Event Control Time Control Statement - 2025.2 English - UG901

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 event (value change or edge) activates an always block if it 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.
• If you substitute an asterisk (*) in the parentheses for a list of signals, you activate the always block on events on any of its signals as described.
• In combinational processes, explicitly assign every signal in all branches of if or case statements. If you fail to do so, 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