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Making Incremental Design Changes

After initially placing and routing a design, you often must go back to the schematic and make slight modifications to the original design. When this situation occurs, you can “recycle” much of the place and route information from the previous design iteration, as much of it does not change. This process is known as incremental design, and it uses the NCD file (containing partition, placement, and routing information) from the prior place and route run as the guide file.

Much of the place and route information extracts from the guide file, greatly reducing the place and route time. The reuse of place and route information also results in more stable timing over a number of guided place and route iterations. After a section of your design passes your timing requirements, guided design ensures that it can pass in the future, even if you modify other parts of the design.

In this section of the tutorial, you make a small change to the schematic and reprocess the design using the guide option in the mapping program (MAP) and the place and route program (PAR).

NOTE

A small design change is the addition, removal, or replacement of only a small amount of logic in the design; the exact amount of “small” depends on the size of the design. If you make radical changes to a design, especially changes to existing portions of the design, guiding the design can produce unpredictable results.

Making an Incremental Schematic Change

Make a simple change to the Calc schematic that you can see immediately on the demonstration board. For example, assume you no longer need the reset opcode and you need to remove it from the design. Do so by grounding the `R' pins (inputs to the FDRE and FD4RE macros in the ALU schematic). The MAP program automatically optimizes out of the netlist the logic that generated the original reset signal, and the logic it drove.

Open Workview Office and load the Calc design.

  1. In ViewDraw, open the alu.1 design.

  2. Use the View commands to zoom in on the lower right quadrant of the schematic.

  3. Select the AND5B2 component that generates the GRESET net feeding the FDRE and FD4RE.

  4. Press the Delete key to delete the component.

  5. Connect a ground symbol to the dangling QRESET net. The GND symbol can be found in the XC4000E library. See the figure that follows.

  6. Check and save the schematic.

  7. Exit ViewDraw and return to Workview Office.

Figure 8.57 Grounding the Reset Logic

Translating the Incremental Design

Translate the guided Calc design by turning on the guide options in Flow Engine. The following instructions demonstrate an alternative method of running Flow Engine that offers more control over the implementation flow.

  1. In the Xilinx Design Manager, select calc, then choose Design New Version.

  2. The New Version dialog box appears with the Name field automatically filled in as “ver2.” You can also add a comment to the new version. This comment appears in the project view next to the version number. Click OK.

    NOTE

    You can add a comment to any version or revision in the project view by selecting that version or revision, then selecting Right Mouse Button Properties.

  3. Select the newly created “ver2” in the project view, then select Design New Revision.

  4. The New Revision dialog box appears with the Name field automatically filled in as “rev1” and the Part field automatically filled in as “XC4003E-4-PC84.” You can add a comment to the new revision if you want. Click OK.

  5. Select the newly created “rev1” in the project view, then select Tools Flow Engine. Alternatively, you can click the Flow Engine icon on the Toolbar. See the figure that follows.

    figures/fe_icon.gif

  6. Flow Engine appears; however, unlike the procedure you used in the first revision, the implementation flow does not start automatically. This allows you to step forward and even backward through the implementation flow by individual stages, using the audio-player-like buttons at the bottom of the Flow Engine window, or the selections underneath the Flow menu.

    Select Setup Options from the menu bar. The Options dialog box appears as before.

  7. Go through the different options as before and verify that the settings you gave in the previous revision carried over into this revision.

  8. In the Guide Design field, select Last. This sets the previous revision of the placed and routed design (in this case, the same effect as selecting ver1 rev1).

  9. Click OK to return to Flow Engine.

  10. Run the implementation as before by clicking the “play” button (on the far left) at the bottom on the Flow Engine window.

  11. When all steps complete successfully, select Flow Close to exit Flow Engine.

Verifying the Change in the Demonstration Board

Verify that the change occurred by downloading the new bitstream to the demonstration board, as you did previously. See the Hardware Debugger Reference/User Guide for more information. Before running through this tutorial, make sure you select the ver2 rev1 revision in the project view.

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