this problem is a relation of the worst case and statistical analysis. For example, if from peak distortion analysis we find that the worst combination of input bits, crosstalk and deterministic noise or jitter reduces the eye size beyond the required mask, has the design failed? Not always. Worst case analysis could be overly pessimistic. It may easily give you the worst case solution that has a probability of 1e-50 or smaller, that simply has no practical meaning. Only combined with statistical analysis, and verified by time domain simulation, worst case estimation provides a practically useful answer. Vladimir -----Original Message----- From: Alfred P. Neves [mailto:al.neves@xxxxxxxxxxx] Sent: Tuesday, March 10, 2009 6:57 PM To: 'QU Perry'; Dmitriev-Zdorov, Vladimir; si-list@xxxxxxxxxxxxx Subject: RE: [SI-LIST] Re: How to simulate worse case eye Am I missing something here? If I examine a full S-parameter description of a LTI passive channel that is relatively behaved, such that there are no deep resonances within the first or second spectral null frequency of the input TX power spectral density, my experience suggests that ISI impacts between n-bits of data is relatively constrained to a reasonable length. In fact I believe this is shown by expanding the impulse response of the channel into a Taylor series such that the higher order terms generally drop off rapidly suggesting there is a reasonable constraint. In other words, higher order derivatives (such as deep VIA or cavity type resonances) relate to a longer interaction ISI length. Worse eye pathology, in my experience, is dramatically impacted by the noise injected into PLL-VCO substrates, crosstalk from long length PRBS aggressors, supply resonance, RJ due to PLL VCO, SSO, SSN, RJ due to input referred noise to the RX, variations of PLL loop dynamics to long 111's or 0000's, etc.,. Adding silicon changes the picture in other words. Is running gazzilions of bits for passive LTI channels (no Pll's or RJ in other words) a solution looking for a problem? My thinking is that be evaluating the channel's signal integrity (including mode conversion, crosstalk as measured with S-parameters, impedance variation, etc,) you will learn a lot more than running LOTS of bits in the time domain. It would be interesting to chart (simulated and measured) peak-peak DJ versus simulation run length in #bits for a host of channel models ranging from very well behaved to very poor S.I. and maybe highly resonant, also including a channel model with 1-7% PRBS crosstalk with varying length (2^7 to 2^31). Alfred P. Neves `·.¸¸.·´¯`·.. ><((((º> .¸¸.·´¯`·.¸><((((º>`·.¸¸.·´¯`·.. ><((((º> 735 South East 16th Avenue Hillsboro, OR 97123 (503) 718 7172 Office (503) 679 2429 Mobile -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of QU Perry Sent: Tuesday, March 10, 2009 1:28 PM To: Dmitriev-Zdorov, Vladimir; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: How to simulate worse case eye Hi, Vladmir: Thanks for the pointer. I will take a look of the training video. Will also check what's new on HL 8.0. Currently we are running 7.7 but we plan to try the 8.0 Beta. Agree that running long PRBS pattern in Hspice or any time domain solver is very time consuming, but as a minimum I hope that I can verify the fasteye algorithm to certain extent by doing some comparisons on selected case with old/plain Hspice simulation and get some confidence on the results. On a side note, I wish we can separate Hspice with Hyperlynx as my limited experience of running Hspice within HL is not very positive. Another issue is encrypted I/O model and it's not very straightforward to run Hspice sim within HL using encrypted models. Will I be able to run a pulse/step response outside of HL and just bring in a voltage/time waveform into HL for fast eye calculation? That would be ideal for me. We can talk more offline or if you have a support person that I should speak to, please let me know. Regards Perry ======================================= Perry Qu IPD Design & Qualification, Alcatel-Lucent Canada Inc. 600 March Road, Ottawa ON, K2K 2E6, Canada DID: 613-7846720 Fax: 613-5993642 Email: perry.qu@xxxxxxxxxxxxxxxxxx ======================================= -----Original Message----- From: Dmitriev-Zdorov, Vladimir [mailto:vladimir_dmitriev-zdorov@xxxxxxxxxx] Sent: Tuesday, March 10, 2009 1:27 PM To: QU Perry; si-list@xxxxxxxxxxxxx Subject: RE: [SI-LIST] Re: How to simulate worse case eye Hi Perry, In fact, you may choose between at least 3 different simulators when running for step or impulse response in Hyperlynx (including HSPICE; they are integrated). The result of this test simulation is fed directly to FastEye engine that is a part of Hyperlynx and performs fast time domain, worst case and statistical analysis for eye diagram/BER etc. >I was thinking about feeding a PRBS31 pattern into a 8b/10B encoder and use >the output of encoder as input excitation in simulation. Yes, this is a good idea. In "fast" time domain analysis, we support pseudo random generation of the 8b/10b pattern, but we do not base this generation on any cyclic source, like LFSR, to avoid periodicity. This way we may get any combination that does fit into 8b/10b constrains. However, even though 8b/10b is a subclass of general unconstrained pattern, it still has sufficient "variability" preventing us from finding the worst case by simply performing long simulation. For example, some time ago, we performed an experiment where such non-cyclic "pseudo random" 8b10b pattern was running on 1 trillion = 1e12 bits (took about 3 weeks to complete). Still, the resulting eye contour was unable to approach to a 100 bit long worst 8b10b combination that can be found from the worst case analysis in a fraction of a second. The eye height that the worst pattern provided was about 12% smaller than we get from this long simulation. My guess is that PRBS7 is often used as a substitute for 8b10b, because it has approximately same running length. However, it still may have 7 ones in a row, and in a long run, it does not support disparity between its 4/6 bit words, and globally: with 127 bit period, it simply cannot contain equal number of ones and zeros. Vladimir There is a tutorial video showing how you use HyperLynx FastEye in v7.7 which is the tool that generates the worst case bit sequence. http://supportnet.mentor.com/reference/tutorials/50024.cfm There is also a possibility to get the license and try the new beta version that is more flexible in terms of providing channel characterization. -----Original Message----- From: QU Perry [mailto:Perry.Qu@xxxxxxxxxxxxxxxxxx] Sent: Tuesday, March 10, 2009 10:32 AM To: Dmitriev-Zdorov, Vladimir; si-list@xxxxxxxxxxxxx Cc: Oh, Dan Subject: RE: [SI-LIST] Re: How to simulate worse case eye Hi, Vladimir: Would you be able to provide some more details on the simulation flow you mentioned using Hyperlynx? If I understand correctly, you extract step response or pulse response separately in Hspice and then feed that waveform into HL for worst case eye simulation ? Do we need the new version of HL for that (8.0)? I also share your observation on using long PRBS pattern on AC coupled serdes channel can be over-pessimistic for 8b/10b encoded system from DC balance point of view. PRBS7 on the other hand does provide enough pattern variation and can be too optimistic. I was thinking about feeding a PRBS31 pattern into a 8b/10B encoder and use the output of encoder as input excitation in simulation. We asked Synopsys to add 8b/10b encoder in LSFR function and they agreed to do that. Dan: I'm interested in your paper. Can you please send me a copy? Thanks! Perry ======================================= Perry Qu IPD Design & Qualification, Alcatel-Lucent Canada Inc. 600 March Road, Ottawa ON, K2K 2E6, Canada DID: 613-7846720 Fax: 613-5993642 Email: perry.qu@xxxxxxxxxxxxxxxxxx ======================================= -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Dmitriev-Zdorov, Vladimir Sent: Saturday, March 07, 2009 10:02 AM To: Dmitriev-Zdorov, Vladimir; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: How to simulate worse case eye Tim/Joel, Perhaps, I should have given more details in my previous reply. Yes, the paper of Brian Casper, and even some earlier works give us a method of building an unconstrained worst case pattern; that is relatively simple. I would also recommend reading this paper as an excellent introduction into the topic. However, this approach does not answer all practical needs, including the following. 1. Encoded worst case pattern. In many important cases the unconstrained worst case solution does not have much value. If e.g. the SERDES channel includes series capacitor(s), we know that the eye will be closed by sufficiently long series of logical 'ones' or 'zeros'. The longer is a series of identical bits, the more closed the eye becomes. In this sense, there is no 'worst' unconstrained solution, unless the pattern length is limited. Of course, in such channels only encoded binary inputs are allowed (8b10b or some others), with their imparity and running length constraints, for which the idea of 'worst case pattern' makes a perfect sense. There exists a solution for such case in HL. 2. Some types of non-LTI behavior. For example, in many cases the responses to rising and falling transitions are not symmetrical, that means their sum R(t) + F(t) is not identical constant. There are several sources of this phenomenon: (a) persistent time shift between R/F transitions (DCD), (b) asymmetry of PU and PD I-V or timing characteristics in a single ended channel, or (c) partial conversion of near end common signal into far end differential signal, that may occur even with identical differential buffers, if linear part of the channel is not ideally symmetric (creates differential skew). In the recent Mentor/Tek paper "New methods of measuring the performance of equalized serial data links and correlation of performance measures across the design flow..." from DesignCon2009 we considered worst case solutions, including cases (1), (2) and combination of (1) and (2). > > > > Joel, I'm sure there will be no shortage of recommendations here, but here's my contribution: It's pretty easy to determine the worst case pattern for a single route, assuming the system is linear-time-invariant. 1. Start by generating the pulse response of the system. 2. Then sample the response at UI intervals from the Peak or cursor sample, or just eyeball the ISI at these intervals. 3. Based on the direction or polarity of the ISI terms, and their relative distances from the cursor (in UI), you can determine what input pattern can be used to maximize the combined contributions of the ISI terms. Positive-going ISI terms eat away from the nominal zero level, and negative-going ISI terms eat away from the nominal one level. 4. You can then move your cursor away from the peak of the pulse response and repeat the process to determine patterns targeting points across the width of the eye if desired. If you haven't read it, check out Bryan Casper's paper describing the use of Peak Distortion analysis to generate Worst Case Eyes. It doesn't explicitly tell you how to generate the worst case pattern, but it may inspire you. B.K. Casper, et al, "An accurate and efficient analysis method for multi-Gb/s chip-to-chip signaling schemes." If you've got IEEE access: http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber15043&isnumber!8 31 Tim Hollis DRAM Design Micron Technology, Inc. -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Joel Brown Sent: Friday, March 06, 2009 3:35 PM To: SI-LIST@xxxxxxxxxxxxx Subject: [SI-LIST] How to simulate worse case eye I was watching a webinar by Mentor on Hyperlynx and how they can quickly generate a prbs pattern that results in a worse case eye diagram. They said without this feature it could take days or even years of simulation to do this. I do most of my simulation in hspice since most of my models are based in Hspice. Is there a way to do what Mentor is claiming in Hspice by generating a certain pattern? I have been using the following code for a prbs sequence: vin inr vcm LFSR(0.1 -0.1 1n 5ps 5ps + 'data_rate' 1 [7,4,1] rout=0) vinn innr vcm LFSR(-0.1 0.1 1n 5ps 5ps + 'data_rate' 1 [7,4,1] rout=0) Thanks - Joel ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu