Re: [SI-LIST] : Number of GND/Power pins in a connector ?

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From: Jim Freeman (freeman@broadcom.com)
Date: Thu Mar 23 2000 - 16:50:07 PST


Yes that is true, but the paths may be much different in length and therefore have a larger loop inductance and loop capacitance.

Jim Freeman

Vinu Arumugham wrote:

> Jim Freeman wrote:
>
> > A second reason for having both pwr and gnds is for return paths for high speed signals. For a signal being pulled high, the return curent is thru the VCC path.
>
> The signal return current will be on the nearest plane regardless of whether the signal is switching high or low. The Vcc path used in a high going transition is the one from the driving device to its
> bypass cap. It therefore does not affect connector pinout.
>
> >
> > You say CMOS and then add that most are PCI bus pins. Cmos levels to me mean that the threshold is VCC/2 and therefore for the signals other than the PCI bus, both pwr and gnd are necessary. PCI is
> > a horse of a different color. Best to follow the PCI backplane to the letter and that will be safe.
> >
> > Jim Freeman
> >
> > Stuart Adams wrote:
> >
> > > > Hi Stuart,
> > > > You don't specify the signalling system going thru the connector.
> > >
> > > Well, in my case the signaling system is all 3.3 volt CMOS (with
> > > 5 volt PCI compliance). The connector is 160 pins and contains
> > > an embedded PCI bus (at 33 MHz), 16 pins of video data, 10/100 ethernet
> > > and various slower discrete I/O.
> > >
> > > I currently have 20 pins allocated for GNDs, spread throughout the
> > > connecter (more less the same as the PCI spec). In most cases the
> > > board will run from a 3.3v LDO linear regulator on the board and I
> > > have 3 pins allocated for unregulated DC in. In some cases the
> > > board will operate from externally supplied 3.3 volts and I
> > > have 3 pins allocated for external VCC. (Worst case steady state
> > > current is only 600 mA - max current spec'd per pin for the
> > > connector is 500 mA)
> > >
> > > Our board is 1.4" x 2.4" and has about 40 bypass caps and four 47uF
> > > bulk tantalums. Our customer's will designing the motherboard it mates
> > > to so we can't predict what it will look like for simulation but we could
> > > provide them some design guidelines for bypassing/layout.
> > >
> > > -- Stuart
> > >
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