[SI-LIST] [Fwd: Re: Re: Adding trace length for timing adjustment]
- From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 21 May 2003 02:12:00 -0700
Tegan,
Are you sure that the simulation accounted correctly for the effects
encountered in a serpentine? Some board level signal integrity tools do
not simulate same net coupling across a serpentined trace. And, even if
they do, no board level tool will account for the corner effects, which
cannot be accounted for in quasi-static field solvers.
regards,
scott
--
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
Tegan Campbell wrote:
>Just a thought.....I agree from a technical perspective if you are talking
>about 10's of picoseconds matching. But on a board I just finished I
>instructed the designer to run three clock lengths on inner layers, and
>match them to within 50ps in simulation. He used serpentining with a rule
>that the net could not come within 2*dielectric thickness of itself. A
>fourth clock net I had him run on the outer layer and have it's flight time
>be 500ps less than the others. This did two things:
>
>1-Satisfied the customer(internal to the company, in this case) that I
>thought about proper timing on the board level and conformed to his specs
>for his chip
>2-Impressed upon people the value of using SI tools along with normal board
>flows. Saved ~2.00 per board(precision delay IC). It helped that we had no
>EMI problems when we got into the chamber.
>
>for what it's worth.
>
>Tegan
>
>-----Original Message-----
>From: Loyer, Jeff [mailto:jeff.loyer@xxxxxxxxx]
>Sent: Monday, May 19, 2003 11:23 AM
>To: Jon Powell; doug@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Adding trace length for timing adjustment
>
>
>
>I agree. The reference to pS came out wrong - today I would state it =
>differently (I wrote the original some time ago). Thanks for pointing =
>out the mis-statement.
>
>When we get up to high enough speeds that pS's count, other techniques =
>are called for. PCI-express, for instance, doesn't have the length =
>matching constraints of past busses (although the matching between =
>halves of differential pairs is specified).
>
>Jeff Loyer
>
>-----Original Message-----
>From: Jon Powell [mailto:jonpowell@xxxxxxxxxxxx]
>Sent: Monday, May 19, 2003 8:59 AM
>To: Loyer, Jeff; doug@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: RE: [SI-LIST] Re: Adding trace length for timing adjustment
>
>
>In my opinion if you are using serpentine nets to provide pico second
>matching, you are fooling yourself and just trying to match some spec =
>that
>is probably suspicious in itself. The coupling of nearby traces and just
>plain irregularities on the constructed board are going to wipe out any =
>ps
>precision matching that you think you are doing.
>
>regards
>jon
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Loyer, Jeff
>Sent: Friday, May 16, 2003 6:54 PM
>To: doug@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Adding trace length for timing adjustment
>
>
>
>First, a question of mine (for anybody): is there some theory that =3D
>declares that the effect I describe below (coupling across serpentine =
>=3D
>legs causes part of the wave to bypass the serpentine) is zero in =3D
>stripline?
>
>Now that I've asked that, here's some of what I've found:
>________________________________
>
>No intense research here, but I did TDR the Front-Side Bus of a product =
>=3D
>board that had a variety of serpentine types (a few loooong legs, many =
>=3D
>short legs, some in-between), and here's what I found.
>
>Conclusions:
>1. Serpentining was >80% effective on this Front Side Bus (FSB). I.E.,
>=
>=3D
>we achieved at least 80% of the expected delay from serpentines.
>2. Vp variation due to serpentining induced about 30pS of skew on this
>=
>=3D
>3", 50ohm bus.
>3. A few long legs of serpentining were better than several short ones.
>4. There won't be any "Rules of Thumb" which will easily describe the =
>=3D
>issue (how effective are serpentines?). When we need to worry about pS, =
>=3D
>simulations have to be performed on expected worst-case nets.
>
>Other Notes
>1. Only layer 1 was represented here. The effects have been less on
>=3D
>stripline.
>2. The dielectric is 4mils thick; serpentine legs are separated by =3D
>20mils center to center (same as trace-to-trace separation); trace width =
>=3D
>=3D3D 7mils.=3D09
>
>Some general conclusions of mine:
>Increasing trace length with serpentines (AKA "meanders") does not give =
>=3D
>an increase in flight-time directly proportional to the increase in =3D
>trace length. Coupling across the serpentine legs causes part of the =3D
>wave to bypass the serpentine (I would refer to it as a "barreling =3D
>through the switchbacks" phenomena), reducing the flight-time. The =3D
>speed-up effects are reproducible in simulations and seem to be only =3D
>weakly tied to rise-time.=3D20
>
>The effect can be lessened by separating serpentine legs, or routing in =
>=3D
>stripline. A "flat spiral" (AKA "bifilar spiral") is an option mentioned =
>=3D
>in some papers, but my experience with it has shown it to be terrible, =
>=3D
>S.I.-wise.
>
>Take pains to avoid serpentines - they're not free.=3D20
>
>If serpentining is necessary, keep the adjacent traces far apart - =3D
>perhaps an S/H (trace separation to dielectric thickness) ratio of about =
>=3D
>5 to 1.=3D20
>
>Routing in stripline reduces the effect.
>
>Jeff Loyer
>
>
>-----Original Message-----
>From: Doug Brooks [mailto:doug@xxxxxxxxxx]
>Sent: Thursday, May 15, 2003 10:09 AM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Adding trace length for timing adjustment
>
>
>
>
>
>I know there are several different views on this subject.
>
>Suppose I need to add some additional length (time) to a trace and =
>am=3D20
>considering three different strategies:
>
>1. a randomly meandering length
>2. a "trombone-like" length down and back
>3. more, shorter snake-like loops
>
>What do people see as the different trade-offs for these three different =
>=3D
>
>strategies under differing conditions?
>
>Doug Brooks
>
>
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>
>
--
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
--
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
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