[PCB_FORUM] Re: Diff pair setup in Constraint Manager
- From: Randy Dawson <rdawson16@xxxxxxxxxxx>
- To: icu-pcb-forum <icu-pcb-forum@xxxxxxxxxxxxx>
- Date: Fri, 15 Feb 2013 11:00:02 -0600
Every time I read a response from David Price, I am reminded he is the Obi-Wan
Kenobi of Cadence tools...
> Date: Fri, 15 Feb 2013 11:27:01 -0500
> Subject: [PCB_FORUM] Re: Diff pair setup in Constraint Manager
> From: davidp@xxxxxxxxxxxxxx
> To: icu-pcb-forum@xxxxxxxxxxxxx
>
> This always seem confusing. It's linear but not obvious how this is setup.
>
> Physical and Spacing are by definition constrained by layer using CSets. Each
> is controlled initially by the built-in Default CSet, so constraints are
> always stackup specific. Even if you put in a value of '8', it's really
> 8,8,8...8 by layer in the Default CSet.
>
> CSet values can be overridden by Net-level values. So if a CSet is assigned
> to a Net - and in one of it's properties you insert a value of '7', all
> layers get this value. Net values trump by layer values. This is the
> important thing to remember when understanding setting Diff properties in
> Electrical vs Physical.
>
> The Electrical Worksheet is ignorant of stackup, it deals with electrical
> behaviors. As such, all values in Electrical are Net-based and trump any
> comparable layer based values.
>
> Because Electrical is Net-based, and not layer-based like Physical, there is
> no Default CSet for Electrical.
>
> The 'physical' Diff values are echo'ed between Physical and Electrical. The
> electrical Diff constraints only exist in Electrical.
>
> As the Default CSt for all Physical constraints includes Diff pair
> constraints, picture completing this first. Many Users create 100ohm Diff
> CSets in Physical, for instance, but this isn't necessary. You can complete
> the Diff Pair physical constraints in the Default Physical CSet, and all
> diffpairs will inherit these.
>
> Starting in 16.6, you can save generic techfiles, with just 4 values for any
> stackup - Top, Bottom, innder signal and inner power. Then the default CSet
> can map to any stackup as appropriate via techfile.
>
> The electrical diffpair props are set in the Electrical worksheet. These are
> unique to this worksheet.
>
> The only time you want to apply physical diffpair constraints via the
> electrical Worksheet is if you have the same value on all layers and want to
> override the Physical by layer values, say as part of an Electrical CSet that
> has variances to your Physical diffpair CSet.
>
> The constraints shared between Physical and Electrical are the diff pair
> constraints. Values by layer must be set in Physical. Any values in
> Electrical, being Net-based, will override all the by layer values set in the
> Physical worksheet.
>
> Regards,
>
> David
>
>
>
>
> -----Original Message-----
> From: "Macindoe, Gary" <Gary.Macindoe@xxxxxxxxxxxx>
> Sent: Friday, February 15, 2013 10:36am
> To: "icu-pcb-forum@xxxxxxxxxxxxx" <icu-pcb-forum@xxxxxxxxxxxxx>
> Subject: [PCB_FORUM] Re: Diff pair setup in Constraint Manager
>
> Thanks for the reply Randy.
>
> One thing I notice is that if you set up your diff pairs under Physical,
> there is no way to specify the phase tolerance (difference in length allowed
> between the positive and negative signals within the diff pair) nor uncoupled
> length (necessary for breakout, etc.). These two parameters can only be
> specified under the Electrical tab.
>
> If I understand it correctly, you could set up diff pairs under Physical, but
> still need to set the above two parameters under Electrical, right? So, just
> set it up completely under Electrical sounds right to me.
>
> Regards,
>
> Gary MacIndoe
> Senior PCB Design Engineer
> EbD R&D Hardware
> Surgical Solutions Group
> Covidien
> 5920 Longbow Drive
> M/S A20
> Boulder, CO 80301
>
> 303.476.7458
> www.covidien.com<http://www.covidien.com/>
>
> From: icu-pcb-forum-bounce@xxxxxxxxxxxxx
> [mailto:icu-pcb-forum-bounce@xxxxxxxxxxxxx] On Behalf Of Randy Dawson
> Sent: Friday, February 15, 2013 8:02 AM
> To: icu-pcb-forum
> Subject: [PCB_FORUM] Re: Diff pair setup in Constraint Manager
>
> Hi Gary,
>
> If you set it up as an electrical constraint, its dimensions will change with
> stackup changes (as the impedance is stackup dependent).
>
> As a physical constraint, they do not.
>
> There are reasons to do it either way, for example you want to preserve or
> reuse some physical routing rules from a prior design, you would set
> constraints as physical, and adjust stackup accordingly.
>
> Likewise, if you are modifying stackup, you want the dimensions to track your
> changes to maintain impedance.
>
> Randy Dawson
> ________________________________
> From: Gary.Macindoe@xxxxxxxxxxxx<mailto:Gary.Macindoe@xxxxxxxxxxxx>
> To: icu-pcb-forum@xxxxxxxxxxxxx<mailto:icu-pcb-forum@xxxxxxxxxxxxx>
> Subject: [PCB_FORUM] Diff pair setup in Constraint Manager
> Date: Fri, 15 Feb 2013 14:47:50 +0000
> Hey guys,
>
> You know, this has always been a mystery to me, where to set up diff pairs in
> CM.
>
> Should you create a differential impedance Electrical Constraint Set under
> the Electrical tab, or a Physical Constraint Set under the Physical tab?
>
> It appears that you can set it up in either place and get it to work.
>
> How do you guys do it, and why?
>
> Regards,
>
> Gary MacIndoe
> Senior PCB Design Engineer
> EbD R&D Hardware
> Surgical Solutions Group
> Covidien
> 5920 Longbow Drive
> M/S A20
> Boulder, CO 80301
>
> 303.476.7458
> www.covidien.com<http://www.covidien.com/>
>
>
>
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