USB routing problems : D+/- length issues

Vincent Vittori schrieb:

I'm a user of Eagle for 2 years now, and i'm trying to make USB with

an Overo and a dsPIC. The thing is that I had read somewhere

(http://www.usb.org/developers/docs/hs_usb_pdg_r1_0.pdf) that the

signals D+ and D- must have the same length (they talk about non

parallelism page 10 figure 5)

 

But here is the point : how you do that on Eagle , to force 2 wire to

have the same length (or being parallel all the way), in autorouter

mode of course but even in manual mode.

First of all, I would never use the autorouter for this. (I don't use it

at all... but here it's essential.)

Then, simply (manually) route your tracks as parallel as possible. You

can check their length with an ULP and correct them if necessary.

In EAGLE, there is no way to /force/ an equally long differential pair

during routing, neither manually nor by autorouter. Maybe V6 will add

something new here, but I don't know.

Tilmann

On 09/23/11 08:08, Tilmann Reh wrote:

...

In EAGLE, there is no way to /force/ an equally long differential pair

during routing, neither manually nor by autorouter. Maybe V6 will add

something new here, but I don't know.

In V6 the (manual) ROUTE command will treat signals that have the

same net class and the same name (one ending with '+' and the other

with '-') as "differential pairs". When picking up one airwire of such

a pair, both signals will be routed in parallel, keeping the distance

as defined in the net class.

This can't guarantee the same length for both signals, so there

will also be a new command that generates "meanders" to compensate

for length differences, or to create a fixed total length of a

differential pair.

Klaus Schmidinger

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Klaus Schmidinger                       Phone: +49-8635-6989-10

CadSoft Computer GmbH                   Fax:   +49-8635-6989-40

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(for some reason my phone nntp reader doesnt quote some msgs)

I think a simple external list of complementary(diff) or grouped(buses)

netnames would handle this issue nicely. It does not have to be tied to any

objects.

On 23.09.2011 16:31, Olin Lathrop wrote:

Klaus Schmidinger wrote on Fri, 23 September 2011 03:46

>> In V6 the (manual) ROUTE command will treat signals that have the

>> same net class and the same name (one ending with '+' and the other

>> with '-') as "differential pairs". When picking up one airwire of such

>> a pair, both signals will be routed in parallel, keeping the distance

>> as defined in the net class.

>

It's good that you are adding the capbility to keep traces together, but

this should not be driven from the net names.  Automatic features like that

may seem like they are saving time, but in the long run always cause

trouble.  If two signals are to be routed together, I want to tell Eagle

that explicitly and never have it think it knows somehow on its own.  For

example, I have some existing designs with signal names D+ and D-, but I

don't necessarily want them routed together.

 

Within the existing Eagle framework, it's probably best if this were a

property of net classes.  When this property is set, then all signals in

the class will be routed together.  It's up to me to explicitly put such

signals in their own net class and then enable the property.  Please,

nothing automatic.

That would mean you need one net class for each differential pair.

Not very practical.

There has to be some way of identifying which two signals form

a differential pair, and that's going to be via the signal names.

We can discuss which way this is done, so if '+' and '-' appear

inappropriate, we could use "_P" and "_N".

Klaus Schmidinger

--

_______________________________________________________________

Klaus Schmidinger                       Phone: +49-8635-6989-10

CadSoft Computer GmbH                   Fax:   +49-8635-6989-40

Pleidolfweg 15                          Email:   kls@cadsoft.de

D-84568 Pleiskirchen, Germany           URL:     www.cadsoft.de

_______________________________________________________________

On 23.09.2011 15:47, Morten Leikvoll wrote:

"Klaus Schmidinger"<Klaus.Schmidinger@cadsoft.de>  wrote in message

news:j5hdh5$va6$1@cheetah.cadsoft.de...

>> In V6 the (manual) ROUTE command will treat signals that have the

>> same net class and the same name (one ending with '+' and the other

>> with '-') as "differential pairs". When picking up one airwire of such

>> a pair, both signals will be routed in parallel, keeping the distance

>> as defined in the net class.

 

You should be a bit careful about using the + and - signs. It works as long

as you dont export netfiles to other tools, but + and - in names are often a

source of confusion and problems. As an example, we export netnames

connected to FPGA's and feed them into the pin assignment files. That tool

will not allow + and - and simply replacing it with something else is a

potential source of conflict.

We could just as well use something more exotic, like "_P" and "_N".

Klaus Schmidinger

--

_______________________________________________________________

Klaus Schmidinger                       Phone: +49-8635-6989-10

CadSoft Computer GmbH                   Fax:   +49-8635-6989-40

Pleidolfweg 15                          Email:   kls@cadsoft.de

D-84568 Pleiskirchen, Germany           URL:     www.cadsoft.de

_______________________________________________________________

Klaus Schmidinger wrote:

 

There has to be some way of identifying which two signals form

a differential pair, and that's going to be via the signal names.

We can discuss which way this is done, so if '+' and '-' appear

inappropriate, we could use "_P" and "_N".

 

As the feature is only for manual routing I would favour a 'gather' command

in which you specify the pair/pairs

This could work a number of ways but something like an alias list. You would

store the signal name pairs in this list and whenever you start to route a

signal the list is consulted to determine if another signal must be routed.

Using this method, signal names are irrelevant and negate the need for + -

_P _N

This list of pairings would need to be stored in the board file

Warren

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Klaus Schmidinger wrote on Fri, 23 September 2011 12:24

That would mean you need one net class for each differential pair.  Not

very practical.

Why?  Does the existance of a new class have a high cost?  Is there some

reason not to make lots of them?  Besides, how many differential pairs do

you envision needing?  Currently I use maybe 4 or 5 net classes at most.

There seem to be a lot more than I need already.

And what about whole busses that should be routed together?

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On 24.09.2011 00:42, Olin Lathrop wrote:

Klaus Schmidinger wrote on Fri, 23 September 2011 12:24

>> That would mean you need one net class for each differential pair.  Not

>> very practical.

 

Why?  Does the existance of a new class have a high cost?  Is there some

reason not to make lots of them?

Net classes have a parameter that defines the minimum distance

between either two of them, which results in a (triangular)

matrix. That matrix would become rather bulky with a large

number of net classes.

Besides, how many differential pairs do

you envision needing?  Currently I use maybe 4 or 5 net classes at most.

There seem to be a lot more than I need already.

I don't know how many differential pairs any future layout

might need. But if we made it so that each and every such pair

would need its very own net class, that would be the wrong way

to go.

And what about whole busses that should be routed together?

The differential pair feature we are currently implementing is

only about two signals being routed in parallel.

Klaus Schmidinger

--

_______________________________________________________________

Klaus Schmidinger                       Phone: +49-8635-6989-10

CadSoft Computer GmbH                   Fax:   +49-8635-6989-40

Pleidolfweg 15                          Email:   kls@cadsoft.de

D-84568 Pleiskirchen, Germany           URL:     www.cadsoft.de

_______________________________________________________________

On 23.09.2011 21:27, Warren Brayshaw wrote:

Klaus Schmidinger wrote:

 

>>

>> There has to be some way of identifying which two signals form

>> a differential pair, and that's going to be via the signal names.

>> We can discuss which way this is done, so if '+' and '-' appear

>> inappropriate, we could use "_P" and "_N".

>>

>

As the feature is only for manual routing I would favour a 'gather' command

in which you specify the pair/pairs

This could work a number of ways but something like an alias list. You would

store the signal name pairs in this list and whenever you start to route a

signal the list is consulted to determine if another signal must be routed.

Using this method, signal names are irrelevant and negate the need for + -

_P _N

 

This list of pairings would need to be stored in the board file

I don't like this.

The fact that two signals are paired has to be a property of these

two signals, and the easiest and most straight forward way to

do it is through their names, since each signal must have a way

of knowing its "companion".

It would be the simplest method to just use a certain naming convention,

like '+'/'-' or "_P"/"_N", but if you're afraid this might clash with

existing projects, we could add a flag to each signal that makes it

"part of a differential pair". However, this still would mean that the two

signals forming the differential pair would have to follow a naming

convention in order to be identified as a pair.

I'd like to avoid this flag, though, because it makes things unnecessarily

complex.

Klaus Schmidinger

--

_______________________________________________________________

Klaus Schmidinger                       Phone: +49-8635-6989-10

CadSoft Computer GmbH                   Fax:   +49-8635-6989-40

Pleidolfweg 15                          Email:   kls@cadsoft.de

D-84568 Pleiskirchen, Germany           URL:     www.cadsoft.de

_______________________________________________________________

Hi!

Am Freitag, den 23.09.2011, 18:24 +0200 schrieb Klaus Schmidinger:

On 23.09.2011 15:47, Morten Leikvoll wrote:

"Klaus Schmidinger"<Klaus.Schmidinger@cadsoft.de>  wrote in message

news:j5hdh5$va6$1@cheetah.cadsoft.de...

>> In V6 the (manual) ROUTE command will treat signals that have the

>> same net class and the same name (one ending with '+' and the other

>> with '-') as "differential pairs". When picking up one airwire of such

>> a pair, both signals will be routed in parallel, keeping the distance

>> as defined in the net class.

 

You should be a bit careful about using the + and - signs. It works as long

as you dont export netfiles to other tools, but + and - in names are often a

source of confusion and problems. As an example, we export netnames

connected to FPGA's and feed them into the pin assignment files. That tool

will not allow + and - and simply replacing it with something else is a

potential source of conflict.

 

We could just as well use something more exotic, like "_P" and "_N".

I second the thoughts on the '+' and '-' chars in names.

As you have written, you don't like to add another kind of a setting to

specify which signals belong together, we might only have the naming.

What do you think of making the naming configurable? The default could

be:

- '-' / '+'

- '_n' / '_p'

(both, but only within its little groups). But this should be possible

to change with a dialog or "set" command. If somebody wants signals with

that name ending, but not have them routed together, just remove the

setting for that schematic/board. If somebody wants special name endings

for differential pairs, e.g. '_PLUS' and '_MINUS', he simply sets it

accordingly.

So it would be nice to have zero to "a few" pair name ends to be setup.

Thanks

  Hansi

Would it be reasonable to make this a bus property, rather than a property of net classes or of individual signals?  As long as you allow nets to be part of more than one logical bus, it's not a big hardship to require users to associate a bus with signals that need to be routed for minimal timing skew.

Using bus notation would permit groups of more than two signals to be routed for equal length, which IMHO is required for this feature to be of any real use.

Klaus Schmidinger wrote on Sat, 24 September 2011 05:04

Net classes have a parameter that defines the minimum distance

between either two of them, which results in a (triangular)

matrix. That matrix would become rather bulky with a large

number of net classes.

I see, it's a squared problem.  I agree, net classes isn't how to define

this then.

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Klaus Schmidinger wrote on Sat, 24 September 2011 05:04

Net classes have a parameter that defines the minimum distance

between either two of them, which results in a (triangular)

matrix. That matrix would become rather bulky with a large

number of net classes.

I see, it's a squared problem.  I agree, net classes isn't how to define

this then.

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