[IBIS-Users] RE: regarding the rising n falling waveform again

Well, lets think about this buffer for a moment.  From your file it calls
this buffer:

[Model type] I/O_open_sink

An open sink buffer does not have a pullup device.  The drain (collector)
node of the pulldown device is not connected to a pullup device.  So, there
is no source of current pulling the load up to Vdd.

So, if you have a resistance tied to ground such as the 500 Ohm resistor in
your example and the pulldown device is on you will have both sides of the
resistor at ground potential. If the pulldown device is off then the  ground
side of the resistor is grounded and the end tied to the buffer is now
floating.  There is nothing to pull that node to any other voltage other
than ground.

Conversely, if your 500 Ohm load is tied to Vdd then the pulldown device can
make one end of the resistor approach ground potential while the end of the
resistor tied to Vdd will remain at Vdd.  So, you should see transitions
when the load is tied to Vdd but you cannot see any transitions when the
load is tied to ground.

For the grounded load to have a transition you need to have a path from the
power supply through the load to ground and back through the power supply.
In electronics there must be a complete circuit of some kind for current to
flow.  Since the impedance between the supply to the load resistor is high
(read open circuit) there is no where for the electrons to flow.  Conversely
one could compute the current as I = E/R.  In your case E = 3.00 Volts in
the typical case but the R is an open circuit - R is close to infinity.
Thus the current is close to zero.  Thus from Ohms Law again (E = IR)
0.0000000000000A * 500 Ohms = 0.000 Volts.

99.9999% of the open sink buffers I have seen do not have a powerclamp or a
pullup thus there is no need for the powerclamp or pullup references.

Since there is no pullup you do not need to have these two lines:

[Rising waveform] 500 0 0 0 NA NA NA NA NA
[Falling waveform] 500 0 0 0 NA NA NA NA NA

For the ramp section IBIS will take the dV/dT_r from the load to ground and
will take the dV/dT_f from the load to Vdd in a push pull buffer. Since you
told the conversion program there was rising falling waveforms with the load
terminated to ground the program is taking the data that cannot be.  If
these two lines are not present then for this buffer the dV/dT_r and dV/dT_f
will be taken from the rising terminated to Vdd and falling terminated to
Vdd.
Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@teraspeed.com
www.teraspeed.com

Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827

  -----Original Message-----
  From: lau yy [mailto:tok47@hotmail.com]
  Sent: Monday, October 17, 2005 5:02 PM
  To: tom@teraspeed.com; ibis-users@eda.org
  Subject: regarding the rising n falling waveform again


  Dear Tom,

  Is me agian, the guy who asking the rising/falling waveform.

  This time, I facing another problem, and wish to have your opinion. The
recent model that I build, where having pullup problem. The problem is no
mather how I change the IN and EN of my IO pins, I cant get the dv/dt_r in
positive value although I set my model as I/O_OPEN_SINK.

  For the falling waveform, although is look fine but also fail for the
checker for 0v references.

  The rising waveform is not working for 0v references and vcc references.
The data is state at some value, it do change in a small scale but not
showing a rising.

  Is my model got problem or the circuit itself got problem? or there were
other problems?

  Below were my setting :
  [Model] IO0
  [Model type] I/O_open_sink
  [Power clamp reference] 3.00 2.70 3.60
  [Gnd clamp reference]   0   0   0
  [Pullup reference]      3.00 2.70 3.60
  [Pulldown reference]    0   0   0
  [Rising waveform] 500 0 0 0 NA NA NA NA NA
  [Rising waveform] 500 3.00 2.70 3.60 NA NA NA NA NA
  [Falling waveform] 500 0 0 0 NA NA NA NA NA
  [Falling waveform] 500 3.00 2.70 3.60 NA NA NA NA NA
  [Vinl] 0.8
  [Vinh] 2.10
  [Cref] 30pF
  [Vmeas] 1.50
  [Rload] 50
  [Model file] hsp.mod hspmin.mod hspmax.mod

  Here, my ramp data :
  [Ramp]
  | variable       typ                 min                 max
  dV/dt_r          -0.6000m/-0.2000n  -1.2000m/0.4000n  -0.6000m/-0.2000n
  dV/dt_f          0.7236/2.6060n      0.4002/4.0761n      1.3476/1.4710n


  ANd here the error message of my Falling waveform :

  ERROR - Model IO0: The [Falling Waveform]
        with [R_fixture]=500 Ohms and [V_fixture_max]=0V
        has MAX column DC endpoints of  0.00V and  3.40v, but
        an equivalent load applied to the model's I-V tables yields
        different voltages ( 0.00V and  0.00V),
        a difference of 100.00% and 178015.78%, respectively.


  Thanks

  rdgs
  yylau



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