[SI-LIST] Re: SV: Total load capacitance
- From: Jim Freeman <freeman@xxxxxxxxxxx>
- To: whizplayer@xxxxxxxxx
- Date: Thu, 10 Jan 2002 11:36:27 -0800
Hi All,
The problem with ignoring the capacitance of the line and relying
totally on the transmission line effects is that there will be different
levels on the receiving end for differeing digital transmission
patterns. If you are considering a signal that is always alternating,
the levels may degrade but are still above and below Vih and VIL. If the
data is coded such that there is always an alternating value, data
transmission is acceptable under these conditions. There are three
classes of digital transmission on any buss, address,data, and control.
rarely can the address be coded and clock signal integrity may be
compromised with the transmission line only approach. If parallel
termination is used for the clock, there will be no reflection but the
levels suffer and may be unacceptable. In most cases, I have found that
a driver that is capable of enough DC current to drive a parallel
termination is easily capable of driving a series termination without DC
load under equivalent loading conditions. there are different
considerations for source synchronous busses as well as busses that use
a clock recovery system.
Thanks
Jim Freeman
Bob Patel wrote:
> Hi! Anders, Even if I take a transmission line
> approach the total capacitance faced by the driver
> will always be there i.e. even if the edge rates are
> faster and the driver is weak then the rise & fall
> time will not be maintained at the receiver and if the
> C is too large then the driver may not be able to
> drive it above Voh. SO, either way this lumped
> capacitance approach has to be taken into account to
> determine the drive needed.
> Thanks for the input.
> Bob
> --- "Anders Ekholm (ERA)"
> <Anders.Ekholm@xxxxxxxxxxxxxxx> wrote:
> > At high speed (HF) this will probably get you a
> > pesimistic result.
> > If you trace is long enough compared to your edge
> > rates the driver will not
> > see the load right away since it will see a
> > transmission line.
> >
> > think what would happen with your formula if you
> > trace was very long
> > like a 1000meter cable, it would give you a
> > capacitive load you would not be able to drive.
> >
> > If you are using the result to select an ASIC driver
> > or something like that, you will
> > choose unnecessary strong drivers if you base your
> > selection on total capacitance of you trace
> > (as long as we are talking fast edges).
> >
> > You might need a transmission line approache rather
> > than a lumped capacitive load approache.
> >
> > Regards /Anders Ekholm
> >
> >
> > > -----Ursprungligt meddelande-----
> > > Fr> ån: Bob Patel [SMTP:whizplayer@xxxxxxxxx]
> > > Skickat: den 9 januari 2002 21:02
> > > Till: si_list
> > > Ämne: [SI-LIST] Total load capacitance
> > >
> > >
> > > Hi! If i am calculating the total load capacitance
> > > faced by the driver, can i use this equation:
> > > Ctotal = C(trace)+ C(via) +Cin(i/p cap. of load)+
> > > Cout(o/p cap of driver)
> > > Now if I add 5% then can I safely assume that if
> > the
> > > I?O is designed to drive this load then on the
> > real
> > > board there should not be a problem fro the driver
> > to
> > > source or sink current.
> > > Thanks in advance
> > > Bob
> > >
> > >
> > > __________________________________________________
> > > Do You Yahoo!?
> > > Send FREE video emails in Yahoo! Mail!
> > > http://promo.yahoo.com/videomail/
> > >
> >
> ------------------------------------------------------------------
> > > To unsubscribe from si-list:
> > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe'
> > in the Subject field
> > >
> > > or to administer your membership from a web page,
> > go to:
> > > //www.freelists.org/webpage/si-list
> > >
> > > For help:
> > > si-list-request@xxxxxxxxxxxxx with 'help' in the
> > Subject field
> > >
> > > List archives are viewable at:
> > > //www.freelists.org/archives/si-list
> > > or at our remote archives:
> > > http://groups.yahoo.com/group/si-list/messages
> > > Old (prior to June 6, 2001) list archives are
> > viewable at:
> > > http://www.qsl.net/wb6tpu
> > >
>
> __________________________________________________
> Do You Yahoo!?
> Send FREE video emails in Yahoo! Mail!
> http://promo.yahoo.com/videomail/
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
