RE: [SI-LIST] : Looking Inside IBIS Model

=?big5?B?VGluZywgU3RldmUgKKRCvW69ZCk=?= ([email protected])
Mon, 17 May 1999 19:07:13 +0800

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I agree with you.
I believe that the test loading value specified in IBIS should be
consistent with datasheet.
If we can get Tco [clock to output] from driver's spec and Tsetup
(Thold) [setup time, hold time]
from receiver's spec ( and with clock's info. available), we can
determine the max. and min.
flight time for the data.

To determine the max. flight time (assume zero skew, jitter),
I'll use the formula:
Tflight_max = Tcycle - Tco_max -Tsetup
And to determine the min. flight time, I'll use:
Tflight_min = Thold - Tco_min

And in my opinion, I think the Tco can be decomposed as IC
internal delay plus
TIME_TO_VM, which depends on loading outside the driver. To avoid double
subtraction with the
TIME_TO_VM on the right side of above formula, the flight time obtained
from simulation (IDD file
generated by Quad's tool) should be corrected by subtracting the

In many cases, where the TIME_TO_VM will be different for rising
and falling.
And sometimes, the asymmetry between rising and falling will be quite
significant, that will result in
some persecution. Because we'll have only one Tco in the spec.

For example, I have
one driver A with Tco =2.5nS
one receiver B with Tsetup=2nS
running speed is 133MHz (Tcycle=7.5 nS).
TIME_TO_VM for the driver is 0.8nS on rising and 1.8nS on falling.
According to above formula Tflight_max =7.5 - 2.5 - 2 = 3nS
What if my simulation result reports the Tflight_rise (rising) = 3.2nS
and Tflight_fall (falling) = 2.8nS for
some signal from layout database? (Not caused by ringback!)

It happened because the real loading in actual application won't
necessarily be the same as the test loading
used in spec. But how should I deal with it? Pass or reject?

Any comments will also be highly appreciated!

With best regards,

Steve Ting
EDA/CAE Engineer

BM__MailData-----Original Message-----
From: Abe Riazi [ mailto:[email protected] <mailto:[email protected]>
Sent: Sunday, May 16, 1999 12:50 AM
To: '[email protected]'
Cc: Walt Otto
Subject: [SI-LIST] : Looking Inside IBIS Model

Dear All,

An IBIS model contains several different sections including
parasitics, pin mapping, DC I-V curves and V-T tables. Each part
performs a useful function and desreves attention when evaluating the
accuracy of an IBIS behavioral model.

Also present in some models are parameters Cref, Rref, Vref
Vmeas, which define the test load used by the semiconductor vendors when

determining propagation delay or switching time of the device. Where:

Cref: A shunt capacitor which connects the driver output to Ground.
Rref: A resistor connecting the output of device to Vref.
Vref: Timing specification test load voltage.
Vmeas: Reference voltage for timing measurements, with a value
between input logic low ( Vinl ) amd input voltage high ( Vinh )

Based on IBIS standards, Vmeas, Cref, Rref, and Vref
parameters are
optional. Therefore, there are many models which lack these parameters.

But if accurately specified they can aid board level timing simulation

I always look for them in driver models for several reasons:

1. For an appraisal of the quality of the model.
2. For a comparison with the test load specified in the AC timing
section of the device's datasheet.
3. If determined to be consistent with the datasheet, then to be
employed in time_To_Vm calibration runs of the Quad models (generated
from IBIS models by means of IBIS2XTK conversion program.)

I have seen numerous models having test load parameters
with the datasheet and have asked following question from several IBIS
model developers:

For timing calibration runs should the test load defined in
the IBIS
model be used or the one specified in the datasheet ?

The answers have varied. Some IC vendors have suggested using
test load defined in the model, while others have recommended the
circuit described by the datasheet. I agree with the latter, and prefer

using the dataheet's test load, whenever there is a diference between
the test loads of the model and the datasheet. Another benefit gained
by referring to the datasheet is that the test loads for the Min, Typ
and Max corners can be different; this variation is often described in
the datasheet, but seldom in the IBIS model.

It is important to note that during development of an IBIS
model the
parameters Rref, Cref, and Vmeas are not utilized for generation of any
of the waveforms (i.e. pullup, pull down, rising, falling, etc.). They

should not be confused with R_fixture, C_fixture, and V_fixture which
are in contrast used for creation of rising and falling tables.

To clarify certain points of this discussion, three examples
been prepared:

Example 1. Following parameters were obtained from IBIS
model of a
Vinl = 1.2 V, Vinh = 1.6 V, Vmeas = 1.4 V, Vref = 1.4 V, Rref = 28
Ohms, Cref = 0.0 pF, R_fixture = 28 Ohms, V_fixture = 1.8 V.

Example 2. Parmameters adapted from behavioral mode1 of a
Accelator chip:
Vinl = 0.8 V, Vinh = 2.0 V, Vmeas = 1.4 V, Cref = 0.0 pF, R_fixture =

50 Ohms, V_fixture = 3.3 V, V_fixture_min = 3.135 V, V_fixture_max =
3.465 V.

Example 3. Extracted from IBIS model of a Programmable Logic
Vinl = 0.8 V, Vinh = 2.0 V, Vmeas = 1.5 V, Rref = 100 K, Cref = 50
pF, Vref = 0.0 V, R_fixture = 50 Ohms, V_fixture = V_fixture_max =
V_fixture_min = 0.0 V.

The fixture parameters define the loading conditions under
which a
rising or falling waveform is generated. According to IBIS standards,
only R_fixture and V_fixture are required, the remaining subparameters
are optional. It is believed that test fixtures composed of R_fixture,
V_fixture, V_fixture_min and V_fixture_max provide the optimum data
needed for production of a behavioral model. When a subparameter is
missing, its value is assumed to be zero by default.

To summarize, test load and test fixture parameters are not
equivalent. When Vmeas, Vref, Rref, and Cref are accurately specified
for a driver model, consistent with the datasheet, then they represent
valuable information which can facilitate the timing synchronization
runs and hence enhance the overall efficiency of a SI simulation. Test
load circuits are not needed for receiver models.

Your comments regarding this topic are highly appreciated.

Best Regards,

Abe Riazi
Anigma, Inc.

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RE: [SI-LIST] : Looking Inside IBIS Model


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