Functions and procedures in VHDL serve to store reusable code. Functions return a single value while procedures can have input, output, and in-out parameters. Functions can only accept constant or signal inputs, while procedures can accept constants, signals, and variables. Functions are used as part of expressions, while procedures are standalone statements. Functions and procedures can be defined in the main code or in libraries and packages.

1. VHDL:
Functions and Procedures
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2. Subprograms
• FUNCTION
• PROCEDURE
• FUNCTIONs and PROCEDUREs have the same
basic purpose
– store commonly used pieces of code, so they can be
reused and shared
• VHDL allow FUNCTIONs and PROCEDUREs to be
overloaded
pieces of sequential VHDL code
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3. PROCESS and FUNCTION vs. PROCESS
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• FUNCTIONs and PROCEDUREs are mainly intended for
LIBRARY allocation
(although it is possible to locate them in the main code)
• PROCESSes are intended for immediate use in the main
code

4. FUNCTION vs. PROCEDURE
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• FUNCTION
– zero or more input parameters and a single return value
– The input parameters can only be constants (default) or
signals (variables are not allowed)
• PROCEDURE
– Any number of IN, OUT, and INOUT parameters which can
be constants, signals, variables
– The default for OUT and INOUT parameters is variable
• A FUNCTION is called as part of an expression, while a
PROCEDURE is a statement on its own

5. FUNCTION/PROCEDURE location
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6. FUNCTION
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7. FUNCTION: Example
Example of Function body:
The word CONSTANT
can be omitted
do not enter RANGE or
TO/DOWNTO
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8. Examples:
FUNCTION
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Function located
in the main
Function located
in a package

9. PROCEDURE
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10. PROCEDURE: example
Example of PROCEDURE body:
std_logic; SIGNAL b,c: IN std_logic;
std_logic_vector(7 downto 0);
NOT an issue with
PROCEDUREs
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11. STD_LOGIC;
Examples:
PROCEDURE
PROCEDURE located
in the main
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STD_LOGIC;
Examples:
PROCEDURE
PROCEDURE located
in a PACKAGE

13. ASSERT
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Example: