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.