Procedure with signal parameters in VHDL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a procedure with a signal parameter.

VHDL

procedure set_signal(signal sig : out std_logic) is
begin
  sig <= [1];
end procedure;

Drag options to blanks, or click blank then click option'

A'1'

B'Z'

C'0'

D'X'

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of '<=' for signal assignment.

Assigning a value without single quotes.

Using a variable assignment style.

The procedure assigns the value '1' to the signal parameter sig.

2fill in blank

medium

Complete the procedure call to pass a signal named 'clk' as an argument.

VHDL

begin
  set_signal([1] => clk);
end;

Drag options to blanks, or click blank then click option'

Asig :

Bsig =>

Dsig

Attempts:

3 left

💡 Hint

Common Mistakes

Using ':' instead of '=>'.

Omitting the parameter name.

Using '=' instead of '=>'.

The correct syntax for named association in VHDL is 'sig => clk'.

3fill in blank

hard

Fix the error in the procedure header to correctly declare a signal parameter named 'data'.

VHDL

procedure update_data([1] : in std_logic_vector(7 downto 0)) is
begin
  -- procedure body
end procedure;

Drag options to blanks, or click blank then click option'

Asignal data

Bdata signal

Cdata variable

Dvariable data

Attempts:

3 left

💡 Hint

Common Mistakes

Placing 'signal' after the parameter name.

Using 'variable' instead of 'signal'.

Omitting the 'signal' keyword.

In VHDL, signal parameters are declared with 'signal' before the parameter name.

4fill in blank

hard

Fill both blanks to complete the procedure that sets a signal 'out_sig' to '0'.

VHDL

procedure reset_signal(signal [1] : out std_logic) is
begin
  [2] <= '0';
end procedure;

Drag options to blanks, or click blank then click option'

Aout_sig

Cin_sig

Dsignal_out

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names in the header and body.

Using a variable name instead of the signal parameter.

Omitting the 'signal' keyword in the header.

The parameter name and the signal used inside the procedure must match.

5fill in blank

hard

Fill all three blanks to create a procedure that toggles a signal 'sig' from '0' to '1'.

VHDL

procedure toggle_signal(signal [1] : inout std_logic) is
begin
  if [2] = '0' then
    [3] <= '1';
  else
    [3] <= '0';
  end if;
end procedure;

Drag options to blanks, or click blank then click option'

Asig

Dsignal

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the signal parameter and inside the procedure.

Using a variable instead of a signal parameter.

Omitting the 'signal' keyword in the parameter declaration.

The signal parameter name must be consistent in the header and body for reading and assigning.

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