Virtuoso XL Layout Editor User Guide

Editing Your Layout with Virtuoso XL Layout Editor

Using Transistor Chaining

Transistor chaining is the process by which a list of MOS transistors (or the fingers of folded transistors) can be abutted with one another in a specified order. Chaining helps reduce layout area and capacitance.

Transistors to be chained must be set up for abutment.

Transistors to be chained must have a property named lxComponentType set on their cell or library. This property stores their attributes. You must set this property on the cell or library using the Edit Component Types form associated with the Design – Component Types command. For complete information about setting the lxComponentType parameter, see Setting Component Types on page 227

You can chain transistors automatically during layout generation or interactively during layout editing.

To chain transistors interactively, follow these steps.

1.From the layout window menu, choose Edit – Transistor Chaining.

The Transistor Chaining form appears.

2.Select in the layout window the transistors to chain or click Add Cell ListBox to see the list of transistors available for chaining.

Virtuoso XL Layout Editor User Guide

Editing Your Layout with Virtuoso XL Layout Editor

The list box appears in the Transistor Chaining form.

3.Select the transistors to chain from the list box.

Note: You can also preselect transistors from the layout window before you choose the

Transistor Chaining command.

4.Click Apply.

The selected transistors are highlighted in the layout. The layout window prompts you to select a destination point in the layout window.

5.Move the cursor into the layout window.

The transistors appear as the image of a transistor stack that follows the cursor. Folded transistors are stacked with the number of fingers as defined.

Stacks consisting of only NMOS or only PMOS transistors are aligned by the bottom edge. If you use the Gen From Source command to create a cluster consisting of both

Virtuoso XL Layout Editor User Guide

Editing Your Layout with Virtuoso XL Layout Editor

an NMOS and a PMOS transistor, the PMOS is placed on the top, aligned to the lower edge, and the NMOS is placed below it, aligned to the upper edge.

PMOS Stack

NMOS Stack

To change the vertical orientation of such stacks, use Move – Upside Down.

6.Click on the layout window where you want to place the chained transistors.

The transistors are placed where you click.

If you want to remove one or more transistor from the stack, follow these steps.

1.Select the transistor(s) you want to remove.

2.Choose Edit – Move.

3.Click where you want to place the transistor.

The transistor is placed where you click in the layout window.