Virtuoso Schematic Composer User Guide

Understanding Connectivity and Naming Conventions

Wire-by-Name Connections

A signal is said to be “connected by name” if more than one physically disjoint wire or group of wires exist that carry the signal.

Instance pin connections

net_name

Instance pin connections

net_name

net_name

To connect instance pins, assign the same name to each wire. The wires do not have to be physically connected. If you use a local net name, you can connect by name on the same page. If you use a global net name, you can connect by name across all levels of the hierarchy.

Be careful not to inadvertently connect a signal across a schematic by using the same name.

You can catch this error before your design has been netlisted by checking your design.

Wire-to-Iterated Instance Connections

Before the editor lets you connect a wire to an iterated instance, it checks to see that the net width equals one of the following:

■Connection Width on page 96

Virtuoso Schematic Composer User Guide

Understanding Connectivity and Naming Conventions

■Pin Width on page 96

■1 on page 97

If the connection cannot be made, the editor generates an error message when you check your design.

Note: The net width has no limit.

Connection Width

If the connection width is equal to the net width, the editor allows the connection without modifying the design. To determine whether net width equals connection width, the editor compares the net width to a computed connection width. The connection width is computed as follows:

instanceIterationWidth * instancePinWidth

In the following example, the width of instance I<0:2> is 3 and the width of pin P<0:2> is 3. The computed connection width equals 9. The net width N<0:8> also equals 9. Therefore, the connection can be made.

Pin Width

If the net width is equal to the pin width, the editor adds a net prefix repeat operator to the net width before it allows the connection. If the net width equals the pin width but does not equal

Virtuoso Schematic Composer User Guide

Understanding Connectivity and Naming Conventions

the connection width, the editor computes a net prefix repeat operator that equals the instance iterations.

The net width (3) equals the pin width but does not equal the connection width (9). Therefore, the editor generates a net prefix repeat operator,<*3>, for the net in order to allow the connection.

1

If the net width is equal to 1, the editor adds a net prefix repeat operator to the net width before it allows the connection. If the connection width is 1, the editor computes a net prefix repeat operator that equals the computed connection width.

N

P<0:2>

The net width equals 1, the pin width equals 3, and the instance width equals 3. The computed width is 9.

I<0:2>

Virtuoso Schematic Composer User Guide

Understanding Connectivity and Naming Conventions

The net width does not equal either the connection width (9) or the pin width (3), but it does equal 1.

<*9>N

P<0:2>

The editor generates a net prefix repeat operator,<*9>, for the net in order to allow the connection.

I<0:2>

Global Net Name Connections

Global nets make connections across the design hierarchy without using pins and wires to explicitly pass the signal throughout the hierarchy. Global nets are used primarily for power and ground connections.

To create a global net, add an exclamation mark to the net name.

gnd!

To make a bus global, add an exclamation mark to the base name of the bus.

A!<0:3>

Another way to make a net global is to connect it to a pin or instance pin that is global. The net inherits the name of the global pin or instance pin. The basic library has various global symbols you can use to indicate global nets.

gnd!

vdd!

In summary, you can create a global signal by

■Using the Add – Wire Name command

■Using the Add – Instance command of normal power and ground symbols