Electric VLSI Design System Tutorials from CMOSedu.com (Return)

Tutorial 1 – Layout and simulation of a resistive voltage divider

This tutorial will introduce you to the Electric VLSI design system.

It’s assumed that Electric (version 8.10 or later) and LTspice have been installed properly on your computer following the instructions here and here.

With this assumption all layout and simulation work will be done (saved) in C:/Electric (where the Electric jar file resides).

Ensure that you have increased the memory in your JVM as instructed above.

Start Electric. You should see the following window.

Note that this shouldn’t be the first time you’ve started Electric since you’ve followed the instructions above and have set LTspice up for use with Electric and have increased the JVM.

This is mentioned since it’s possible to simply save the Electric jar file to the desktop, double-click on it, to start/use Electric as discussed here.

Next go to menu item Window -> Color Schemes -> White Background Colors

Using a white background will be useful in these tutorials so that ink is minimized if they are printed out

It’s often preferable to use a black background colors to ease the stress on your eyes ;-)

Adjust the sizes of the windows to fill the available space as seen below.

We’ll set Electric up for use in ON Semiconductor’s C5 process and fabrication through MOSIS.

This process has two layers of polysilicon to make a poly1-poly2 capacitor, 3 layers of metal, and a

hi-res layer to block the implant, and thus decrease in resistance, of poly2 to fabricate higher-value

(than what we would get with poly1) poly2 resistors.

This tutorial uses the MOSIS scalable CMOS (SCMOS) submicron design rules.

While the C5 process is an n-well process we’ll still draw the p-well, which will be ignored during fabrication, just to make the layouts more portable between processes.

Next, go to File -> Preferences (or just hit the wrench/screwdriver menu icon) then Technology -> Technology to get to the window seen below.

Change the information to match what is seen below.

Note that the “Analog” Technology is selected.

This selection shows the resistor and capacitor Nodes in the Component menu (discussed shortly).

Next, the scale (lambda) for the C5 process is 300 nm using the MOSIS Scalable CMOS (mocmos technology in Electric, see image above) submicron design rules.

To set the scale go to File -> Preferences -> Technology -> Scale and set mocmos scale to 300 nm as seen below.

Press OK to exit.

Select Mark All Libs in the next Window to indicate you want all of the libraries marked with these changes.

Here are my preferences (right click and save as), electricPrefs.xml which can be loaded if there are problems or to ensure consistency through the remainder of the tutorial.

File -> Import -> User Preferences followed by navigating to where my preferences were saved imports these preferences if there are problems.

We now have Electric set up to fabricate a chip in the C5 process via MOSIS (technology code is SCN3ME_SUBM with a lambda of 0.3 um, see the MOSIS page here at CMOSedu.com).

Go to File -> Save Library As -> tutorial_1.jelib

Next let’s begin to draw the schematic of a resistive divider.

Go to Cell -> New Cell and enter the cell name (R_divider) and view (schematic) seen below.

After selecting the Component tab on the left side of the window we get the following.

The library name and cell name are seen above the Components, Explorer (for looking at the cells in your library), and Layers (useful in layouts to turn on/off the display of certain layers).

In the Component menu there is a box containing a resistor and the word “Normal.”

Click on the arrowhead in this box and select N-Well. This selects the N-Well schematic resistor Node.

In Electric-speak a Node is a component used in a schematic or layout. Examples of Nodes include transistors, resistors, capacitors, etc.

An Arc, which we’ll discuss shortly, is used to connect Nodes together to form schematics or layouts. A wire is an example Arc in a schematic.

Place the N-Well schematic resistor Node into the drawing area as seen below (left click the mouse button to place the Node).

Use the Window menu commands to zoom in/out, fit, etc. the view after placing the Node.

All Nodes have a highlight box that turns on, to indicate that the Node may be selected, when the cursor is placed over it.

When a layout/schematic contains Nodes whose highlight boxes overlap the selection of a particular Node can be cycled through by pressing the Ctrl+mouse click (very useful in complex layout).

Pressing Shift while clicking the left mouse button selects/de-selects an item (again, very useful).

Select, by clicking on Node, the N-Well resistive Node.

Next go to Edit -> Properties -> Object Properties (or simply Ctrl+I) to edit the properties of this Node, see below.

The sheet resistance of n-well in the C5 process is roughly 800 ohms.

The minimum width of n-well is 12 lambda so let’s make a 10k resistor using a width of 15 and a length 187.5 (since sheet resistance varies we could round to 185).

Enter the values as seen below.

If the field for the Well resistance isn’t showing hit the More button.

We’ll use these same values when doing the corresponding layout.

After pressing OK (not the X in the top right side of the window which would ignore your changes) we get the following.

This is a schematic representation of a 10k N-Well resistor.

Before moving on go to Tools -> DRC -> Check Hierarchically (or just hit F5) to check the schematic for errors.

The Electric Message window will indicate that there aren’t any errors.

It’s a good idea to get used to looking at the messages in this window.

Listening to Electric can really save time ;-)

Let’s next make a layout corresponding to this schematic-view cell.

Again, go to the menu item Cell -> New Cell and enter the Name and View seen below.

This will create a group of cells.

Clicking on the “+” sign adjacent to the cell group (R_divider) name gives the following.

The red circle indicates a schematic view while the yellow circle indicates the cell’s layout view.

Blue indicates an icon while black may indicate a Verilog view (among others).

Notice, above the tabs, that the library name is tutorial_1 while the active cell name is R_divider{lay}.

Also notice at the bottom of the screen is an indication of the technology and scale.

When we start doing layout this area will also the indicate x and y position of the cursor.

Next, go to the Components tab and select the N-well resistor Node in the bottom left-hand side of the menu (click on the arrowhead).

Note that if this menu item isn’t available, as seen below, then you didn’t select the Analog option in the preferences near the beginning of this tutorial.

Set the size (Edit -> Properties -> Object Properties or better yet simply hit Ctrl+I) of the N-Well resistor to, as above, L=15, W=187.5, and a resistance of 10k.

After fitting the display using the Window menu item we get the following.

Notice, like the schematic N-Well resistor Node, that this Node is selected by moving the mouse over the Node’s highlight box and left clicking on the Node.

Also note, again like the corresponding schematic Node, that this Node has two ports for connection to Arcs.

In the figure below we moved the cursor towards the left port of the Node so that clicking on the Node selects this port.

To verify the layout doesn’t contain design rule errors go to Tools -> DRC -> Check Hierarchically (or just hit F5) to perform a design rule check.

From this point on “pressing F5” will be equivalent to saying that we are doing a design rule check of a layout or checking a schematic.

After pressing F5 we see in the Electric Message window that there aren’t any errors (so let’s make one).

Edit the properties of the N-Well resistor Node above so that the width is 5.

Press F5 to run the DRC. We get the following.

Notice how the Electric Message window tells us that to step through the errors we press the greater-than key, >, to go forward through the errors or the less-than key, < to go backwards through the errors

Pressing OK above and then > gives the following view (after zooming in around the flashing red and black box indicating the error’s location).

Notice the type of error is indicated in the Message window.

Pressing Ctrl+Z a few times gets back to the case where W =15 (or selecting the resistor Node and hitting Ctrl+I allows us to change it back manually)

Press F5 to verify the layout is DRC free.

Then use the menu item Window -> Fill Window to zoom back out.

At this point let’s verify that the schematic and layout views of the R_divider cells are equivalent.

This layout versus schematic (LVS) verification is performed in Electric using Network Consistency Checking (NCC).

To perform an NCC go to Tools -> NCC -> Schematic and Layout Views of Cell in Current Window.

There shouldn’t be any errors. However, if there are a table pops up that allows you to click on the error to view it in a new window.

Since we’ll be doing an NCC quite often let’s setup a key that we can press on the keyboard to perform this menu selection.

In other words let’s bind a key to this menu selection.

In the menu go to File -> Preferences -> General -> Key Bindings as seen below

On the right, navigate to Tools -> NCC -> Schematic and Layout Views of Cell in Current Window

Once this menu item is selected Add and bind L to this menu item (so we can do an NCC, aka LVS, each time we press L)

Note that although we pressed lowercase l it shows up as uppercase L below. So lower case l will be bound to the NCC command.

Press Add again in the window seen below.

This next part is important.

If a conflict exists when binding a key then Electric will tell you after you have added the key and pressed Add.

You need to “Remove All” conflicts (important).

If you don’t select Remove All Conflicts then a key may be bound to two or more menu items causing crazy behavior!

Before connecting resistors together to form a voltage divider let’s talk about the connection of the n-well and p-substrate.

Since the C5 process used in this tutorial is an n-well process the p-type substrate is common to all NMOS devices and grounded.

One of the electrical rule checks (ERCs) is to verify that the p-well (in our case this means p-substrate) is always connected to ground.

Further, in this n-well process, if the design contains only digital circuits then we always want the n-well to be connected to VDD.

To setup the ERC Well Check go to Preferences -> Tools -> Well Check as seen below

In all cases we want to verify that a contact is found in every area (floating wells are bad!).

We also want to verify that the p-substrate (p-well) is always tied to ground.

However, as just mentioned, we only want to verify that the n-well is tied to vdd (power, yes, we’ll use lowercase) if the chip is a digital only design (no N-Well resistors)

Running the Well Checker (Tools -> ERC -> Check Wells) on the resistor layout above reports the following errors.

To eliminate these errors, since our design isn’t only digital (hence why we check the analog box when we setup our preferences at the beginning of the tutorial) change the settings to the following.

Mark All Libs to indicate you want all currently open libraries to use these preferences.

Running the Well Checker again on the resistor’s layout above doesn’t report errors (which is good).

Next let’s wire up a resistive divider.

Go back, using the Explorer, to the schematic view of the R_divider cell.

Select the N-Well resistor Node and then hit Ctrl+C to copy the Node.

Next click somewhere in the drawing area to deselect the Node. Press Ctrl+V and then left-click the mouse to paste the copied Node.

Don’t worry about making a mistake. Ctrl-Z works really well to back you up for another try.

An example result is seen below (where I zoomed out a bit prior to capturing the screen image).

Next select the bottom resistor Node and use the menu item Edit -> Rotate -> 90 Degrees Counterclockwise (or just simply Ctrl+J) to get the following.

Move the resistors apart so the horizontal Node is above the vertical Node.

Next select the top Nodes right port by clicking near it with the left mouse button.

This makes this port active and thus ready of a wire connection.

Move the mouse cursor so it’s above the vertical Node and RIGHT click the mouse (using the RIGHT mouse button may take a little getting used to)

The result is seen below.

Next move the mouse cursor over the top port of the left (vertical) resistor Node and RIGHT click the mouse button.

The following is the result (zoom out or in as required).

Now is a good time to check the schematic for errors by pressing F5.

Note that if you put your cursor over the corner in the wire you will see a Pin

Pins can be moved as well as the wires in schematics

A common error in a schematic is having unnecessary Pins.

Unnecessary Pins can be removed by going to the menu item Edit -> Cleanup Cell -> Cleanup Pins

I set up my key bindings so that F4 cleans up unnecessary Pins everywhere (available in electricPrefs.xml, right click to save as then File -> Import -> User Preferences)

Next let’s add a couple more wires as seen below.

Remember to left click for selecting a port on a Node and right click to add the Arc (wire).

We could add symbols for ground and a voltage source from the component menu but instead let’s simply label the Arcs (the wires) in the schematic.

Note that the SPICE components are accessed by clicking on the arrowhead in box under the Component menu labeled SPICE

Select an Arc and press Ctrl+I to edit the properties of the Arc or simply just double-click on the Arc.

Label the Arcs as seen below. Note that the bottom Arc is label gnd (a universal name for ground in SPICE, yes, use lowercase as we did for vdd)

Move the Arc names until they are aesthetically pleasing (see example below).

Next, click on the arrowhead in the Misc box in the Component menu to add SPICE code to the schematic as seen below.

Place the SPICE code in the schematic and use Ctrl+I to edit its properties.

Ensure, in the SPICE code property box, that the Multi-line Text box is checked.

Add the text seen below for specifying a SPICE transient analysis and an input voltage source.

Press F5 to check the schematic.

Also save the library.

Again, as mentioned at the beginning of this tutorial, LTspice must be setup with Electric.

To simulate this schematic go to Tools -> Simulation (Spice) -> Write Spice Deck

The following LTspice window will open.

Click on the circled icon above to show the waveforms available for plotting.

Select V(vout) and V(vin) to display the following.

The LTpsice waveform plotter is very useful but doesn’t allow for cross-probing between SPICE results and Electric layout/schematics.

To plot SPICE results using Electric’s probe first ensure, via (in LTspice, not Electric) Tools -> Control Panel -> Operation (see below) that waveform files (.raw files) are not automatically deleted.

When LTspice is closed, with the setups seen here, Electric’s probe will run as seen below (after the explorer was used to select vin and vout).

Note that these Windows can be tiled and the same commands used for zooming and expanding in the layout/schematic views can also be used here.

To bypass the LTspice Window and use Electric’s probe only change from the –i (interactive) to –b (batch) modes as mentioned when setting LTspice up for use with Electric

Next let’s layout the resistive divider.

Open the layout view of the R_divider cell then copy/paste (Ctrl+C/Ctrl+V) an additional resistor as seen below.

To move the resistor Node (the layout of an N-Well resistor) either the mouse can be used or you can select the Node and use the keyboard arrows.

The menu items circled in this figure above increase, set, and decrease the grid alignment. It’s set at 0.5 scale above.

Running a DRC (pressing F5) on the above layout results in the following figure.

By pressing > we see that there is too little space between the N-wells.

Move the Nodes apart until the layout passes the DRCs.

Next move the mouse over the right side of the top Node’s highlight box and left click (to select the top node and its right port).

RIGHT clicking (over the right side of the highlight box on the bottom resistor node) will connect the metal1 Arc to the bottom resistor.

Note that when you RIGHT click to connect the node, you have to be over the right side of the highlight box else you won’t connect the two ports

and the resistor will fail DRCs (you will generate an Arc that isn’t connected).

DRC your layout to verify there aren’t errors.

Before labeling the Arcs in this layout and simulating (we’re almost done!) let’s provide some comments.

First, how many Pins are found in this layout? Answer, 2.

The ends of the metal Arcs that aren’t connected to anything contain Pins.

There would also be Pins at any bends or corners in the metal Arcs.

Below is a zoomed in view of the top Pin (this Pin has been selected).

Also, in the Component menu below the metal1 Pin is circled.

Placing a Pin in a layout is useful for drawing an Arc without first having a Node.

Of course, with this Pin selected, you can RIGHT click somewhere in the layout and a metal1 Arc is drawn to that point.

Please experiment with drawing Arcs from Pins, these Nodes, and to other Arcs.

Remember you can always it Ctrl+Z to undo your work so don’t worry about “mistakes”

Next zoom in on the area seen below.

Let’s increase the width of the metal1 Arc so that it matches the connection to the N-Well resistor.

Select the Arc and hit Ctrl+I (or Edit -> Properties -> Object Properties noting that in my key bindings use Q in place of Ctrl+I)

The following window should appear.

Change the width of the Arc to 4 and hit OK (not the X at the top right of the window which cancels your change).

Note the field “End Extension”

This field specifies how the ends of the Arc are drawn. We’ll talk about the End Extensions in greater detail in the coming tutorials.

After changing the Arc’s width to 4 the layout changes to the following.

Change all of the Arcs in the layout so that they are 4 wide and then fit the layout to the window, as seen below.

Remember to DRC the cell when you are done.

Next, just like we did in the schematic, let’s label the Arcs: vin, vout, and gnd.

To label the Arc we select it and press Ctrl+I (or just double click on the Arc).

Change the name of each Arc (knowing the Arc connecting the two resistors has to have the name vout) as indicated below.

The result is seen below.

The names are circled but we can’t see them!

Zoom in around the Arc name and select it as seen below.

Using Ctrl+click may be very useful here.

Edit the properties of this text, the Arc’s name, by pressing Ctrl+I.

Change the text size from 1 to 10 as seen below.

Change the size of all of the other Arc names resulting in the following.

At this point this cell should be DRC clean (meaning when you press F5 you don’t get any errors). Verify this now.

This layout cell should also match the schematic cell. Verify this by running the NCC (aka LVS check).

Note that the names of the Arcs don’t have to match for the cells to pass NCC. The Arc names are useful for humans but don’t affect circuit operation ;-)

We are ready to simulate the layout view off this cell.

Let’s open up the schematic view of the cell and copy the SPICE code (select, Ctrl+C).

Go back to the layout view and paste the SPICE code as seen below (the SPICE code is circled).

Change the size of the text used in the SPICE code to 10 (you should know how to do this now).

Also, let’s change the Text size of the resistor’s value to 10 so we can see the resistor’s value.

The results are seen below.

Run a DRC, NCC, and a Well Check to ensure that there aren’t any errors (there shouldn’t be)

This cell can be simulated following the same steps used for simulating the schematic view above.

Simulate this cell using LTspice now.

This is the end of the first tutorial.

For your reference the final jelib (Electric library) used in this tutorial is located in tutorial_1.jelib (right click to save to C:\Electric).

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