The Logical Effort tool examines a digital schematic and determines the optimal transistor size to use in order to get maximum speed. The tool is based on the book Logical Effort, by Ivan Sutherland, Bob Sproull, and David Harris (Morgan Kaufmann, San Francisco, 1999). It is highly recommended that the user be familiar with the concepts of this book before using the Logical Effort Tool.

To control Logical Effort, use the Logical Effort Project Settings (in menu File / Project Settings..., "Logical Effort" tab). This lets you control a number of settings for Logical Effort analysis.
Figure 9.14

Logical Effort Gates

A design that is intended to be analyzed with Logical Effort must be composed of special Logical Effort gates. A Logical Effort gate is simply a schematic or layout cell that conforms to the following specifications:

Figure 9.25

On the input and output exports of the cell, we can define an attribute named "le" (use the Add LE Attribute to Selected Export command in menu Tool / Logical Effort to add this attribute). The value of this attribute is the logical effort of that port. For example, a NAND gate typically has a logical effort on each input of 4/3, and an output logical effort of 2. An inverter is defined to have an input logical effort of 1, and an output logical effort of 1.

The size assigned to the logical effort gate is retrieved via the "LE.getdrive()" call. This value can then be used to size transistors within the gate. The size retrieved is scaled with respect to a minimum sized inverter (as are all other logical effort parameters). So a size of "1" denotes a minimum sized inverter.

While these attributes are defined on the layout or schematic cell definition, they must also be present on the instantiated icon or instance of that definition. By default this will be so.

Finally, there must be at least one load that is driven by the gates in order for them to be sized. A load is either a transistor or a capacitor. Gates that do not drive loads, or that do not drive gates that drive loads, will not be assigned sizes.

Logical Effort Libraries

Electric comes with a set of libraries that are specially designed for Logical Effort. Use the Load Logical Effort Libraries (Purple, Red, and Orange) command (in menu Tool / Logical Effort) to read these libraries.

Advanced Features

There are several advanced features that may be added to the cell definition:

Commands

These commands may be given to the Logical Effort tool (in menu Tool / Logical Effort):