Here is a discussion and definition of A_gnd based on the presentation and
discussion we had today. I think this could go somewhere in the beginning
All voltages in IV curve, VT, curves, Measurement Thresholds, Model Spec
voltages in this document are voltage that are either measured in hardware
or simulations reference to an appropriate location in the hardware or
node in a simulation. IBIS originally defined voltage measurements at pins
are measured relative to a reference pin. Data Books usually refer to
these reference pins as "Ground" or supply pins with a zero voltage. As
IBIS has evolved to define rules at the die pad, and with BIRD 189 at the
buffer itself it is necessary to move reference point to the die pad or to
the buffer rail voltage. IBIS defines the reserved name A_gnd to refer to
Note that "measurement" in this document shall either refer to
1. A physical measurement of a voltage between a probe point and a
2. A simulation "probe" of a voltage between a probed node and a
A connection between two or more I/O buffer models can be defined as an
IBIS-ISS netlist which is essentially a list of IBIS-ISS element
instances. The terminals of these elements can reference nodes for the
other terminals of these elements. There is no requirement that the
reference A_gnd for one measurement on a buffer be the same SPICE node as
the reference A_gnd for a measurement at a buffer either in the same
component or a buffer in a different component.
All data in the specification is based on the assumption that rail
voltages are static. [Pin Mapping] allows the EDA tool to determine which
pin(s) supply the voltages to all of the rail terminals of a [Model], and
therefore all of the references for all of the interconnect between
[Model]s and pins.
Note that in the case of some ECL, MECL, PECL and RS232, IBIS models may
have voltages that are referenced to some point outside of the component.
In this case the reference location (A_gnd) is external to the component,
and the EDA tool must choose a reference node for rail voltages supplied
to the component and for measurements made at the I/O pin, pad or buffer.
This reference "Node 0" is commonly used in this case.
One must take great care in defining reference node connections when doing
Power Aware Simulations. A simulation is a Power Aware Simulation when the
rail voltages referenced to their local reference node or referenced to a
simulator reference node (e.g. Node 0 ) change in time.
Almost every use of the word "ground" in this document refers to a point
in the hardware or a node in a simulation that is the reference for signal
and supply voltage measurements for a Device Under Test. In IBIS 7.0 or
7.1, all of these reference to "ground" will be clarified to mean the
reference point for measurement (A_gnd).
978.461-0449 x 133