1. Ground Recommendations Review of Recent E-mail Discussion
Walter Katz
Signal Integrity Software, Inc.
IBIS AMI/Interconnect
March 13, 2018

2. Overview IBIS ISS Ground Correct Voltage Measurements
IBIS is an Electronic Data Book
“Name” of the Reference
How to Change “ground” to “reference” in IBIS 6.1
A_gnd and Node 0
Reference Nodes in BIRD 189
“Mixed Reference” Simulations 2

3. IBIS ISS and “ground” IBIS ISS does support Node 0
There are many occurrences of the word “reference”
There are very few occurrences of the word “ground”
Suggested changes in the following slides to eliminate “ground” 3

4. IBIS ISS Page 17
Now
Nodes are the points of connection between elements in the input circuit description.  If entirely numeric, node numbers shall be between 1 and (1 to 1e16-1).  A node number of 0 is permitted but is interpreted as ground.  Letters that follow a leading number in a node name are ignored; this means that node strings such as '3n5' and '3' shall be interpreted as referring to the same node. …
To indicate the ground node, use either the number 0 or the names GND, !GND, GROUND, or GND!.  Every node shall have at least two connections, except for transmission line nodes (unterminated transmission lines are permitted).
Suggested Change
Nodes are the points of connection between elements in the input circuit description.  If entirely numeric, node numbers shall be between 1 and (1 to 1e16-1).  A node number of 0 is permitted but is interpreted as the “simulator reference node”.  Letters that follow a leading number in a node name are ignored; this means that node strings such as '3n5' and '3' shall be interpreted as referring to the same node.
To indicate the simulator reference node, use either the number 0 or the names GND, !GND, GROUND, or GND!.  Every node shall have at least two connections, except for transmission line nodes (unterminated transmission lines are permitted). 4

5. IBIS ISS Page 36 Now Suggested Change refin
Ground reference for the input signal.
refout
Ground reference for the output signal.
Suggested Change
refin
Reference node for the input signal.
refout
Reference node for the output signal. 5

6. IBIS ISS Page 39 Now Suggested Change Lgnd=val
Defines val as the DC inductance value, per unit length for ground (reference line).
H/m
Rognd=val
Defines val as the DC resistance value, per unit length for ground (reference line).
Ω/m
Rsgnd=val
Defines val as the skin effect resistance value, per unit length for ground (reference line).
Ω/(m•√Hz)
Suggested Change
Lgnd=val
Defines val as the DC inductance value, per unit length for reference line.
H/m
Rognd=val
Defines val as the DC resistance value, per unit length for reference line.
Ω/m
Rsgnd=val
Defines val as the skin effect resistance value, per unit length for reference line.
Ω/(m•√Hz) 6

7. Correct Voltage Measurements
Voltage measurements are made between a point in a circuit (or node in a simulation) and a second reference point in a circuit. This reference point is located as near as possible to the point being measured.
The manufacturer of a component is required to specify these two points, and today he does that in a data book. The data book specifies signal names that are the reference voltage for I/O voltage measurements and the data books traditionally call these signal names “ground” or rail voltage =0.
A proper voltage measurement at an I/O pin is made between that pin and a nearby reference pin. Some components have multiple “reference” signal names, and the measurement (physical or simulation) must be made between the I/O signal pin and a nearby “reference” pin that is on the appropriate “ground” for that I/O signal.
All of this can be generalized to measurements at an I/O buffer which must have a reference point for measurements that should be local to the I/O buffer itself (e.g. ground_clamp_reference). 7

8. IBIS is an Electronic Data Book
We need to go back to IBIS basics as an “electronic” representation of an IC Data Book. In Data Books that name specific signals on pins as “Ground”. I have to except that this is the traditional way of saying that these are the reference nodes for measurements of signal (and other rail) pins. We of course now know that we need a nearby reference node, but 20 years ago nearby was someplace on the chassis. We know better now.
Unfortunately “ground” is in the vernacular of IBIS. Whether we call it “local ground”, “local reference” or “signal ground” we need to define this concept of how voltages are measured with respect to all of the voltage rules we have in IBIS.
We cannot get mired in some of the declarations that everything can be measured using Node 0, or that since some weird ECL/MECL/PECL/RS232 buffers that do not have a buffer rail or a pin that is a node that voltages can be measured to. 8

9. “Name” of the Reference
Candidate names for the reference of a voltage measurement
Signal Ground
Local Ground
Local Reference
Reference Node
Reference
Because of the desire to remove the word “ground”, I strongly recommend Local Reference 9

10. IBIS Describes Measurements to Generate IV and VT Curves and Thresholds
IBIS does state that when generating IBIS IV and VT curves and thresholds the reference nodes are connected to a simulator reference node which is uses names such as Node 0, A_gnd, absolute ground.
IBIS does not describe how to measure voltages during simulation of actual channels. 10

11. How to Change “ground” to “reference” in IBIS 6.1
There are over 100 occurrences of “ground” in IBIS 6.1.
I would prefer to add to the beginning of IBIS that “ground” was used as a reference node of all measurements 20 years ago.
Measurements to generate IBIS data (IV and VT curves and Thresholds) assume that all reference nodes are tied to the same Node in SPICE to IBIS simulations.
Model makers and EDA tools shall interpret the word “ground” in the IBIS 6.1 specification as a Reference Node for each voltage measurement. 11

12. A_gnd and Node 0
A_gnd was added to IBIS as part of [External Model] where the analog signals for the Stimulus and Enable terminals would normally be to a different reference node than the I/O and Rail terminals of the model.
IBIS formally supports Berkley SPICE and IBIS-ISS which both support Node 0.
We can certainly say that A_gnd is an alias for Node 0. (???? Some issues here)
A_gnd (????) and Node 0 can be a reference for Interconnect Models, but this limits the way Power Aware simulations can be performed. 12

13. Reference Nodes in BIRD 189
BIRD 189 model circuit elements can reference either a Buffer, Pad, or Pin rail terminal, or Node A_gnd or Node 0.
Simulation results may be incorrect if some models (I/O buffer or Package Interconnect or Board Interconnect) in a simulation reference Node 0 or A_gnd and other models (I/O buffer or Package Interconnect or Board Interconnect) reference rail signals that are not specified as 0.V voltage sources. (“Mixed Reference”) 13

14. “Mixed Reference” Simulations
Interconnect Models for a Single Channel can be generated by multiple vendors and EDA tools:
Tx on-die
Tx package
Mother board interconnect
Connector
Daughter board interconnect
Rx Package
Rx on-die
What to do if one or more of these models reference Node 0 14

15. What to do if one or more of these models reference Node 0
Scott McMorrow says:
Circuit theory says that we can go from a partial element system where ground and power loops are fully modeled to a ground referenced system where node 0 ground is applied to every element in the path.  But, ground bounce inductance and resistance is then lumped into power circuit and signal path circuits, and the discrimination between the these is lost.  From a differential node voltage perspective at the receiver, the result is the same.  The voltage between the signal and ground will remain the same.  If there is a difference, then somewhere in the circuit, the ground partial inductance has not been reduced into the loop inductance for the signal path and power paths.
This is a pretty standard transformation from partial inductance/resistance matrices to loop inductance/resistance matrices, and is covered quite extensively in Brian Young's book, which is still the best on the subject. He's been at Motorola and TI.  I believe he's at TI now running their ASIC packaging group. 15

16. Conclusions
We should make minor changes to IBIS-ISS to remove “ground”
We should add a section to the beginning of IBIS to define that voltage measurements are always made to a “Reference Node”. This document defines the “Reference Node” to be Node 0 is limited to describing methods to generate IBIS data.
BIRD 189 should recommend that Interconnect Model elements reference local rail voltage terminals, and suggest that all reference terminals be connected to a common reference node if any reference (that current flows to) is A_gnd or Node 0. 16