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9: Networks |
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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:
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.
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.
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.
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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