Exercise
As an exercise, now that you know how to create Electrical Csets and add the appropriate settings for them, let's set the
DDR_ADDR Electrical Constraint Set.
Once finished, your Electrical CSet will look like the one above.
Set Impedance Targets
The next step in any high-speed design is to define your impedance targets (e.g., 50Ω for single-ended, 100Ω for differential).
To do that:
1. In the Constraint Manager, go to Electrical > Routing > Impedance (as shown below):
2. Set your value under the Target column for the row, DDR3_DATA to 50 Ohms and Tolerance to 15% as most manufac-
turers can accommodate this level of precision. However, choose whatever tolerance your specific manufacturer tells you
to.
3. To set an impedance target for your differential pairs, say, 100 Ohms, you still set the single-ended impedance for now to
half that differential (100/2 = 50 Ohms) and at 15%
Important Note: If possible, avoid using Ohm as the Tolerance. Constraint Manager can take % or Ohm. However, we want it to
automatically calculate the right tolerance for us instead of a hard-set value.
Jitter
If your license allows it, set maximum allowed jitter (e.g., 5-8% of the clock period), depending on the version of the software.
This setting is not available in the version used for this guide.
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OrCAD X High-Speed Digital Design Guide
