Sunday, 11 October 2015

Timing Exceptions

Timing exceptions are nothing but constraints which don’t follow the default when doing timing analysis. The different kinds of timing exceptions are

1. False path: If any path does not affect the output and does not contribute to the delay of the circuit then that path is called false path.

2. Multi-cycle Path Multicycle paths in a design are the paths that require more than one clock cycle. Therefore they require special Multicycle setup and hold-time calculations

3. Min/Max Path : This path must match a delay constraint that matches a specific value. It is not an integer like the multicycle path. For example:Delay from one point to another max: 1.67ns; min: 1.87ns

4. Disabled Timing Arcs : The input to the output arc in a gate is disabled.
 For e.g. 3 input and gate (a, b, c) and output (out). If you want you can disable the path from input ‘a’ to output ‘out’ using disable timing arc constraint.

Clock Path:

Please check the following figure:


In the above fig its very clear that for clock path the starts from the input port/pin of the design which is specific for the Clock input and the end point is the clock pin of a sequential element. In between the Start point and the end point there may be lots of Buffers/Inverters/clock divider.

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