National Semiconductor Corporation

Amir Yashfe

Tathagato Rai Dastidar

Partha Ray

The state nodes in a sequential digital circuit are identified by identifying the minimal combinatorial feedback loops that are present in the digital circuit. Each minimal combinatorial feedback loop has at least one driver node, and one driver node from each minimal combinatorial feedback loop is assigned to be the state node for the loop.

Method of identifying state nodes at the transistor level in a sequential digital circuit

An integrated circuit design simulation method is provided that takes advantage of the fact that, when an instance of a circuit module has been simulated under a given set of input conditions, and the resulting output values and delays have been evaluated, another instance of the same module need not be re-simulated when it has the same input combination as the prior circuit module instance. The results computed earlier for the earlier circuit module instance can be re-used for the current circuit module instance.

Use of state nodes for efficient simulation of large digital circuits at the transistor level

Sequential digital integrated circuits have stable state nodes that are capable of retaining their state (logic value) even in the absence of any input directly driving these points. However, in addition to stable state nodes, some custom-designed digital circuits have so-called transient state nodes. A transient state node is defined as node that can directly affect the value of a stable state node and is combinatorially driven by inputs of the circuit, but the transition delay from at least one input to the node is greater than a predefined threshold value. Identifying such transient state nodes, along with the stable state nodes, is critical for the efficient simulation of custom digital circuits by a hierarchical device level digital simulator. A method is provided herein for identifying transient state nodes in a digital circuit, given the circuit's netlist and the identity of the stable state nodes in the circuit.

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Cupertino, CA, United States of America

Amir Yashfe