Stimulus process with wait statements in VHDL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to wait for 10 ns before changing the signal.

VHDL

process
begin
    signal_a <= '1';
    wait for [1];
    signal_a <= '0';
    wait;
end process;

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

A10 s

B10 ns

C10 ms

D10 ps

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect time units like seconds or milliseconds which are too large for typical signal delays.

Omitting the time unit after the number.

The wait for statement pauses the process for the specified time. Here, 10 ns is the correct time unit for typical VHDL simulation delays.

2fill in blank

medium

Complete the code to wait until the rising edge of the clock signal.

VHDL

process
begin
    wait until [1] = '1' and [1]'event;
    signal_b <= not signal_b;
end process;

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

Aclock

Breset

Cenable

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Waiting on the wrong signal instead of the clock.

Using wait for instead of wait until for edge detection.

The wait until statement waits for a condition. To wait for the rising edge of the clock, we check when clock = '1' and clock'event is true.

3fill in blank

hard

Fix the error in the wait statement to correctly wait for 20 ns.

VHDL

process
begin
    signal_c <= '1';
    wait for [1];
    signal_c <= '0';
    wait;
end process;

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

A20 ns

B20

C'20 ns'

D20ns

Attempts:

3 left

💡 Hint

Common Mistakes

Writing the time value as a string with quotes.

Omitting the space between number and unit.

The wait for statement requires a number followed by a space and a time unit without quotes. So wait for 20 ns; is correct.

4fill in blank

hard

Fill both blanks to create a process that waits for 5 ns, sets signal_d to '1', then waits for 10 ns.

VHDL

process
begin
    wait for [1];
    signal_d <= '1';
    wait for [2];
    signal_d <= '0';
    wait;
end process;

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

A5 ns

B10 ns

C15 ns

D20 ns

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the wait times.

Using incorrect time units.

The process first waits 5 ns, sets the signal, then waits 10 ns before resetting it. So the blanks are filled with 5 ns and 10 ns.

5fill in blank

hard

Fill all three blanks to create a process that waits for a rising edge of clk, sets signal_e to '1', then waits for 15 ns.

VHDL

process
begin
    wait until [1] = '1' and [2]'event;
    signal_e <= '1';
    wait for [3];
    signal_e <= '0';
    wait;
end process;

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

Aclk

Bclock

C15 ns

Dclk_signal

Attempts:

3 left

💡 Hint

Common Mistakes

Using different signals for the clock in the wait until condition.

Incorrect time unit or missing space in the wait for statement.

The process waits for the rising edge of clk (so both blanks are clk), then waits for 15 ns before resetting the signal.

Practice

1. What is the main purpose of using wait for statements inside a stimulus process in VHDL?

easy

A. To declare variables inside the process

B. To pause the process and apply inputs step-by-step over time

C. To define the clock period for the design

D. To reset all signals to zero immediately

Solution

1. Step 1: Understand the role of stimulus processes

   Stimulus processes apply test inputs to signals during simulation.

2. Step 2: Identify the function of wait for

   The wait for statement pauses the process for a specified time, allowing signals to hold values before changing again.

3. Final Answer:

   To pause the process and apply inputs step-by-step over time -> Option B

4. Quick Check:

   Stimulus process uses wait for to pause and apply inputs [OK]

Hint: Wait statements pause input changes to simulate time steps [OK]

Common Mistakes:

• Thinking wait for resets signals
• Confusing wait for with variable declaration
• Assuming wait for defines clock period

2. Which of the following is the correct way to end a stimulus process in VHDL to stop it from running indefinitely?

easy

A. wait;

B. stop;

C. end process;

D. wait for 10 ns;

Solution

1. Step 1: Review how stimulus processes terminate

   Stimulus processes often run indefinitely unless explicitly stopped.

2. Step 2: Identify the correct wait statement to stop the process

   The statement wait; causes the process to suspend forever, effectively stopping it.

3. Final Answer:

   wait; -> Option A

4. Quick Check:

   Use wait; to stop stimulus process [OK]

Hint: Use plain wait; to stop stimulus process execution [OK]

Common Mistakes:

• Using wait for with a time instead of wait;
• Trying to use end process; as a stop command
• Using stop; which is not valid VHDL

3. Consider the following VHDL stimulus process snippet:

process
  begin
    signal_a <= '0';
    wait for 10 ns;
    signal_a <= '1';
    wait for 20 ns;
    wait;
end process;

What is the value of signal_a at 25 ns after simulation start?

medium

A. '0'

B. Undefined

C. '1'

D. Signal does not change

Solution

1. Step 1: Trace signal assignments over time

   At 0 ns, signal_a is set to '0'. After 10 ns, it changes to '1'.

2. Step 2: Determine signal value at 25 ns

   At 25 ns, which is after the 10 ns and before the 30 ns (10+20), signal_a remains '1'.

3. Final Answer:

   '1' -> Option C

4. Quick Check:

   Signal changes at 10 ns to '1', stays until 30 ns [OK]

Hint: Add wait times cumulatively to find signal value at given time [OK]

Common Mistakes:

• Assuming signal_a is still '0' at 25 ns
• Ignoring the second wait for 20 ns
• Confusing wait; with wait for

4. Identify the error in the following stimulus process code:

process
begin
  signal_b <= '1';
  wait for 5 ns
  signal_b <= '0';
  wait for 5 ns;
  wait;
end process;

medium

A. Missing semicolon after first wait for statement

B. Signal assignment syntax is incorrect

C. wait; statement is misplaced

D. Process missing sensitivity list

Solution

1. Step 1: Check syntax of wait statements

   The first wait for 5 ns is missing a semicolon at the end.

2. Step 2: Confirm other syntax elements

   Signal assignments and the final wait; are correct. Sensitivity list is not required in a process with wait statements.

3. Final Answer:

   Missing semicolon after first wait for statement -> Option A

4. Quick Check:

   Every wait for must end with semicolon [OK]

Hint: Check semicolons after wait for statements carefully [OK]

Common Mistakes:

• Thinking sensitivity list is required with wait
• Misreading signal assignment syntax
• Assuming wait; must be last line only

5. You want to create a stimulus process that toggles clk every 15 ns and stops after 90 ns. Which code snippet correctly implements this behavior?

hard

A. process
begin
clk <= '0';
wait for 15 ns;
clk <= '1';
wait for 15 ns;
wait;
end process;

B. process
begin
clk <= '0';
wait for 90 ns;
clk <= '1';
wait;
end process;

C. process
begin
for i in 0 to 5 loop
clk <= '0';
wait for 15 ns;
clk <= '1';
wait for 15 ns;
end loop;
wait;
end process;

D. process
variable count : integer := 0;
begin
while count < 6 loop
clk <= not clk;
wait for 15 ns;
count := count + 1;
end loop;
wait;
end process;

Solution

1. Step 1: Understand toggling every 15 ns for 90 ns

   Toggling means changing clk from 0 to 1 or 1 to 0 every 15 ns. For 90 ns total, toggling happens 6 times (90/15).

2. Step 2: Analyze each option's logic

   process variable count : integer := 0; begin while count < 6 loop clk <= not clk; wait for 15 ns; count := count + 1; end loop; wait; end process; uses a loop with a counter to toggle clk 6 times, waiting 15 ns each time, then stops with wait;. Others either toggle once or use incorrect timing.

3. Final Answer:

   variable count : integer := 0; while count < 6 loop clk <= not clk; wait for 15 ns; count := count + 1; end loop; wait; -> Option D

4. Quick Check:

   Loop with wait for 15 ns toggles clk 6 times [OK]

Hint: Use loop with counter and wait for to toggle signals repeatedly [OK]

Common Mistakes:

• Using single wait for without loop for repeated toggling
• Incorrect loop ranges causing wrong toggle count
• Not stopping process after toggling