All,
IBIS is very unclear when naming the rail terminals of IBIS models and the
voltages that are applied to these terminal in Device Under Test (DUT)
conditions, and during simulations.
This is another attempt to clarify this in the IBIS introduction section.
I suggest that the following be included in the introduction section of
IBIS 6.2.
A simulation that uses an IBIS model must supply rail voltages to the rail
terminals of an IBIS model. IBIS also describes the DC Voltage values
parameters that are applied to the buffer to derive that data that is in
the model (e.g. IV curves, VT curves, Thresholds, C_comp). In simulations
using IBIS models, the EDA tool is not required to use these same values.
IBIS 6.1 is not consistent in naming the rail terminals. The following
table contains the various names of these terminals
DUT Voltages
[Pulldown Reference]
[Pullup Reference]
[GND Clamp Reference]
[POWER Clamp Reference]
[External Reference]
[Pin Mapping]
pulldown_ref
pullup_ref
gnd_clamp_ref
power_clamp_ref
ext_ref
[Receiver Thresholds]
Pulldown_ref
Pullup_ref
Gnd_clamp_ref
Power_clamp_ref
Ext_ref
[External Model]
A_puref
A_pdref
A_gcref
A_pcref
A_extref
A_gnd
IBIS 6.1 in some places uses the keyword for the DUT Voltages as a name
for a terminal.
A node in a SPICE simulation does not, by itself, have a voltage. A
voltage is the potential difference between two nodes in a simulation.
Twenty years ago (when data rates were 20 Meg) IBIS assumed that "Ground"
existed, and was universal, and that it was "Node 0" in simulators. So
IBIS has always assumed that voltages at model terminals were defined as
the voltage at that terminal relative to the simulator reference node
(e.g. Node 0).
Data rates are now 56,000 Meg (56 Gig). Voltages must be measured relative
to a reference node in the simulator that is located as close as possible
to the terminal being measured (or simulated).
For Devices Under Test, the reference node (Local Signal Reference) for
all measurements at the pad, and all voltages supplied to the device rail
terminals are reference to the test fixture reference node. Therefore, if
any one of the DUT voltages are 0.0 V, then that corresponding model
terminal is the reference node for generating all test data and
thresholds, and should be the reference node for all voltage measurement
made during IBIS simulations. If none of the DUT voltages are 0.0 V,
simulators may use the simulator reference node (e.g. Node 0) for all
voltage measurements, or they can make measurement to one of the model
rail terminals, and then adjust that voltage by the DUT value of the
voltage at that terminal.
For example:
If the value of [GND Clamp Reference] is 0.0 V, then if simulating a model
using DUT rail voltages, then V(pulldown_ref, gnd_clamp_ref) = [Pulldown
Reference]
A_gnd is a terminal that is used only in [External Model] models. A_gnd is
a universal reference node, similar to SPICE ideal node "0."
One of the primary goals of IBIS 6.2 is an editorial rewrite of IBIS to
clean up the naming of I/O Buffer Model rail terminals, and declare that
voltage measurements at all I/O Buffer Model terminals be defined relative
to a Local Signal Reference.
In real-world designs, "ground" not treated as a single, universal
reference node, but rather as a network of nodes connected by various
parasitic impedances (resistance, inductance) to each other and to other
circuit nodes (capacitively or inductively). The I/O Buffer Model
reference terminal is one of the nodes of this network of nodes, and the
operation of the I/O Buffer Model is determined by the voltage of all of
the other I/O Buffer Model terminals relative to the I/O Buffer Model
reference terminal. You will note in IBIS 6.2, the "earth ground" symbol
in the figures containing schematics is replace by the Signal Reference
symbol.
Walter
Walter Katz
<mailto:wkatz@xxxxxxxxxx> wkatz@xxxxxxxxxx
Phone 303.449-2308
Mobile 303.335-6156
