Abrar, I am going to reply publicly, beaus this is a common question, and I would like other people to benefit from this reply also. The main reason for the rule of sweeping the IV curves from -Vcc to 2*Vcc is based on the basic reflection phenomena of transmission lines. If you have an open line, the voltage can double due to reflections. If your driver could drive a rail to rail signal into a T-line (0 to Vcc, or Vcc to 0), there is a possibility that you could see a reflection that goes to 2*Vcc or -Vcc. However, this will usually not happen in real life, because 1) in order to be able to drive rail to rail, the driver impedance and T-line impedance ratio would have to be extreme (low driver Z and high T-line Z) 2) in order to get a doubling at the end of the T-line, the T-line impedance and the receiver's impedance, including its clamps, would also have to be extreme (low T-line Z and high receiver Z) For one, these two conditions pose a conflicting requirement on the T-line impedance, and second, practical designs usually cannot have extremely low driver and extremely high receiver impedances. In addition, most high speed buses today are terminated anyway, so reflections are not going to double because of that. So, the bottom line is that the doubling of the signal is only an extreme or absolute worst case situation, and considering this background, it really doesn't matter whether the IV curve is swept -Vcc to 2*Vcc using the typical or maximum supply voltage conditions. There is another problem. The IBIS specification doesn't spell out which voltage we should use for calculating -Vcc or 2*Vcc. To be safe you could use the max Vcc values, but I usually use the typical only. There is no rule for this in the spec. To add another twist to this subject, consider the low voltage swing devices. For example, in GTL signaling the high level is at 1.5 V, and the low level is anywhere between 0 and 550 mV depending on how strong the driver is. If the driver was a super conductor, it could drive to 0 V. In this case a full reflection would give you -1.5 V on the low side, and 3.0 V on the high side. But GTL is a terminated bus, so you will never see full reflections. So considering a 3.3 V I/O supply on a GTL device, does it really make sense to sweep the IV curves from -3.3 V to 6.6 V? Absolutely not. Yet the IBIS specification doesn't say anything about that. If you obey the letter of the law, you will use up a lot of the 100 points in areas of the IV curve which will never be used. So my recommendation is to seep the double of the signal swing to the top and bottom. In this GTL case this would be no more than -1.5 V and 3.0 V. Similar situations can arise with other low voltage swing buffers, such as LVDS, etc... Since this is illegal according to the IBIS spec, I also recommend that after you are done, just add to more points with extrapolation to the ends of the IV curve at -Vcc and 2*Vcc (without going over the maximum limit of 100 points) to satisfy the spec and avoid any possibilities for the parser to give you error messages. I hope this helps your understanding of the IV curve ranges. Arpad Muranyi Intel Corporation --------------------------------------------------------------- ________________________________ From: Abrar Ahmed [mailto:abrar_techie@yahoo.com] Sent: Tuesday, April 11, 2006 11:05 PM To: Muranyi, Arpad Subject: RE: [IBIS-Users] Sweep voltage listing query Dear Mr.Arpad, You mean to say that the sweep range is fixed at the "typical case" values only? If that is the case, what is the wisdom behind selecting only typical case ? What about using "max case" as the sweep range, we get more values, would we not. Regards Abrar ====================================================== "Muranyi, Arpad" <arpad.muranyi@intel.com> wrote: Abrar, The pullup and power clamp tables are Vcc relative, so the origin of these IV tables are at the supply voltage. When the supply voltage moves for min and max, the origin of the IV tables moves together with it. The end points of the tables only determine the sweep range, which has nothing to do with what the supply voltage variation is. I hope this clarifies your question. Arpad |------------------------------------------------------------------ |For help or to subscribe/unsubscribe, email majordomo@eda.org |with just the appropriate command message(s) in the body: | | help | subscribe ibis <optional e-mail address, if different> | subscribe ibis-users <optional e-mail address, if different> | unsubscribe ibis <optional e-mail address, if different> | unsubscribe ibis-users <optional e-mail address, if different> | |or email a written request to ibis-request@eda.org. | |IBIS reflector archives exist under: | | http://www.eda.org/pub/ibis/email_archive/ Recent | http://www.eda.org/pub/ibis/users_archive/ Recent | http://www.eda.org/pub/ibis/email/ E-mail since 1993Received on Wed Apr 12 09:10:35 2006
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