Stimulus process with wait statements in VHDL - Step-by-Step Execution

Concept Flow - Stimulus process with wait statements

Start process

↓

Apply stimulus

↓

Wait for time or event

↓

Apply next stimulus

↓

Wait again

↓

Repeat or end

The process applies signals, then waits for a specified time or event before continuing to the next step.

Execution Sample

VHDL

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

This process sets signal_a to '0', waits 10 ns, sets it to '1', waits 20 ns, then stops.

Execution Table

Step	Time (ns)	Action	signal_a Value	Wait Statement
1	0	Set signal_a to '0'	'0'	wait for 10 ns
2	10	Wait ends, set signal_a to '1'	'1'	wait for 20 ns
3	30	Wait ends, process waits indefinitely	'1'	wait (infinite)
4	-	Process stops here, no further action	'1'	no further wait

💡 Process stops at infinite wait statement, no further stimulus applied

Variable Tracker

Variable	Start (0 ns)	After 1 (10 ns)	After 2 (30 ns)	Final
signal_a	'0'	'1'	'1'	'1'

Key Moments - 2 Insights

Why does the process stop after the last wait statement?

Because the last wait statement is 'wait;' with no condition or time, it causes the process to pause indefinitely, as shown in execution_table step 3.

Does the signal change immediately after the wait statement?

No, the signal changes happen right after the wait period ends, as seen between steps 1 and 2 in the execution_table.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of signal_a at 15 ns?

A Undefined

B '0'

C '1'

D Process stopped

💡 Hint

15 ns is between step 2 (10 ns, '1') and step 3 (30 ns, '1') in the execution_table.

At which step does the process enter an infinite wait?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the Wait Statement column in the execution_table for the infinite wait.

If the first wait was changed to 'wait for 5 ns;', how would signal_a change at 7 ns?

A signal_a would be '1'

B signal_a would still be '0'

C signal_a would be undefined

D Process would stop

💡 Hint

Refer to variable_tracker and execution_table timing for signal changes after wait periods.

Concept Snapshot

Stimulus process uses wait statements to control timing.
Signals change, then process waits for time or event.
'wait for time;' pauses process for that duration.
'wait;' pauses process indefinitely.
Useful for testbenches to apply signals step-by-step.

Full Transcript

This VHDL stimulus process example shows how signals are assigned values and then the process waits for specified times before continuing. Initially, signal_a is set to '0' at time 0 ns. The process then waits for 10 ns. After 10 ns, signal_a changes to '1'. The process waits another 20 ns. At 30 ns, the process reaches an infinite wait statement and stops executing further. The variable tracker shows signal_a starting undefined, then becoming '0' at 0 ns, and '1' at 10 ns, remaining '1' thereafter. Key points include understanding that signals change right after wait periods end, and that a plain 'wait;' causes the process to pause forever. The quizzes test understanding of signal values at specific times and the effect of wait statements on process flow.

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