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Null Statement

Used In

  • Process
  • Function
  • Procedure

Reference Manual

VHDL-93:
  • Section 8.12

Syntax

null;

Rules and Examples

The null statement performs no action. It is usually used with the case statement to indicate that under certain conditions, no action is required.

case ENCRYPTION is
    when "00" =>
        CPU_DATA_TMP := (B & A) - OPERAND;
    when "01" =>
        CPU_DATA_TMP := (B & A) + OPERAND;
    when "10" =>
        CPU_DATA_TMP := (A & B) - OPERAND;
    when "11" =>
        CPU_DATA_TMP := (A & B) + OPERAND;
    when others =>
        null;
end case;

The null may have an optional label:

label null;

Synthesis Issues

The null statement is supported by synthesis tools.

Note: using a null statement in a “combinational process” can result in latches being inferred, unless all signals driven by the process are given unconditional default assignments.