[PCB_FORUM] Length matching for DDR - was - RE: Re: Allegro V6.01 Constraint Manager

Hi Richard,

> > I'd like to send datasheets, but I can't.  See
> >  http://focus.ti.com/lit/an/spraac5f/spraac5f.pdf ,
> > page 19.  Not exactly our designs, but the same idea.
> > It is to force reflections to cancel.

> I looked at that page/diagram.  It doesn't say a thing about making the
length
> matching from the T to the destination tighter than from the source to the
> destination, at least that I saw. They only provide source to destination
> length match numbers.  Did I perhaps miss something?

Actually, I did miss something.  Their diagram confused me, they used
rectangles to indicate the segment lengths (yet they say "length A to C",
when they really mean A + C), instead of lines.  But, none the less, I
believe I understand it now, and they specify:

"Length B should match length C within 100 mils."

Which is apparently the T to destination...so they do specify this...but...

"Length A to C and A to B should match within 100 mils within ADDR_CTRL net
class."

The A segment is the same for both A to C and A to B, which means it is only
the B and C segments that length match to the 100 mils...so stating that B
should length match C to within 100 mils is redundant.

The proof:

A to C = A + C, A to B = A + B, so they want the following:

100 = ABS((A + C) - (A + B))
100 = ABS(C - B)

So:

ABS((A + C) - (A + B)) = ABS(C - B)

for simplicity let's remove ABS for now...and since no order of precedents
exist in addition/subtraction:

Let's substitute - (A + B) = - A - B in the equation:

A + C - A - B = C - B

A - A makes A drop out so we are left with:

C - B = C - B

...and means the two constraints are equal, which means they aren't
constraining the T to destination tighter than the overall length match.
Unless I made a mistake somewhere...but I think I got that right.

Best Regards,

Austin

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