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what is variable assignment in vhdl

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Assignment Symbol in VHDL

VHDL assignments are used to assign values from one object to another. In VHDL there are two assignment symbols:

Either of these assignment statements can be said out loud as the word “gets”. So for example in the assignment: test <= input_1; You could say out loud, “The signal test gets (assigned the value from) input_1.”

Note that there is an additional symbol used for component instantiations (=>) this is separate from an assignment.

Also note that <= is also a relational operator (less than or equal to). This is syntax dependent. If <= is used in any conditional statement (if, when, until) then it is a relational operator , otherwise it’s an assignment.

One other note about signal initialization: Signal initialization is allowed in most FPGA fabrics using the := VHDL assignment operator. It is good practice to assign all signals in an FPGA to a known-value when the FPGA is initialized. You should avoid using a reset signal to initialize your FPGA , instead use the := signal assignment.

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COMMENTS

  1. Assignment Symbol

    In VHDL there are two assignment symbols: <= Assignment of Signals. := Assignment of Variables and Signal Initialization. Either of these assignment statements can be said out loud as the word “gets”. So for example in the assignment: test <= input_1; You could say out loud, “The signal test gets (assigned the value from) input_1.”.

  2. VHDL Logical Operators and Signal Assignments for

    The VHDL code shown below uses one of the logical operators to implement this basic circuit. and_out <= a and b; Although this code is simple, there are a couple of important concepts to consider. The first of these is the VHDL assignment operator (<=) which must be used for all signals.