Re: [SI-LIST] : Non-standard bus termination (2nd posting)

sweir (weirsp@a.crl.com)
Thu, 23 Sep 1999 00:07:58 -0500

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Ron,

I have to agree with D.C. that this sounds like a really bad idea. Let's
assume that you have an arbitrarily large capacitor. Then, the voltage on
the capacitor side of the termination resistors will be essentially
constant for any signal period we care to evaluate, but may wander from GND
to Vcc over time. Unless the bus has a state-space average value which
happens to be a good terminating value, we get the worst of all worlds:

1. The power consumption of a parallel termination to either Vcc or GND for
the fully charged and discharged states.
2. Non-uniform termination current, and therefore variable settling time at
the load, which depends on the states of adjacent lines, ( read crosstalk
distortion ).

In the best case, the state-space average is a convenient terminating
value, in which case, this just emulates an active
termination. Considering that active termination can be had fairly easily,
why rely on the average signal content to set the voltage?

Regards,

Steve.
At 02:48 PM 9/22/99 -0700, you wrote:
>Hi DC
>
>Make it a big cap.
>
>The cap is used to keep from DC loading the line.
>
>The resistor provides the line AC termination.
>
>Ron Miller
>
>"D. C. Sessions" wrote:
>>Ron Miller wrote:
>> >
>> > Use tie all the common sides of the resistors together, and use one
>> capacitor from the
>> > common point to ground.
>>
>>Not exactly a Real Good Idea unless you have lots of timing margin.
>>The problem is that this creates a lot of jitter (edge times depend
>>on prior state of all the lines.)
>>
>> > Kassem Abdallah wrote:
>> >
>> > > > Has anybody out of there any suggestion about the following
>> problem? (I think it
>> > > > is quite a common one).
>> > > >
>> > > > I?ve got a board with a non-standard bus of 34 traces. The bus
>> drives a group of
>> > > > memories.
>> > > > The lenght of the bus is necessarily higher than the critical
>> lenght (lcr =
>> > > > tr/2tpd): a termination could be useful. The problem: there are
>> so many traces
>> > > > that one has to use too many components (34 resistors and 34
>> capacitors for AC
>> > > > parallel termination).
>> > > >
>> > > > What shall I do?
>> > > > Are there any special components (array) to terminate this type of
>> bus?
>> > > > Shall I terminate the bus or not? (In past experience not always
>> a termination
>> > > > was mandatory)
>> > >
>> > > I am not the expert on SI Issues, but there are several resistor_packs
>> > > for termination available from a variety of vendors. However, You have
>> > > to be very carefull, because these packs sometimes cause noise and make
>> > > things worst. It depends on how the resistors network is packaged for
>> > > current/GND leads.( I suggest to look carfully at the schematics and do
>> > > some noise analysis).
>> > >
>> > >
>> > > Also, I "think" a 34-bit bus is not that WIDE, and if it is critical
>> > > data path, you might be better off terminating with single loads( may
>> > > be you can stick them on the backside of your board ?? It is a
>> > > compromise).
>> > >
>> > > ********************
>> > > |Kassem M. Abdallah|
>> > > |Digital Designer |
>> > > |EMC Corp. |
>> > > |Hopkinton MA 01748|
>> > > ********************
>> > >
>> > > **** To unsubscribe from si-list: send e-mail to
>> majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
>> si-list, for more
>> > > help, put HELP. si-list archives are accessible at
>> <http://www.qsl.net/wb6tpu/si-list>http://www.qsl.net/wb6tpu/si-list ****
>> >
>> > --
>> > Ronald B. Miller _\\|//_ Signal Integrity Engineer
>> > (408)487-8017 (' 0-0 ') fax(408)487-8017
>> > ==========0000-(_)0000===========
>> > Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose,
>> CA 95131
>> > rmiller@brocade.com, rbmiller@sjm.infi.net
>> >
>> >
>>
>>--
>>D. C. Sessions
>>dc.sessions@vlsi.com
>>
>>**** To unsubscribe from si-list: send e-mail to
>>majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
>>si-list, for more help, put HELP. si-list archives are accessible at
>><http://www.qsl.net/wb6tpu/si-list>http://www.qsl.net/wb6tpu/si-list ****
>
>--
>Ronald B. Miller _\\|//_ Signal Integrity Engineer
>(408)487-8017 (' 0-0 ') fax(408)487-8017
> ==========0000-(_)0000===========
>Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA 95131
>rmiller@brocade.com, rbmiller@sjm.infi.net

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Ron,

I have to agree with D.C. that this sounds like a really bad idea.  Let's assume that you have an arbitrarily large capacitor.  Then, the voltage on the capacitor side of the termination resistors will be essentially constant for any signal period we care to evaluate, but may wander from GND to Vcc over time.  Unless the bus has a state-space average value which happens to be a good terminating value, we get the worst of all worlds:

1. The power consumption of a parallel termination to either Vcc or GND for the fully charged and discharged states.
2. Non-uniform termination current, and therefore variable settling time at the load, which depends on the states of adjacent lines, ( read crosstalk distortion ).

In the best case, the state-space average is a convenient terminating value, in which case, this just emulates an active termination.  Considering that active termination can be had fairly easily, why rely on the average signal content to set the voltage?

Regards,


Steve.
At 02:48 PM 9/22/99 -0700, you wrote:

Hi DC

Make it a big cap.

The cap is used to keep from DC  loading the line.

The resistor provides the line AC termination.

Ron Miller

"D. C. Sessions" wrote:
Ron Miller wrote:
>
> Use tie all the common sides of the resistors together, and use one capacitor from the
> common point to ground.

Not exactly a Real Good Idea unless you have lots of timing margin.
The problem is that this creates a lot of jitter (edge times depend
on prior state of all the lines.)

> Kassem Abdallah wrote:
>
> > > Has anybody out of there any suggestion about the following problem? (I think it
> > > is quite a common one).
> > >
> > > I?ve got a board with a non-standard bus of 34 traces. The bus drives a group of
> > > memories.
> > > The  lenght  of  the  bus  is necessarily higher than the critical lenght (lcr =
> > > tr/2tpd):  a  termination could be useful. The problem: there are so many traces
> > > that  one  has to use too many components (34 resistors and 34 capacitors for AC
> > > parallel termination).
> > >
> > > What shall I do?
> > > Are there any special components (array) to terminate this type of bus?
> > > Shall  I  terminate the bus or not? (In past experience not always a termination
> > > was mandatory)
> >
> > I am not the expert on SI Issues, but there are several resistor_packs
> > for termination available from a variety of vendors. However, You have
> > to be very carefull, because these packs sometimes cause noise and make
> > things worst. It depends on how the resistors network is packaged for
> > current/GND leads.( I suggest to look carfully at the schematics and do
> > some noise analysis).
> >
> >
> >  Also, I "think" a 34-bit bus is not that WIDE, and if it is critical
> >  data path, you might be better off terminating with single loads( may
> >  be you can stick them on the backside of your board ?? It is a
> >  compromise).
> >
> > ********************
> > |Kassem M. Abdallah|
> > |Digital Designer  |
> > |EMC Corp.         |
> > |Hopkinton MA 01748|
> > ********************
> >
> > **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more
> > help, put HELP.  si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****
>
> --
> Ronald B. Miller  _\\|//_  Signal Integrity Engineer
> (408)487-8017    (' 0-0 ') fax(408)487-8017
>      ==========0000-(_)0000===========
> Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
> rmiller@brocade.com,  rbmiller@sjm.infi.net
>
>

--
D. C. Sessions
dc.sessions@vlsi.com

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP.  si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

-- 
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017                 
     ==========0000-(_)0000=========== 
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net
 
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