Hierarchical Design

This page contains information about creating designs in Mentor Graphic using a hierarchical approach. Hierarchy in circuit design refers to the creation of higher level cells based on lower lever cells. For example, if you create a cell that is composed of inverters and nand gates, you can instantiate (add) your previously designed inverter and nand cells rather than recreating them in the higher level cell. This page provides some commands that are necessary to create hierarchical designs with instantiated cells.

Hierarchy in Schematic Capture (DA)

Hierarchy in Layout (IC)

Hierarchy in Schematic Capture (Design Architect)

Once you launch DA and open a New Sheet for your new higher-level cell
File > Open > Sheet
you can start adding the cells you have previously created. From the schematic_add_route palette, select
Choose Symbol
This opens up a dialog box in which you can select the symbol for the desired lower-level cell. Scroll down to locate the cell/symbol you need to add. Select it and click OK. A ghost image of the symbol will appear and should be dragged and placed where it’s needed in your new cell.

If desired, you can look down into the symbol to see the schematic of the instantiated (added) cell. This action is called 'peeking'. To peek the symbol, select it and press Control+F8.

Hierarchy in Layout (IC Station)

Add Cell:
Any previously-crated cell can be added to the layout of a higher-level cell with the Add Cell function. With the mouse pointer on the layout window, press F5. An ‘add cell’ dialog box pops up and you need to enter the layout cell name, e.g., inv. You must enter the complete path if the cell to be added is in a different directory than your current directory (from which you launched IC). Click OK and you will see a ghost image of the cell with an outline box. Place the cell where you want it by clicking on the layout window. Cells will be placed with their origin (recall the Set Origin command) at the positing where you click to place the cell. Press F2 to unselect the added cell. Notice that only the ports of the cell are visible at this time.

You can moved added cells just like any polygon in IC station and the entire cell will move. To move these cells, select them and from easy edit menu, use ‘move’ function, or use the function keys. As all these basic cells have been assigned pitch match of 50L or 50l, the VDD and VSS/GND rails should align. If you set cell origin (as recommended) at the bottom left corner of GND rail, it’s quite easy to align the power the rails.

Peeking Cells:
To view all of the layers within an added cell, select the cell (the outline and ports are selected) and press control+F6. Press F2 to unselect all. Note that although you can see the lower-level layers, you can not edit them from this cell (there is a way, but for now, assume you can not). To edit a lower-level cell you should open that cell and edit it there. However, be warned that any changes you make in a lower level cell will change anywhere you have added that cell. For this reason, it is highly recommended that you do NOT edit lower-level cell layouts once that layout is completed.

Making Arrays:
If you need to duplicate an added cell over one or two dimensions, first select the added cell. Then, from the main menu select
Objects > Make > Arrays
A dialog box will show up at the bottom of the screen. Enter the # of row and columns that you want the array to be. Click on OK and the new structure will be shown in the layout.