Using the Electric VLSI Design System

Chapter VI: ADVANCED EDITING

| ## 9: Networks |

A collection of electrically connected components defines a network. Networks may span many arcs, or they may reside on only a single port of a single node. Since networks are stored in the Electric database, they can be immediately accessed when needed.

Whenever a port on a node is selected, the highlighting indicates the entire network that is connected to that port. Another way to see an entire network is to use the Show Network subcommand of the Network command of the Tools menu. This will highlight all arcs on the currently selected network. If the design is very dense, you can select a network by its name with the Select Network... subcommand of the Selection command of the Edit menu.

There are many commands that can be used to get information about the networks in a facet:

- The List Networks command of the Info menu shows a list of the named nets in the current facet.
- The List Ports on Network command of the Info menu lists all exported port names on the currently highlighted network. This list contains the names of ports at all levels of the hierarchy, above and below the current facet. The facility is useful if, for example, you have propagated clock lines throughout the circuit and wish to make sure that all of the exported port names on this network have some variant of the name "phi". By quickly examining this list, you can see all of the names that have been used on the network, throughout the hierarchy.
- The List Connections on Network command of the Info menu lists all nodes that are connected to the current network. This list includes only those nodes at the ends of the net, and not the pin or contact nodes used inside of the network. The command is useful if you are at one end of a wire and want to check to see what is at the other end.
- The List Geometry on Network command of the Info menu lists all geometry that is connected to the current network. This reports the area and perimeter of all attached layers.

### Naming Networks

Network names are derived from exported ports and named arcs in a facet. The name given to an exported port becomes the network name of all arcs connected to that port. Similarly, the name given explicitly to an arc (by setting the name field in the Get Info command) becomes the name of the network for all connected arcs.

If no named arc or exported port is connected to a network, then it has no name. If, however, you check "Automatically name networks" in the Network Options... subcommand of the Network command of the Tools menu, then a name will automatically be generated.

Two phenomena can occur in network naming: a network can be multiply named, and it can span disjoint circuitry. A network has multiple names when two or more connected arcs or ports are named with different names. For example, if you export a port on a contact node and call that port "clock", then you select an arc connected to that contact node and name it "sig", the circuitry will be on the network "clock/sig."

The other phenomenon of network naming is that a single network can include unconnected parts of the circuit. This happens when arcs in unconnected parts of the circuit are given the same name. This causes the two arcs to be implicitly joined into one network. Because this network naming phenomena is most commonly used in schematics, the unification of like-named networks only happens in facets with the "schematic" view. To cause this same effect in all views (such as "layout"), use the Network Options... subcommand of the Network command of the Tools menu and check the "Unify all like-named nets" item.

### Bus Naming

The Bus arc of the Schematics technology is a special arc that can carry multiple signals. When giving a network name to Bus arcs, it is possible to specify complex bus names. Bus names can be lists (for example, "clock,in1,out" which aggregates 3 singals into a 3-wide bus) or they can be arrays (for example, "A[0:7]" which defines an 8-wide bus). Arrays indexes can be individual values, or ranges of values (for example, the bus "b[0],c[3,4],d[1:2],e[8:6]" is an 8-wide bus with signals in this order: b[0], c[3], c[4], d[1], d[2], e[8], e[7], e[6]). Finally, it is possible to use symbolic indices in bus naming (for example, the bus "r[x,y]" defines a 2-wide bus with the signals r[x] and r[y]).

Individual wires that connect to a bus must be named with names from that bus. As an aid in obtaining individual signals from a bus, the Rip Bus Signals subcommand of the Network command of the Tools menu will automatically create such wires for the selected network.

### Power and Ground

Identification of a power network is done by finding (1) a Power node from the Schematic technology; (2) an exported port in the current facet that has the "power" characteristic; (3) an exported port in the current facet that begins with the letters "vdd", "vcc", "pwr", or "power"; or (4) a port on a component in the current facet that has either of the above two properties. Ground networks use the same rules, except that the acceptable port names begin with "vss", "gnd", or "ground".

By default, supply networks defined with the Power and Ground nodes of the Schematic technology are combined into one network. This means, for example, that two arcs, each connected to a separate Ground node, appear on the same network regardless of their actual connectivity in the circuit.

Although this unification is the proper thing to do for schematics, it is not always proper for IC layout. For example, in MOS technologies, two ports exported with the "power" characteristic are not on the same net unless they are actually connected (there may be multiple power rails that do not connect). As a circuit debugging aid to ensure that power and ground networks are properly connected, Electric can be instructed to unify power and ground networks in ALL technologies, regardless of their actual connectivity. The Network Options... subcommand of the Network command of the Tools menu has the "Unify Power and Ground" item which causes all power and ground networks to be combined. This unification of all supply rails can be disabled by unchecking the menu entry. By highlighting power and ground networks with and without this option, designers can see whether all of their supply rails are fully connected.

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