ELEC451 Integrated Circuit Engineering
Using Virtuoso Schematic Editor
Contents
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1. General
2. Adding Components
3. Connecting Components
4. Selecting/Deselecting Components or Wires on the Schematic
5. Adding Wire Names
6. Editing
o 6.1 Moving Components or Wires
o 6.2 Deleting Components or Wires
o 6.3 Undoing/Re-doing an Action
o 6.4 Editing Component Properties (Parameters)
7. Printing a Schematic
8. Saving a Design
9. Creating a Symbol Cellview for Your Design
1. General
Virtuoso Schematic Editor is a schematic-entry tool. Is allows you to define
your design in terms of a graphical picture of connected components (a
"schematic'').
All user designs, as well as components and cells prepackaged as part of the
environment are stored in libraries, which are just Unix directories. Any
components you need are picked from these libraries.
To start Schematic Editor, select File->Open from the icfb window menu. Now
change the Library Name to the one you want, then click on the cell you want (or
type in its name), verify that the SCHEMATIC view of that cell is selected and
click OK. You can also use the Browse button to browse through the contents of
other libraries.
What you really need to know about using Schematic Editor is covered in
the CAD assignments themselves, but for a better understanding, or as a
reference guide, please read the following sections.
2. Adding Components
You can add one or more "instances'' of any component that you can find in a
library. Use Add->Instance from the window menu. Like most actions, this one
can be taken either by selecting the menu item, selecting an icon from the toolbar
at the left, or using an equivalent keyboard key (in this case the i key). The Add
Instance dialog appears to allow you to locate the component you would like to
add, and you can then Browse different libraries to find it.
Once you have clicked on a component, you can drag the mouse back into the
Schematic Editor window, and a silhouette of the picked component follows,
which you can then drop by clicking at the desired insertion point. Once you have
added this first component, CDS is in the "add component'' mode, and is
immediately ready for you to drop in another one right away. If you only wanted
the one, you can click Cancel in the Add Instance dialog (or hit the ESC key on
the keyboard if that dialog box is not longer visible). If you want to add multiple
but different components, you do not need to close the Add Instance window, just
click on another component in one of the libraries to prepare to add it.
3. Connecting Components
You connect components with wires. Adding wires is part of the default mode for
the Schematic Editor, so you can just click on the terminal of one component
(one of the little red boxes) to select it, drag the mouse to the terminal of the
component you want to connect to, and release to complete the wire. You can
also select Add->Wire from the Schematic Editor main menu, then click at the
desired endpoints. But if you type w as a shortcut, you have to have the mouse
already positioned over the first terminal, or else you will end up with a wire
hanging in space (hit Cancel or ESC to get out of that situation).
4. Selecting/Deselecting Components or Wires on
the Schematic
To select an existing wire or component on the schematic, left-click on it until you
see it highlighted with a solid box (or highlighted line for a wire). Note that a
dashed box (or a dashed line for wires) appears when you hover above a
component (or wire), but you must then click to select it. If you fail to left-click at
this point, the "currently selected component'' (or wire) remains at the last one
you clicked on. Remember this to avoid inadvertently modifying or deleting the
wrong component or wire.
To deselect a wire or component, click in an empty region of the schematic - the
highlighting of the wire or component will disappear to let you know it is no longer
selected.
To select a group of adjacent components, click and drag to define an outline box
with the mouse. When you release the mouse, all components and wires inside
the box are selected as a group.
Personally, I recommend against selecting single objects by clicking on
them, since you then have to click in the empty background to deselect
them before moving on to a new object. Instead, I find it faster and more
convenient to "hover" the mouse over the object of interest until its border
is dashed-highlighted, then use a keyboard key to take my desired action (
m to move it, c to copy it, q to query/change it, and Del to delete it). You still
need to hit ESC when done to get out of the mode for the last action you
took.
5. Adding Wire Names
Although CDS generates automatically numbered "net'' names for each wire
(circuit node), you can override these with explicit names that are more
informative. Hover over the wire till it is dash-highlighted, then type the l key (for
"label"), then type in the desired name, then click OK, and click the mouse on the
wire to be named. You can also first click to select the wire, then use the WireName toolbar tool, or the Add->Wire Name menu item.
6. Editing
You can move, copy, delete, rotate, and take other actions on components from
the Edit menu, but most of these have shortcut keys that are faster. The more
important edit actions are described below.
6.1 Moving Components or Wires
With a single component or wire, you can just click and hold down the left mouse
button, and drag to the desired position - you don't need to select the component
(or wire) first, and the action does not leave the component selected when
completed. But to move a whole group of components to another spot, you first
need to select that group by dragging a box around it, then move it to the new
position. To move the selected group (solid-white-highlighted), type the m key,
then move your mouse (the group will follow), then click to drop it. You can only
make a vertical OR a horizontal move at any one time. Be sure to deselect the
group when done. You can also use the m key with single objects that you have
first dash-highlighted by hovering over them with the mouse.
6.2 Deleting Components or Wires
To delete, hover over the component till it is dash-highlighted, then press the Del
key (this also works if you first click to select the component then use the DEL
key or the Edit->Delete menu item, or the eraser icon at the left of the display.
Caution: Once you delete an item, CDS is in the "delete'' mode, and
anything you next click will also be deleted - to escape from this mode, hit
the ESC key as usual.
6.3 Undoing/Re-doing an Action
You can undo any change by typing in a u, clicking the undo icon, or using the
corresponding Edit menu item. You can actually undo a series of actions by
repeatedly typing u . The depth of this undo capability is set under Options>User Preferences in the icfb main window.
If you accidentally undo something you didn't want undone, just do a "redo'' (type
key U (capital "U") or select it and use the Edit menu).
6.4 Editing Component Properties (Parameters)
Hover over the component to dash-highlight it, then type the q key (for "query") .
Alternatively, you can select the component first by left-clicking on it, and then
use either the q key, or click the Property icon on the toolbar, or select Edit->
Properties. If you don't click to select the component first, you may end up
inadvertently editing the properties of a different component - the component you
had last selected.
From the Edit Properties window, you can change the values of the component's
parameters, for example, to change the voltage of a voltage supply, or to change
the width of a transistor, etc. The units for each property are defined by a letter
which you are not allowed to change (even though it appears you can change it).
For example, transistor dimensions are in metres (capital "M''), voltage sources
are in volts (capital "V'') etc. The usual SI modifiers are accepted by typing them
following the numeric value you enter. For example, 3u means 3 x 10 -6 . Letters
m,u,n,p,f stand for milli, micro, nano, pico, and femto, respectively.
Some properties are specified in terms of expressions involving variables or
other property names. This allows, for example, a transistor width to be specified
as a fixed multiple of its length, or instead to be specified by a variable whose
value will be filled in later by the user during a simulation run.
With CDS, the units are an integral part of the Property, so that when filling in
variable values (for example, later on in a simulation), the unit must not be typed
in with the value of the variable. It is therefore incorrect to type in 3V for the value
of a voltage variable - CDS would interpret this to mean "3V" volts, which is
undefined , rather than the "3" volts you intended.
7. Printing a Schematic
Select Design->Plot from the main Schematic Editor menu, and set up for
printing as described under "Using the Cadence Design System". You will need
to click on the Plot Options button in the dialog box that appears in order to
choose the printer type, plus other options.
8. Saving a Design
You can save a design from the Design menu, but the Cadence documentation
recommends that you always use Check and Save rather than just Save. You
can also save the design under a different name (for later editing to make a
different version, say), but note that the current file name stays what it was. So if
you want to save to a new name, and edit that newly-named design, you will first
have to open it by using Design->Open. This behaviour is different from most
editors and programs you may be used to, so be careful you are working on the
properly named schematic.
9. Creating a Symbol Cellview for Your Design
You can "package'' your designed cell into a convenient module that has a
graphical symbol (a "symbol''), which allows you to then incorporate it as a whole
into another design schematic. For example, you could package an inverter
design schematic named, say inv_small, containing an NMOS and a PMOS
transistor into a normal inverter symbol (i.e. a triangular package with one input,
and one output, with a bubble at the output).
Before you can make the symbol cellview, you need to add "pins'' (from the Add
menu, or by using the pin tool on the toolbar) to define where the inputs and
outputs are. Then when you select Design->Create Cellview->From
Cellview, a cellview editor opens allowing you to define the shape of the
symbol that will house your design, labels, etc.
Now you could open a new design, and by picking inv_small from your library,
drop in an instance that would appear as the triangular inverter symbol, having
terminals to which you will connect. By using this feature, you can build up
designs hierarchically, with simple logic cells having few transistors at the bottom
of the hierarchy, and complex cells ( that contain simpler lower-level cells) at the
top of the hierarchy. You should already be familiar with this approach if you
have used the Altera Design software.