Procedure with signal parameters in VHDL

Introduction

A procedure with signal parameters lets you change signals inside a reusable block of code. This helps organize your design and avoid repeating code.

When you want to update signals in multiple places with the same logic.

When you want to group signal assignments to keep your code clean.

When you want to pass signals to a procedure to modify them directly.

When you want to reuse signal-changing code without copying it.

When you want to separate complex signal updates into smaller steps.

Syntax

VHDL

procedure procedure_name(signal param1 : inout std_logic; signal param2 : out std_logic) is
begin
  -- statements that assign to param1, param2
end procedure;

Signal parameters must be declared with the keyword signal before the parameter name.

Parameter modes can be in, out, or inout. Use inout if you want to read and write the signal.

Examples

This procedure toggles a signal between '0' and '1'.

VHDL

procedure toggle_signal(signal s : inout std_logic) is
begin
  if s = '0' then
    s <= '1';
  else
    s <= '0';
  end if;
end procedure;

This procedure sets a signal to '1'. It uses out mode because it only writes to the signal.

VHDL

procedure set_high(signal s : out std_logic) is
begin
  s <= '1';
end procedure;

Sample Program

This VHDL design toggles a signal each clock cycle using a procedure with a signal parameter. The procedure changes internal_sig, and the output port reflects its value.

VHDL

library ieee;
use ieee.std_logic_1164.all;

entity proc_signal_example is
  port(
    clk : in std_logic;
    out_sig : out std_logic
  );
end entity;

architecture behavior of proc_signal_example is
  signal internal_sig : std_logic := '0';

  procedure toggle_signal(signal s : inout std_logic) is
  begin
    if s = '0' then
      s <= '1';
    else
      s <= '0';
    end if;
  end procedure;

begin
  process(clk)
  begin
    if rising_edge(clk) then
      toggle_signal(internal_sig);
      out_sig <= internal_sig;
    end if;
  end process;
end architecture;

Important Notes

Procedures with signal parameters allow direct signal updates inside the procedure.

Remember that signal assignments inside procedures take effect after the process suspends (delta cycle).

Use procedures to keep your VHDL code modular and easier to read.

Summary

Procedures with signal parameters let you change signals inside reusable code blocks.

Use signal keyword and in, out, or inout modes for parameters.

This helps organize and reuse signal-changing logic in your VHDL designs.

Practice

1. What is the main purpose of using a procedure with signal parameters in VHDL?

easy

A. To allow the procedure to directly read and modify signals inside a process

B. To declare variables local to the procedure

C. To create constants that cannot be changed

D. To define new data types

Solution

1. Step 1: Understand signal parameters in procedures

   Signal parameters allow procedures to access and modify signals directly, unlike variables which are local copies.

2. Step 2: Identify the purpose of procedures with signal parameters

   They help reuse code that changes signals inside processes, making designs cleaner and modular.

3. Final Answer:

   To allow the procedure to directly read and modify signals inside a process -> Option A

4. Quick Check:

   Signal parameters modify signals directly [OK]

Hint: Signal parameters let procedures change signals directly [OK]

Common Mistakes:

• Confusing signal parameters with variables
• Thinking procedures cannot modify signals
• Assuming procedures only read signals

2. Which of the following is the correct way to declare a procedure with an out signal parameter in VHDL?

easy

A. procedure update_signal(constant s : out std_logic) is

B. procedure update_signal(variable s : out std_logic) is

C. procedure update_signal(signal s : out std_logic) is

D. procedure update_signal(signal s : in std_logic) is

Solution

1. Step 1: Recall signal parameter syntax

   Signal parameters must be declared with the keyword signal and direction in, out, or inout.

2. Step 2: Identify correct declaration

   procedure update_signal(signal s : out std_logic) is correctly declares a procedure with an out signal parameter. The others use incorrect keywords like constant or variable, or wrong direction in.

3. Final Answer:

   procedure update_signal(signal s : out std_logic) is -> Option C

4. Quick Check:

   Signal parameters use 'signal' keyword and direction [OK]

Hint: Use 'signal' keyword with direction for signal parameters [OK]

Common Mistakes:

• Using 'variable' instead of 'signal' for signal parameters
• Omitting the direction (in/out/inout)
• Using 'constant' keyword incorrectly

3. Consider the following VHDL code snippet inside a process:

signal a : std_logic := '0';

procedure toggle_signal(signal s : inout std_logic) is
begin
  if s = '0' then
    s <= '1';
  else
    s <= '0';
  end if;
end procedure;

begin
  toggle_signal(a);
end process;

What will be the value of signal a after the process runs once?

medium

A. Signal assignment error

B. '0'

C. Undefined

D. '1'

Solution

1. Step 1: Analyze initial value and procedure logic

   Signal a starts at '0'. The procedure toggles it: if '0', assign '1'; else assign '0'.

2. Step 2: Apply procedure to signal a

   Since a is '0', the procedure sets it to '1'.

3. Final Answer:

   '1' -> Option D

4. Quick Check:

   Toggle '0' to '1' [OK]

Hint: Toggle flips '0' to '1' and vice versa [OK]

Common Mistakes:

• Confusing signal assignment timing (<= is scheduled)
• Assuming value stays '0' after toggle
• Thinking procedure cannot modify signals

4. Identify the error in the following VHDL procedure declaration:

procedure set_high(signal s : in std_logic) is
begin
  s <= '1';
end procedure;

medium

A. Missing variable declaration

B. Signal parameter cannot be declared as in if it is assigned inside

C. Procedure must return a value

D. Signal parameters cannot be used in procedures

Solution

1. Step 1: Check signal parameter direction

   The parameter s is declared as in, meaning it cannot be assigned a new value inside the procedure.

2. Step 2: Identify assignment conflict

   The procedure tries to assign s <= '1';, which is illegal for in signals. It should be out or inout.

3. Final Answer:

   Signal parameter cannot be declared as in if it is assigned inside -> Option B

4. Quick Check:

   Only out or inout signals can be assigned [OK]

Hint: Assign only to out or inout signal parameters [OK]

Common Mistakes:

• Assigning to an 'in' signal parameter
• Thinking procedures must return values
• Confusing variables and signals

5. You want to create a procedure that increments a 4-bit signal counter by 1 each time it is called. Which of the following procedure declarations and bodies correctly achieve this?

hard

A. procedure increment_counter(signal cnt : inout std_logic_vector(3 downto 0)) is
begin
cnt <= std_logic_vector(unsigned(cnt) + 1);
end procedure;

B. procedure increment_counter(signal cnt : in std_logic_vector(3 downto 0)) is
begin
cnt <= std_logic_vector(unsigned(cnt) + 1);
end procedure;

C. procedure increment_counter(variable cnt : inout std_logic_vector(3 downto 0)) is
begin
cnt := std_logic_vector(unsigned(cnt) + 1);
end procedure;

D. procedure increment_counter(signal cnt : out std_logic_vector(3 downto 0)) is
begin
cnt <= cnt + 1;
end procedure;

Solution

1. Step 1: Check parameter declaration

   The signal parameter must be declared as inout to read and write the signal inside the procedure.

2. Step 2: Verify increment logic and syntax

   procedure increment_counter(signal cnt : inout std_logic_vector(3 downto 0)) is begin cnt <= std_logic_vector(unsigned(cnt) + 1); end procedure; correctly converts cnt to unsigned, adds 1, then converts back to std_logic_vector and assigns with <=.

3. Step 3: Identify errors in other options

   The in version cannot assign to the signal, the variable version is not for signals, the out version lacks type conversion and reads uninitialized value.

4. Final Answer:

   procedure increment_counter(signal cnt : inout std_logic_vector(3 downto 0)) is begin cnt <= std_logic_vector(unsigned(cnt) + 1); end procedure; -> Option A

5. Quick Check:

   Use inout signal and proper type conversion [OK]

Hint: Use inout signal and convert types for arithmetic [OK]

Common Mistakes:

• Using 'in' direction when assigning signals
• Confusing variables and signals in procedures
• Skipping type conversion for arithmetic