Hi Steve, Sorry for my empty message. Although I've always been a bit leery about this kind of behavioral modeling that merely depends on a set of numbers under restricted conditions, I think this whole IO buffer standardization should cover these issues as well. If not, we should restrict the usage of these models to signal integrity for overshoot/undershoot, termination requirements, etc. Timing is too serious an issue to go with uncertainty in a digital circuit... Ihsan On 10/20/05, steve weir <weirsi@xxxxxxxxxx> wrote: > > Ishan, while what you say about buffer construction is true, it really > doesn't go to Andy's point which my interpretation is: > > KNOW YOUR MODEL'S FRAME OF REFERENCE! > > One would hope that the model maker constructed the model in question such > that it can be correlated to observable nodes on the board. But there > isn't anything that forces them to be so reasonable. > > Steve. > > At 09:32 PM 10/20/2005 -0400, Ihsan Erdin wrote: > >In contemporary IC devices, the buffer stage operation is distinctly > >separated from that of the core logic. IO buffers are multistage analog > >(OPAMP) amplifiers that interface to the core logic with a differential > >amplifier stage. Their output stage is a highly linear class-AB amplifier > >with a low impedance to drive the load. They are usually operated at > higher > >voltage levels than the core circuit because in microprocessors and such, > >the internal logic has to switch at much higher rates than the output, > hence > >the necessity of lower voltage levels at the core. With the advent of low > >voltage signaling like LVDS, PECL, HSTL, etc. for gigabit interfaces, > >devices that employ them have started to operate at the same voltage as > the > >core but that doesn't merge the IO and core logic. In this context, > >subtracting the buffer delay from the output timing is justified for the > >accuracy of timing analysis. > >As for the cases where buffer delay may be larger than the min. output > >timing parameter, we know for sure that the data cannot be at the buffer > >output before the min delay following the clock edge. Rounding the > internal > >logic delay to zero will yield a higher than spec'd min output time, > which > >may mess up the whole fast process timing computations. So using the > >negative number is the right thing to do. > > > >Ihsan Erdin > >Nortel > > > >On 10/20/05, Andrew Ingraham <a.ingraham@xxxxxxxx> wrote: > > > > > > > According to the TCO measurement, TCO include Internal > > > > Logic delay and external buffer delay. > > > > > > I am assuming by "external buffer" you mean the buffer, internal to > your > > > device, that drives the output (external) signal. Is that what you > meant? > > > > > > > I use the IBIS model and the measurement circuit to > > > > measure the external buffer delay. > > > > > > Be careful about this. IBIS does not necessarily model the external > buffer > > > delay. The IBIS model may represent just a portion of the buffer, or > it > > > may > > > include more than the buffer. All you know is that IBIS describes how > the > > > buffer drives the output node. If you set up a simulation and measured > the > > > delay from the "input" of the IBIS model to its output, it is a > relatively > > > meaningless measurement. > > > > > > Aside from that, there are many reasons why the internal logic would > > > appear > > > to have a negative delay. One is if the device incorporates a PLL for > the > > > clocks. Another happens because of different loading conditions. If > your > > > calculation results in a negative number and you know you are doing > the > > > right calculations and using the right methods, then use the negative > > > number, not zero. > > > > > > Why do you need to separate the internal logic delay from the output > > > buffer, > > > anyway? Why not just treat your device as one block that has no such > > > boundary? (Perhaps my assumption above was incorrect?) > > > > > > Regards, > > > Andy > > > > > > ------------------------------------------------------------------ > > > 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 FAQ wiki page is located at: > > > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > > > > > > List technical documents are available at: > > > http://www.si-list.org > > > > > > 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 FAQ wiki page is located at: > > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > > > >List technical documents are available at: > > http://www.si-list.org > > > >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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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