RE: C_comp confusion

Steve,
 
I think you are, perhaps, being misled by a mistake or a misinterpretation
on the part of the developer of the example IBIS file you refer to.
 
It is normal, in my experience, that the C_comp parameter arguments should
be listed in the order of the actual capacitance value to match typ, min,
max. This is how it is in your original C_comp line in your example lines
below.
 
How each is used should be set by functionality in the simulation tool that
you use when it comes to defining "Corners" and "Corner Cases". For example,
a certain corner will be defined to use "min C_comp" or "max C_comp", as
appropriate.
 
The one parameter that does follow your reasoning (which, by the way, is
entirely plausible - but this is a spec we are dealing with!) is
[Temperature Range]. This is refelected in your example lines. The arguments
to this parameter are intended to reflect the value which gives rise to the
fastest/slowest, strongest/weakest characteristics in the V/I and V/T tables
which follow in the [Model] description.
 
Best regards,
Tony Dunbar
tony_dunbar@mentor.com <mailto:tony_dunbar@mentor.com>
 
-----Original Message-----
From: Steve Constable [mailto:steve@tundra.com]
Sent: Tuesday, February 15, 2000 4:30 PM
To: ibis-users@eda.org
Subject: C_comp confusion

I'm confused about C_comp. All of the data tables in an IBIS file are
ordered as TYP, MIN then MAX.
But it looks like C_comp is ordered as TYP, MAX then MIN....that's were the
confusion is.
The reason I think C_comp is ordered differently is that the MAX should have
the smallest capacitance associated with it _not_ the greatest. In the
example below (taken from the 21152_aa.ibs model from Intel), the C_comp has
a max value of 0.50pF, this will slow down the I/O buffer (compared to the
TYP case of 0.40pF), this contradicts what the rest of the model is trying
achieve when the simulator is referring to the MAX columns (i.e. MAXimize
speed).
Likewise I think the C_comp_min should be 0.50pF NOT what the model
indicates as 0.30pF.
And the C_comp_max should be 0.30pF NOT 0.50pF as the model indicates.
 
Just in case I've not made myself all that clear....below I have added a
line to the original C_comp line:
 
| TYP MIN MAX
[Temperature Range] 25.00 100.00 0.000
[Voltage Range] 3.30V 3.00V 3.60V
C_comp 0.40pF 0.30pF 0.50pF
| should be: C_comp 0.40pF 0.50pF 0.30pF
I.E swap the 0.30pF with the 0.50pF......
 
I've checked with several different models and they all seem to have C_comp
as TYP,MAX,MIN
Even the IBIS spec has the MAX value as the largest and not the smallest
capacitance...as below:
Vref = 0 |Timing specification test load voltage
| variable typ min max
C_comp 12.0pF 10.0pF 15.0pF

If that's the way the spec has it, then I guess everyone will follow the
spec, but I'm still confused as to why the MAX parameter has the largest
capacitance when the objective of using MAX is to get the fastest switching
time possible.....
 
Any ideas or suggestions....?????
 
 
Steve Constable
Tundra Semiconductor
 
                               
    

 
Received on Tue Feb 15 20:30:21 2000