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

Saturday 29 December 2012


Formal Definition

A Boolean-valued expression associated with a block statement that controls assignments to guarded signals within a block. A guard expression defines an implicit signal GUARD that may be used to control the operation of certain statements within the block.

Simplified Syntax

some_signal_in_a_block <= guarded expression;


The characteristic feature of the block statement is the guard expression. It is a logical expression of the Boolean type, declared implicitly after the reserved word block whenever a guarded expression appears inside the block (Example 1).

The guard expression implies a signal named 'guard' at the beginning of the block declaration part. This signal can be read as any other signal inside the block statement but no assignment statement cant update it. This signal is visible only within the given block. Whenever a transaction occurs on any of the signals on the right hand side of the guardexpression, the expression is evaluated and the 'guard' signal is immediately updated. The 'guard' signal takes on the TRUE value when the value of the guard expression is true. Otherwise, 'guard' takes on the FALSE value.

The 'guard' signal may also be declared explicitly as a Boolean signal in the block statement. The advantage of this approach is that more complex (than a simple Boolean expression) algorithm to control the guard signal can be used. In particular, a separate process (Example 2) can drive the guard signal.

If there is no guard expression and the guard signal is not declared explicitly, then by default the guard signal is always true.

The guard signal is used to control so called guarded concurrent signal assignment statements contained inside the block. Each such statement contains the reserved word guardplaced after the symbol "<=". They assign a new value to the signal only when the guard signal is true. Otherwise, the signal assignment statement does not change the value of the given signal. In Example 1, the signal OUT_1 will take on the value of not IN_1 only when the value of the expression CLS'EVENT and CLK='1' will be true.


Example 1

RISING_EDGE : block (CLK'EVENT and CLK='1')
  OUT_1 <= guarded not IN_1 after 5 ns;
end block RISING_EDGE;

The assignment to the signal OUT_1 is guarded, which introduces the implicit GUARD signal into the block.

Example 2

ALU : block
signal GUARD: Boolean := False;
  OUT_1 <= guarded not IN_1 after 5 ns;
  P_1: process
      GUARD <= True;
    end process P_1;
end block ALU;

Signal GUARD is declared explicitly and can be assigned value like any other signal.

Important Notes

  • Guarded blocks are usually not supported for synthesis.

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