[SI-LIST] Re: negative propagation delay
- From: Perry Qu <perry.qu@xxxxxxxxxxx>
- To: a.ingraham@xxxxxxxx
- Date: Fri, 03 Oct 2003 11:22:06 -0400
Andrew:
Thanks for your comments. I understand that delay is relative. Negative delay
means that the Tco in datasheet is too big for my real load and we should take
it into account accordingly. Agreed that negative delay number does not cause
hold time violation alone but the fact that real loading is smaller should be
taken into account, rather than just relying on datasheet Tco number for hold
time calculation.
0 pF will be ideal for Tco spec but that's not practical as you pointed out.
Regards
Perry
"Ingraham, Andrew" wrote:
> > As a few people pointed out to me offline, negative delay should be ok as
> > it
> > reflected the situation that Tco for my real load should be smaller since
> > my
> > real load is smaller.
>
> And it's not really negative delay. It's just one number that is smaller
> than another number. So if you express the first number with respect
> to the second number as your reference, then the first number would
> be negative.
>
> It's like driving a car at 90 km/h and noticing that the car next to you,
> which is going only 80 km/h, seems to be driving backwards. The faster
> you go, the more he seems to be going backwards.
>
> But (total) delay is always positive. The car next to you still gets to the
> exit ramp after the entrance ramp.
>
> > I guess the only
> > reason I need to watch out negative delay is for hold time margin, since
> > if
> > I plugged in a negative delay for Hold time margin calculation, I may end
> > up
> > with negative margin.
>
> I don't follow this. If the delays are such that your value for "wire"
> delay (or whatever you or your tool calls it) is negative, then that's
> what it is. Yes, Hold time problems do happen, they can happen
> whether the propagation delay number is positive, negative, or zero.
> Having a negative delay number does not necessarily mean there is
> a Hold time violation.
>
> You should never "correct" or modify a propagation delay number
> just because it's negative.
>
> The reason this stuff happens, is that the delay of a driver part is not
> independent on the load you connect to it. You can't separate out the
> delay of the driver alone, from the delay of the interconnect/load. So
> the approach we take, is to measure and specify the delay of the driver
> with some sort of "standard" or "test" load attached.
>
> A load of zero (0pF) might be better than 50pF from the point of view
> of partitioning the part's internal delay from the load's effect on delay,
> except that 0pF loads are impractical to use, and ultimately everything
> on the part's data sheet needs to be tested on an IC tester with realizable
> loads. The data sheet generally reflects what is testable.
>
> Also, there are people who do back-of-envelope timing analysis without
> simulating their interconnects, and it's better to use a test load that is
> similar to the actual load one might typically attach to a part, so that the
> customer doesn't get unrealistic expectations.
>
> Regards,
> Andy
>
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--
Perry Qu
Design & Qualification | 600 March Road
Alcatel Canada | Ottawa, ON K2K 2E6, Canada
DID: (613) 7846720 | FAX: (613) 5993642
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