Chapter VII: DESIGN ENVIRONMENTS

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The Schematic Technology

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Electric is primarily oriented for IC layout technologies. Nevertheless, the Schematic technology allows schematic capture to be done. To obtain this technology, use the Change Current Technology command of the Technology menu and select "schematic, digital". The Schematic technology contains both digital components, which appear in the component menu on the left, and analog components, which are always available from the New Analog Part and New SPICE Part commands of the Edit menu. The analog components may be placed in the component menu by selecting the "schematic, analog" technology.

Figure 7.3
There are two arcs in the Schematic technology: the wire (in blue) and the bus (in green). These arcs can be drawn at 45 degree angles. One typically names busses with array names (for example "insig[0:7]"), and then names wires with scalar names (for example "insig[1]"). In this way, they make an electrical connection, even if not physically connected in the circuit. To make a physical connection of a wire to a bus, the bus pin can connect to either, so it acts as a tap. Use the Rip Bus Signals subcommand of the Network command of the Tools menu to automatically add taps to a bus.

Digital schematics are built with the And, Or, Xor, Buffer, Multiplexor, and Flip-Flop nodes that appear in the component menu on the left. By attaching arcs to these components and negating them (with the Negated command of the Arc menu), these turn into NAND, NOR, Inverter, and many other specialized components. Note that the size of the negating bubble can be controlled with the Schematic: Negating Bubble Size subcommand of the Parametrize Technology command of the Technology menu.

The And, Or, and Xor nodes can accept any number of input connections on the left, and there are three possible output connections on the right, so they require some care in wiring. The right side has a normal output port in the center, and two differential output ports above and below. The left side has one large input port that allows an arbitrary number of connections. Initially, wires may attach at only three input locations, spaced evenly along the left side. However, when all three locations are connected, the node automatically expands, adding additional space along the side for new arcs. The Multiplexor node also has a variable-sized port on the left side.

To properly wire inputs to an And, Or, Xor, or Multiplexor node, cursor placement is very important, for it determines which of the locations to use on the left side. If an arc gets connected in the wrong location, try connecting more arcs until one appears in the right place, and then delete the unwanted ones.

The Schematic technology also contains analog components. To obtain them, select from the New Analog Part and New SPICE Part commands of the Edit menu or use the Change Current Technology command of the Technology menu and select "schematic, analog" to get the menu shown here.
Figure 7.4
The analog nodes can have user-settable values displayed on them. When a node is created, you are prompted for an appropriate value.

The Switch node can take an arbitrary number of poles. Simply stretch it along the line of the poles and their number will grow. To do this, use the Size command of the Edit menu.

For SPICE simulation, some special purpose nodes exist. When creating a Source, you are prompted for a SPICE fragment (i.e. "DC 5" on a Voltage Source). The Power and Ground nodes have standard definitions. Additional SPICE components can be found in the New SPICE Part command of the Edit menu. More information about the use of these parts can be found in the Simulation section of Chapter 9.

As a graphical aid to schematic design, frames can be displayed in a facet. The A and D size frames are available in both landscape and portrait mode with the A-size Landscape Frame, A-Size Portrait Frame, D-Size Landscape Frame, and D-size Portrait Frame commands of the View menu. Use the Remove Frame command to turn off this frame display.


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