Tutorials:Cadence:CreatingSchematicInverter

Creating a New Cell: Inverter

Click Library Manager: New → Cell View...

The New File form should appear as shown below. We will be making an inverter, so type inv in the Cell field. In the View field type schematic or from the Type pull-down menu choose schematic and the View field will be automatically filled. In the Application section, select the following: Open with Schematic XL and check the box next to Always use this application for this type of file. Once you make these two selections, in the future they will be automatically selected.

Left click the OK button.

The Virtuoso Schematic Editor window should be showing on your screen as shown in the figure below:

Left click: Schematic Editor: Create → Instance... (bind key <i>)

Shortcuts: You may notice letters by some of these menu choices. Rather than clicking through all the menus, you can just hit that button on the keyboard to the same effect. These are called bind-keys. In the future, you can press <i> in order to insert an instance.

A Command Browser window appears. In this window, select NCSU_Analog_parts under Library pull down menu. Next click on P_Transistors → pmos4. The window should look as shown in the figure below.

Type pmos4 to choose a four terminal PMOS transistor in the Cell box and symbol in the View box. Note that you can use the Browse button in order to browse through the libraries and find the cell you want. You can edit the parameters of the pmos4 cell, such as width, length, etc. For this exercise, keep the default parameters. The Add Instance window is shown in the figure below.

Move the cursor into the editing window. Notice that there is an PMOS transistor there instead of the normal cursor. Position it where you want to put the transistor, and left click to place it. You can right click to rotate the transistor if you want it to face a different direction (this is especially useful with pins). While placing, stretching, etc., you can press F3 to show the options form for the command if it is currently hidden.

Press <Esc> to return to a normal cursor after you have finished placing all the transistors you want. For this inverter example, place it on the bottom half of the screen on the right side of the center-line. To rotate press <r>.

Click Schematic Editor: Create → Instance...

Follow the same steps as before, but choose a nmos4 transistor. Use the default values for length and width. Place the NMOS transistor somewhere below the PMOS transistor.

To make life easier: Before trying to place a component, left click the Hide button on the Add Component window. This will move it into the background so it's out of your way.

Repeat the same procedure as above and add both gnd and vdd symbols. Place the gnd symbol below the nmos4 transistor and place the vdd symbol above the pmos4 transistor. Refer to the final figure at the end of this section to see the placement of all components. When adding the gnd and vdd symbols, the Add Instance window appears and it should look like the ones shown below.

 

Now, we'll add the pins for the inverter. Click Schematic Editor: Create → Pin (bind key <p>).

The Add Pin dialog box comes up. In the Pin Names field enter the pin name in (note, multiple pin names can be given, separated by a space) and ensure that Direction is set to input. The Add Pin window should look as follows:

Place the in pin somewhere towards the left side of the editing window, between the two transistors.

Click Schematic Editor: Create → Pin again. The Add Pin dialog box comes up. In the Pin Names field enter the pin name out and ensure that Direction is set to output. The Add Pin window should look as follows:

Place the out pin somewhere towards the right side of the editing window, between the two transistors.

Now, we'll add all the wires to make the inverter work. Click Schematic Editor: Create → Wire (bind key <w>). The Add Wire form should appear. Just click its Hide button. You can refer to the figure at the end of this section to see how everything is connected together.

Notice that as you get closer to one pin than another (including those on devices), a small diamond will show up inside of or around that pin. That is where you want to click to connect a wire. Also, when wiring the schematic, leave the wire width at 0, Route method at full, and (usually) Draw Mode as route. This tells the software to auto-route the wires for you.

For the first wire, left click on the gate terminal of the pmos4 transistor, and left click again on the gate terminal of the nmos4 transistor. You have just connected the gates.

Now, move the mouse until the little square is inside the diamond on the in pin. Left click in the diamond. Move the cursor over to the wire you connected the two gates together with. A diamond will form around the cursor, as long as it's on the wire. Left click on the wire. You have just connected the input to the gates of both transistors.

The body of the nmos4 and pmos4 transistors are the center pins. On the pmos4 transistor, connect this pin to vdd. On the nmos4 transistor, connect the body to gnd.

If you put a wire where you don't want it to go, you can delete the wire by left clicking Schematic Editor: Edit → Delete (bind key <del>) and then left click on the object you want to delete (wire, pin, component, etc.).

There should now only be one pin left on each transistor (the drains of both transistors). Connect these two pins together. Finally, connect this last wire to the out pin. A picture of what this should all look like is shown below:

Once you are done editing, left click the "check mark" icon on the left side of the screen. This will check your work for connection errors and will save your work in the library. You can accomplish the same thing by left clicking Schematic Editor: File → Check and Save.

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