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VHDL (VHSIC-HDL, Very High-Speed Integrated Circuit Hardware Description Language) is a hardware description language used in electronic des...

Sunday 9 September 2012

Edge Sensitive Path

Formal Definition

The edge sensitive path is a module path described with an edge transition at the source.

Simplified Syntax


  ([edge_identifier] input_terminal =>

  output_terminal [polarity]:data_source) = delays;

Full_path :

  ([edge_identifier] input_terminal *>

  output_terminal [polarity]:data_source) = delays;

Edge identifier:




  + -


The edge sensitive path is the same as simple module path. The difference is that in the edge sensitive path, the source edge transition is used. The edge identifier can be either aposedge or a negedge. If an input is declared as a vector port then the least significant bit is used to detect the edge transition. If no transition is specified, then the edge sensitive path works as a simple module path (i.e. at any transition that occurs on an input terminal).

The polarity operator describes if data passing from the source to the output is inverted (-) or not inverted (+).

The keywords posedge and negedge can be used both in parallel and in full paths.


Example 1

(posedge clk => (q +: d)) = (3,1);

At a positive edge on a 'clk' signal the value of 'q' will change, using the rising delay of 3 and the falling delay of 1 time unit. The data path travels from 'd' to 'q' and data is not inverted.

Important Notes

· The polarity operator is used by timing analysis tools and is ignored by the simulator.

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