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Simple Cadence Software Tutorial Part 4


Making the Inverter Schematic

In this fourth part of the tutorial, we will discuss how to layout the schematic design of our Inverter. As stated in the last part, after creating the new cell, a window opens in which we will create our schematic. This window is seen in figure 5. This window is the Virtuoso Schematic Editing window.
Figure 5. Virtuoso Schematic Editing Window

Lets begin our Inverter design by adding the transistors. To add a component click Add->Instance. This brings up the Component Browser and a window in which you can specify the component to be added. These windows are shown below:

Figure 6. Component Browser and Add Instance Windows

The first part we will add is the nmos transistor. To select this transistor, select NCSU_Analog_Parts then select the category N_Transistors and select the nmos4 transistor from the list. In the Add Instance window, enter the width as 4u and the length as 2u.  Cadence will automatically change this to 4u M and 2u M respectively. Move the cursor into the editing window. Notice that there is an nmos transistor there instead of the normal cursor. To place the component, you need to left click.  To rotate the component you need to right click. To exit from adding the current component or any other action press <Esc>. Notice that there are letters next to many of the choices in the menus, these are hot keys and can make you work progress a lot faster if they are learned. Place the nmos transistor in the bottom half of the screen on the right side of the center line.

Next, we will add a pmos transistor.  This is located under NCSU_Analog_Parts->P_Transistors and select the pmos4. Give it the same width and length and place it on the top half of the screen above the nmos transistor.

The next step is to add the pins.  Click Add->Pin and a window appears for adding pins to the schematic.  We will add pins vdd, vss, in, and out. Ensure that the direction is set to inputOutput. This can be seen below:

Figure 7. Add Pin Window

The order in which the pins are added does not matter. You can also add only one pin at a time. In the current method, the pins will be placed in the order they appear in the list. First, place the vdd pin above the pmos transistor. Note the small diamond, that is the actual pin so rotate the object until the diamond faces down toward the nmos transistor. Add the vss below the nmos, the in to the left of the screen, and the out to the right of the screen. In call cases make sure the small diamond faces the transistors.

Now we will add wires to connect the entire Inverter so it will work. Click Add->Wire to add wire or you can use the hot key ‘w’. Refer to the figure below to see how the connections are made. Notice that as you get closer to a device or node when placing wire, a small diamond appears. This is where you need to click to place a wire. If you make a mistake click on the error and select Edit->Delete.

Figure 8. Finished Schematic of the Inverter

Once you have completed adding all components and wire, click the “check mark” icon on the left of the window. This will check your work for errors and save your work to the library. The same can be done with Design->Check and Save. Any errors will be reported in the CIW.

Printing Your Schematic

Now that we have finished the schematic, we would like to print it out. To do this click Design->Plot->Submit. The window that appears allows you to select the options for printing. You can print directly from here, or if desired, the schematic can be saved as a file, such as a PostScript. To save to a file click Plot Options on the bottom of the window. In the new window, select Send Plot Only to File and enter the desired file name, such as plot.ps.
Figure 9. Submit Plot and Plot Options Windows

We need to create a symbol for our inverter. We will discuss this in Part 5 of this tutorial.

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