next up previous contents
Next: Incorporating external netlists : Up: Hierarchical design Previous: Automatic creation of symbols

Rules for local and global connections within schematics.

                 

The connectivity of schematics is defined by nets, global pins (gnd, vss,vdd,vss1, vdd1), named globals, ports, named pins and symbol contacts.

Global pins and named globals may be placed on all hierarchy levels. However, it is necessary to place each used global pin at least once on the top level. For example, if you try to place voltage supply sources in a symbol(schematic) on a deep hierarchy level and connect them via global symbols to the rest of your schematic, you will receive an error message. You must place at least one global pin of each kind or name on top level if you use them on deeper hierarchy levels.

Named pins and ports can also be placed in all levels, but different to global pins, named pins and ports are local. When a port ( SPICECAD version < 1.7.4 : named pin ) is placed on a hierarchy level n and is related to a symbol contact with the same name, the net on level n attached to this port will be connected to a net on level n-1 which is connected to the symbol contact. For the upward-compatibility, SPICECAD does not differentiate between named pins and ports. So, every named pin can be replaced by a port, and each port can be replaced by a named pin, both with the same name. To make schematics easier understandable, ports, however, should be used to define inter-schematic connectivity, and named pins should be used to define intra-schematic connectivity (see picture 16.3).  

 

If you have placed a named pin more than once on different nets, these nets (on the same hierarchy level) will be connected to each other. This allows you to save space which you might want to use for large nets running from one end of your schematic to the other.


  
Figure 16.3: Usage of 'named pins'
\fbox{
\includegraphics{use_namedpins.ps}
}


next up previous contents
Next: Incorporating external netlists : Up: Hierarchical design Previous: Automatic creation of symbols
Martin Maschmann
1999-10-10