Event Control Time Control Statement - 2024.1 English

Vivado Design Suite User Guide: Synthesis (UG901)

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2024.1 English

The following statements describe modeling combinatorial logic with the event control time control statement [@ (at)].

  • A combinatorial always block has a sensitivity list appearing within parentheses after always@.
  • An always block is activated if an event (value change or edge) appears on one of the sensitivity list signals.
  • The sensitivity list can contain:
    • Any signal that appears in conditions, such as if or case.
    • Any signal appearing on the right-hand side of an assignment.
  • By substituting a * (asterisk) in the parentheses for a list of signals, the always block is activated for an event in any of the always block's signals as described.
  • In combinatorial processes, if a signal is not explicitly assigned in all branches of if or case statements, Vivado synthesis generates a latch to hold the last value.
  • The following statements are used in a process:
    • variable and signal assignments
    • if-else statements
    • case statements
    • for-while loop statements
    • function and task calls