DC constraints

  1. niks8285

    1) Though max_fanout and max_capacitance depends on load capacitance why do we need these two constraints separately

    2) Why do we need min_capacitance constraint in DC?

    3) Why do we need slew_threshold ?

    Thanks in advance...!!
  2. kaushikrvs
    1) You need max_fanout and max_capacitance to constrain the design , so that each cell can drive only a particular number of gates. Pleas look at the example below,
    If one of your AND gates is trying to drive 100 other gates, it might work for simulation but it will fail after fabrication. Because, every gate can drive only 10 - 20 other gates(assumption) based on their strength. So when you constrain
    max_fanout and max capacitance in DC, it partitions the entire design to make sure every gate drives only 10 other gates. Hence to drive 100 gates, DC will replace the 1 AND gate with 10 AND gates.

    I am not sure about 2 and 3. Hope that was helpful
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