μWaveRiders: Setting Up a Successful AWR Design Environment Design - Layout and Component Libraries

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When starting a new design, it's important to take the time to consider design recommendations that prevent problems that can arise later in the design cycle. This two-part compilation of guidelines for starting a new design is the result of years of Cadence AWR Design Environment platform Support experience with designs. Pre-design decisions for user interface, simulation, layout, and library configuration lay the groundwork for a successful and efficient AWR design. This blog, part 2, covers the layout and component library considerations designers should note prior to starting a design.

Layout Considerations

• Determining Database Resolution: The database resolution unit is important when setting up a process. This step is automated for designers using Process Development Kits (PDKs). A database unit is defined as the smallest unit of precision for a layout. All layout coordinates in the AWR Design Environment platform are stored with 1nm resolution. However, most processes cannot resolve 1nm geometry, so the database resolution allows you to match the layout precision with your process precision. To set the database resolution for a process, choose Options > Layout Options to open the Layout Options dialog box Layout tab. Under Grid Options, select an LPF from Grid to modify and then click Edit Grid to display the Options - LPF dialog box. Click Units to specify the Length > Multiplier for the Database Size. Next, click Grids to specify the Database Size > Spacing.
  

• Setting Snapping Options: Snapping functions connect layout cell faces in different ways. To set snapping options, choose Options > Layout Options to open the Layout Options dialog box Layout tab. Under Layout Cell Snap Options, select the appropriate Snap together option. Using manual snapping rather than automatic snapping for complex layouts is recommended.

- Auto snap on parameter changes: Automatically performs a snap together operation after a parameter that affects layout is changed. This mode is good for simple layouts but can cause problems with more complex circuits, especially those with closed loops. When using this mode, you typically need to use anchors and the freeze function to help control your layout snapping. To access these settings, right-click a layout object and choose Shape Properties to open the Cell Options - Layout dialog box. Snapping commands start with anchored objects, and do not move frozen objects.

- Manual snap for all objects: Snaps together all objects in a layout only when you choose Edit > Snap Objects > Snap Together. This mode is more controlled than auto snap because you decide when to snap. The same problems arise with complex layouts, however, especially closed loops. When using this mode, you typically need to use anchors and the freeze function to help control your layout snapping. To access these settings, see above.

- Manual snap for selected objects only: Snaps together selected objects in a layout only when you choose Edit > Snap Objects > Snap Together. This mode provides the most control, as not all layout objects are snapped together, only those you select.

• Inset Faces: In some designs, connections between cells are made on faces rather than pins. If layout options are not fully understood, gaps can be created in layout when rotating cells. Because the layout system stores shapes on a grid, there is shape point rounding when shapes are rotated at other than 0-, 90-, 180-, or 270-degrees. This rounding can cause very small gaps in the layout. The Auto Face Inset feature fixes this issue by auto-insetting the connection faces to create an overlap at connection points. After selecting an initial value, you should not change it, to prevent needing to reconnect the entire layout. To set the auto face inset, choose Options > Layout Options to open the Layout Options dialog box Layout tab. Under Auto face inset, select the appropriate option.
  

• Selecting Cell Library Properties: Familiarity with Cell Library Properties dialog box options such as the When to save cells with the project? option is important. To access this option, in the Layout Manager, right-click a library name and choose Properties to open the Cell Library Properties dialog box.
  
      - Always is the only option available if you create a library in a project. It saves all library cells (used and unused) in the project.

- In Use is the default if you import a library into your project. Only library cells used in schematic layouts in the project are saved. If the linked library is accessible, all cells from that library are available for use.

- Never does not save cells in the project, and is useful when using cells that are included in a PDK, ensuring that you always have the latest cells available from the PDK.

• Exporting your Design: It is important to understand Layout Options if you plan to export your layout. You do not need to select these options at the beginning of a project, however, you should consider them prior to tapeout.

To set layout export options, choose Options > Layout Options to open the Layout Options dialog box Export/LPF tab. Under Layout Export Options, select the appropriate combination of options as follows:

- Union layout shapes and Subcircuits as instances are not selected: Subcircuit layout instances are flattened, but artwork cells are not, and nothing is unioned.

- Union layout shapes is selected, Subcircuits as instances is not selected: Subcircuit layout instances are flattened, but artwork cells are not. Schematic layout instances (except for artwork cells) are unioned, and only objects within each artwork cell are unioned, individually.

- Union layout shapes is not selected, Subcircuits as instances is selected: Subcircuits and artwork cells are exported as instances (not flattened).

- Union layout shapes and Subcircuits as instances are selected: Subcircuits and artwork cells are exported as instances (not flattened), however only elements within a subcircuit are unioned.

Select the appropriate Artwork instance export options to address potential duplicate cell names:

- Do not change cell names: When there are multiple same-name cells from different libraries, only one of the instances is written to the GDSII file.

- Append number to duplicates: Allows multiple same-name cells to be written to a single GDSII file. Previously written cell names are appended with "_n", where n is a sequential number.

- Append lib name to duplicates: Allows multiple same-name cells to be written to a single GDSII file. Previously written cell names are appended with "_library" where library is the origin library name of the cell instance.

- Append lib name to all: Allows multiple same-name cells to be written to a single GDSII file. All cell names are appended with "_library" where library is the origin library name of the cell instance.

Select the appropriate Layout cell export options for any object drawn from a parameterized cell (PCell):

- Export flattened: Exports PCells flat.

- Export all as instances: Exports all PCells as instances, as separate GDSII or DXF cells for each unique PCell instance.

- Export specified as instances: Exports only the PCells selected after clicking the Select cells to export as instance button.

Select or clear the Write layout cell parameter map option to write a text file to the exported artwork location. The text file maps individual PCells and the instance they use:

Component Library Considerations

• Determining library creation and management: Decide on a vendor library management approach if you augment the components available in the AWR Design Environment platform. Creation of PCB designs typically requires the purchase and mounting of vendor components such as models, layout cells, and symbols to properly create a design. The AWR Design Environment platform includes a library of vendor components in the Elements Browser. This data is collected from vendor websites and should be verified as current by the vendor before use. 
  

Related Resources

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• μWaveRiders: Cadence AWR Design Environment/Project Browser Tips & Tricks
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Product Specs

• AWR Design Environment Platform

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