Re: [SI-LIST] : Does IBIS describe output transition which both MOS turned on?

Fred Balistreri (fred@apsimtech.com)
Wed, 13 May 1998 10:06:08 -0700

D. C. Sessions wrote:
>
> Jean-Claude Perrin wrote:
>
> [wrt: IBIS v/t curves modelling crowbar current in CMOS]
>
> > This is not really true. The V/T curve in the IBIS model definition does not
> > give any information about the PMO/CMOS overlapped current.
> > The V/T describes the behavior of the buffer output versus frequency but does
> > not give any information on the current passing internaly through the output
> > transistors.
> > To know the value of the through current it is necessary to know the "Ron"
> > variation of the two transistors during the transition periode. The value of
> > this current depends upon the equivalent impedance of the two transistors,
> > impedance which is connected between Vdd and Vss power supply.
>
> The IBIS v/t curves (four, note: one each with the load to power
> and one with the load to ground for both rising and falling edges)
> give the full picture. The turnon of the pullup device is given
> by the rising edge/grounded load curve; the turnoff of the pullup
> is given by the falling edge/grounded load curve. The turnon of
> the pulldown device is given by the rising edge/pullup load curve;
> the turnoff of the pulldown is given by the falling edge/pullup
> load curve. Crowbar current on the rising edge is just the
> overlap between the pullup turnon and pulldown turnoff, and on
> the falling edge between the pulldown turnon and pullup turnoff.
>
> Also, the crowbar current in CMOS outputs (esp. tristate ones, and for
> practical purposes that means all of them) is very low by design. At
> least the ones I design are, and I have yet to see any others that act
> differently. Unlike internal gates output drivers have separate paths
> for turning on the pullup, turning off the pulldown, turning on the
> pulldown, and turning off the pulldown. As a result it's easy to turn
> the driver devices OFF faster than ON, and since crowbar current not
> only wastes power but slows down the buffer I have a hard time imagining
> a competent designer shipping a driver that has more than trivial
> crowbar current.
>
> --
> D. C. Sessions
> dc.sessions@tempe.vlsi.com

Excuse my ignorance D.C. but I thought the V/T curves are a function of
voltage vs time during the respective tr/tf periods. By itself this
represents a behavior for the given load and the given load only. In the
past the V/T curves were used by the simulation vendors as a means to
coorelate the data. In other words one could look at the v/t data and
make it match IBIS under that load. Now enter 4 v/t curves. We can use
the data to test and make that match IBIS. But now the final output wrt
v/t will depend on the vendors interpretation, algorithms, topology,
and the phase of the moon. There seems to be a lot of decisions that
need to be made left up for grabs. Although my company attends the
IBIS meetings I do not. Is there talk of publishing final V/T curves
for say a resistively unloaded device for example. This would serve as
a means of coorelation for the vendors sake.

One problem I see is that the v/t information by itself does not contain
the current information. That's buried in the I/V information. In fact
we can only get the answer for the given loads not dynamically. Your
presumption seems to be based on pullup/pulldown simple topologies. In
fact I can tell you we are dealing with very complex I/O stages with
feed backs, multiple current switching techniques, and the like. I don't
see such a simple solution to this problem. Real devices seem to be much
more complex and dynamic than IBIS can support. I am encouraged after
reading 3.1 spec and the formation of an independent IBIS users group
though.

Best Regards,

-- 
Fred Balistreri
fred@apsimtech.com

http://www.apsimtech.com