[print this page]

Introduction: Cell Abstract Generation

The next thing we need to do is convert our cell layouts to cell abstracts. These abstracts will be used by First Encounter for place-and-route.

Cell abstracts will be stored in two locations. The first is within a LEF (Library Exchange Format) file, which combines abstract versions of all your cell layouts into a single file. The abstracts are stored as abstract views in the Cadence IC-Tools library.

An abstract is essentially a cell layout without the silicon (active) layers. In other words, the abstract version of a cell contains the cell geometry (X and Y), internal route obstructs (internal metal and via polygons), and I/O pin locations (X and Y).

In order to create the abstracts, we'll need to generate GDS versions of our cells and read them into a tool called "Abtract Generator" to convert the GDS versions of the layouts into abstracts.

There are two problems that we must deal with. First, the version of Abstract Generator that is included in the latest version of Cadence IC-tools, was recently redesigned such that it is more tightly integrated with the IC-tools environment. This would theoretically allow seamless operation with the Cadence environment that we're now accoustomed to.

However, Abstract Generator requires detailed information concerning our fabrication process, which includes layer information as well as spacing rules. Although the NCSU CDK does contain layer information in its Cadence technology library, it doesn't contain spacing rules here. The spacing rules are only contained within the Diva DRC file, which itself is not a part of the Cadence technology library (Diva is a separate tool).

As a result of this problem, we cannot use the version of Abstract Generator that's included with this version of the Cadence IC-tools. We'll need to turn to an older version which runs completely independent from the IC-tools framework. However, this brings up two additional problems.

Abstract Generator

First, this older version of Abstract Generator was never ported to Linux, so we'll have to use the Solaris version (Cadence package DSMSE54USR14). Okay, that's not a big deal.

Second, this version of Abstract Generator only accepts technology information from a file format called "tech.dpux". The NCSU CDK doesn't include this file. However, Abstract Generator does include a tool to convert LEF files containing technology information to a tech.dpux file (LEF files may contain technology information as well as cell abstracts, which are called "macros" when in a LEF file).

Okay, but the NCSU CDK doesn't contain a LEF technology file either! The good news is that LEF files are simple, human-readable, easy-to-write text files. Therefore, it is entirely possible to manually type in the technology information in LEF-format. This involves defining layers, vias, and spacing rules.

Fortunately, we don't have to do this. Dr. James Stine's research group at Oklahoma State University has already done this for the AMI C5N process. I have obtained their LEF file and made some changes to it that will allow us to use it for our purposes.

Okay, still with me?

Exporting GDS

The first thing we should do is create a new directory to do our abstract work. Create a new subdirectory under your Cadence work directory called "abstract_work".

We need to export our cell layouts from Cadence in a standard layout file format called GDS. Although the GDS file format is a widely-used industry standard format for exchanging layout, the format has several annoying disadvantages.

First, GDS files are binary files and thus cannot be viewed or edited with a text editor. Second, GDS files contain raw layout only, without any technology information. In fact, GDS files don't even contain layer names!

GDS files associate layout polygons with layer NUMBERS instead of layer NAMES. Since we use layer names when we design our cells, we need a "mapping file" that maps the layer names with layer numbers. This file is normally part of a process design kit, and there is one included in the NCSU CDK. However, their GDS mapping file is incomplete because it doesn't contain non-fabrication layers, including (most importantly) the poly and metal PIN NAME layers.

Luckily, Dr. Stine's group has also written a good GDS map file for the AMI C5N process. Let's start out by copying the GDS map file and the techology LEF file to your abstract work directory. These files are called "ami06_layers.map" and "ami06.lef", and are both contained in "~jbakos/cds_work/abstract_work".

Now, back in your Cadence CIW window, select File | Export | Stream. ("Stream" is another word for "GDS".)

Click the "Library Browser" button to select the layout view of one of your cells.

Set the output file such that the path is your abstract work directory and the filename is in the format ".gds".

Now click the "User-Defined Data" button. In this window, specify the GDS layer map file and click OK.

Now click Options. In this window, turn on "Convert PCells to Geometry" and click OK.

Now click Apply on the main "Stream Out" window. This will perform the export. If you didn't get any errors, you should see a window like the following.

Repeat this procedure for each one of your layout cells. Do this for all of the cells you characterized with SignalStorm with the addition of the FILL cell. Note: in this tutorial, I also exported my non-pull tristate despite the fact that this was unnecessary as it will not be used in the synthesized netlist.

As you're converting, remember to change the GDS output filename immediately after you change or browse-to each cell name. The GDS output filename will not change automatically when you change the input cell name.

The goal here is to have a GDS file for each of your cells.

Using Abstract Generator

Now we need to start up Abstract Generator. Log in to a Solaris machine, change to your abstract work directory, and startup the tool by typing "abstract".

The first thing you'll see is the following window. Click OK.

You will now see the main Abstract Generator window as well as a log window.

Creating Technology File

Select File | Technology. You will see the following window.

Select File | Load Lef File. Double-click "ami06.lef". Once the technology infomation is read from the LEF file, let's create a new Abstract Generator library.

Replace the default "hldLib" library with "usc_scells" and set the path field to the path of your usc_scells library, but append "_ab" to the end of the path. For example, I used "/acct/f1/jbakos/cds_work/usc_scells_ab" for my path.

We don't want to give Abstract Generator the path to the "real" usc_scells library. If you do, Abstract Generator will do some very unfavorable things to your design data.

Now select the "Layers" section and go to the "Mapping" tab.

You'll notice here that our layers are defined under the "Layer" and "Purpose" column, but the "Gds Number" column is all zeros. The layer names came from the LEF technology file. We need to read in our GDS mapping file in order to associate each of the AMI C5N layer names with GDS layer numbers.

Click the "Map" button. A file open dialog will appear. Double-click the "ami06_layers.map" file. Once you do this, the GDS layer numbers will be shown for each layer name. You'll also notice additional layers that were added, corresponding to the layers specified in the map file that weren't present in the LEF file. These layers include pin layers.

Select the "Grid" section. Make sure the Manufacturing Grid is set to .15 (half-lambda).

Set the pitch to be 4.5 and offset to be 2.25 for all three metal layers.

Now do a File | Save. When asked, save your new technology file as "tech.dpux". After this, do a File | Close.

Reading GDS Cell Layouts

Now we need to read in our GDS files. First, let's open our library.

Select File | Library.

Next, select File | Import | Stream (GDSII). For each of your cells, click Browse and double-click the corresponding GDS file name. Once you've added all the GDS files for your cells, click OK.

Once you do this, your cells should be read into Abstract Generator.

There are three steps to complete in order to generate our abstracts: generating the pins view the extract view, and the abstract view.

Pins Step

Let's perform the pins step. Select all of your cells and select Flow | Pins. Fill in the next window as shown and click Run.

Your main Abstract Generator window should look as shown. If you received any errors or warnings (shown as 'X'- or '!'-symbols instead of the check-marks), select Cells | Report to view the report for each cell.

The FILL cell has a warning sign (!) because it doesn't have any input or output terminals. This is expected and thus not a problem.

Extract Step

Now let's perform the extract step. Make sure all the cells are selected and select Flow | Extract. The default settings will work. Click Run.

Now your main Abstract Generator window should look as shown. If you received any errors or warnings, select Cells | Report to view the report.

Abstract Step

Now let's perform the abstract step. Make sure all the cells are selected and select Flow | Abstract. In the new window, click the Site tab and enter "core" under "Define new site name:".

Now go to the Grids tab. Turn on "Save routing grids to technology file". Enter the routing grids, which is 4.5 pitch and 2.25 offset for both metal layers.

Click Run. This step will take some time to complete. Afterwards, your main Abstract Generator window should look as shown. If you received any errors or warnings, select Cells | Report to view the report.

Defining Symmetry

As a last step, make sure all your cells are selected. Select Cells | Cell Properties. Change "class" to "symmetry" and change "R0" to "X Y". Click Apply then OK.

You may also want to check out Cells | Terminal Properties. This will show you all your cell I/O terminals with their properties. However, there's nothing here you need to change.

Exporting Combined Technology/Abstract LEF File

Finally, select File | Export | LEF. Change the LEF filename to "usc_scells.lef" and click OK.

Your new LEF file contains both technology information as well as the abstracts (macros) for each of your standard cells. You may now exit Abstract Generator.

Creating Abstracts-Only LEF File

This LEF file is used for First Encounter, but we also need to import the abstracts into the Cadence IC-Tools. To do this, open usc_scells.lef into a text editor. Delete all the text between the following line... 

		MANUFACTURINGGRID 0.15 ;

...and the first line that begins with "MACRO". This effectively removes the technology information from the LEF file, leaving only the cell abstracts (macros).

Save the resulting file as "usc_scells_abstract.lef".

Importing Abstracts into Cadence IC-Tools

Back in the Cadence IC-Tools CIW, select File | Import | LEF. Type in the path for your abstracts-only LEF file ("usc_scells_abtsract.lef"), then type the name of the target library ("usc_scells"). Click OK.

You will see the following warnings in your CIW window. It is okay to ignore them.

Note that this import procedure will not overwrite (or even modify) existing abstract views. Therefore, if you are performing this step after you've already imported your abstracts (for the purpose of correcting mistakes), you'll first need to delete all your existing abstract views one at a time using the Library Manager.

You will now see abstract views for each of your cells. As an example, open the abstract view for the INVX1 cell. You will see the cell border, pins, and metal1 route obstructs.

My DFFX1 cell also shows examples of metal2 route obstructs.

Fixing the Combined Technology/Abstract LEF File (usc_scells.lef)

There is at least one manual change that we must make to our combined technology/abstract LEF ("usc_scells.lef") file.

For some reason, Abstract Generator will not write the routing grid offset for the metal layers into the combined technology/abstract LEF file. We must enter this information manually or the place-and-route tool will refuse to route.

Open the combined LEF file ("usc_scells.lef") in a text editor. In the metal1 layer, metal2 layer, and metal3 layer LAYER section, insert the following line: 

		OFFSET	2.25 ;

Here's an example metal1 section. As you can see, I inserted the line after the PITCH line. 

		LAYER metal1
		  TYPE		ROUTING ;
		  DIRECTION	HORIZONTAL ;
		  PITCH		4.5  ;
		  OFFSET	2.25 ;
		  WIDTH		0.9 ;
		  SPACING	0.9 ;
		  RESISTANCE	RPERSQ 0.09 ;
		  CAPACITANCE	CPERSQDIST 3.2e-05 ;
		END metal1

Also, make sure all of your clock inputs (CLK) are CAPITALIZED. If not, you can open the combined technology/abstract LEF file and replace "clk" with "CLK", although you should make this change in the original layout and repeat the entire Abstract Generator procedure.

That's all!