An IBIS model contains several different sections including package
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 and
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 inconsistent
with the datasheet and have asked following question from several IBIS
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 the
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 have
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 Graphic
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 =
Example 3. Extracted from IBIS model of a Programmable Logic Device
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.
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