Virtuoso XL Layout Editor User Guide

Editing Your Layout with Virtuoso XL Layout Editor

Using Transistor Folding

You can use the Transistor Folding command to interactively divide a transistor or a transistor stack (an abutted group of MOS transistors) into two or more fingers, layout instances with terminals all connected in parallel to the same nets. Folding transistors and stacking them allows you to change their aspect ratio for design efficiency.

Transistors to be folded must have a property named lxComponentType to store their width attributes set on the cell. You must use the Edit Component Types form associated with the Design – Component Types command to set this property on the cell. Using this form, you can definelxDeviceWidth, the MOS device gate width parameter used to edit the transistor folding (required), and lxMaxWidth, the maximum transistor width (optional). For more information about setting these parameters, see Setting Component Types on page 227

You must have the environment variable mfactorSplit set to t in a setup file for the design, or the Fold Transistors command does not work.

Controlling the Folding Grid

To keep folded devices from becoming off-grid set the lxRounding property. This property is set on either the cell master, CDF, or schematic instance. Rounding is currently only done on w, lxDeviceWidth (Folding), and sFactor (series connection factor) properties.

A lxRounding property on the master of the PMOS and NMOS devices will yield the desired rounding. For example, lxRounding = (w 0.05 round) will result in rounding the value of the width to the closest multiple of the 0.05.

The format is (paramName roundingFloat keyword)

paramName - the name of the parameter to be rounding in case Virtuoso XL changes the respective value. The parameter value must evaluate to a numeric value. When returning the value to the instance the original property type is reserved. Allowable paramNames are w, lxDeviceWidth, and sFactor.

rounding - float is a number. In case the property is rounded the returned value is an integer multiple of that value.

keyword - the rounding method used. Allowable values are;

floor - round to the integer multiple at or below the current value

cell - round to the integer multiple at or above the current value.

round - round to the integer multiple at or closest the current value.

Virtuoso XL Layout Editor User Guide

Editing Your Layout with Virtuoso XL Layout Editor

Folding Transistors

To fold one or more transistors, follow these steps.

Note: You can preselect the transistors you want to fold in the layout or schematic window. If you select a transistor that has already been folded, you must select it from the schematic or select all of its fingers at the same time in the layout.

1.Choose Edit – Transistor Folding.

The Set Transistor Folding form appears.

If you did not preselect any transistors, no transistor-specific information is shown in the form until you select at least one.

If you preselected transistors to fold, the name of first one you selected is shown in the

Transistor Name field.

The width of the transistor, as specified in the schematic, is shown in theTransistor Width field.

2.For the selected transistor, type in the Number of Fingers field the number of fingers into which you want it divided.

The Set Finger Widths option button appears on the form.

3.To set the finger widths to unequal widths, clickSet Finger Widths.

Virtuoso XL Layout Editor User Guide

Editing Your Layout with Virtuoso XL Layout Editor

The form changes to let you enter the finger widths.

You can add new fingers with theAdd Finger button, delete fingers with theDelete button opposite each finger, and assign the same width (dividing the total width of the transistor by the number of fingers) to all the fingers with theSame Width button.

Note: Interactive folding might overwrite the default lxMaxWidth parameter; by using the specifications you supply. For example, if you fold the transistor in too few fingers, the

fingers might end up longer than thelxMaxWidth parameter you must specify using the Edit Component Types form associated with the Design – Component Types command. Automatic folding always respects the lxMaxWidth parameter.

4. Type the finger widths you want.