Report statement for debug output in VHDL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the report statement in VHDL?

The report statement is used to display messages during simulation, helping to debug and understand the behavior of the VHDL design.

beginner

How do you write a simple report statement in VHDL to show the message 'Signal value changed'?

Use:
report "Signal value changed";
This prints the message during simulation.

intermediate

What are the severity levels available in a VHDL report statement?

Severity levels include:
- NOTE (default)
- WARNING
- ERROR
- FAILURE
They indicate the importance of the message.

intermediate

How do you add a severity level to a report statement in VHDL?

Syntax:
report "Message" severity WARNING;
This sets the message severity to WARNING.

intermediate

What happens if a report statement with severity ERROR or FAILURE is executed during simulation?

The simulator usually stops or flags an error, helping to catch serious problems early in the design.

Which VHDL statement is used to print debug messages during simulation?

A report

B print

C display

D echo

The report statement is the correct way to output messages in VHDL simulations.

What is the default severity level if none is specified in a report statement?

A WARNING

B ERROR

C NOTE

D FAILURE

If no severity is given, the default is NOTE.

How do you write a report statement with a WARNING severity?

A report WARNING "Warning message";

B report "Warning message" severity WARNING;

C report "Warning message" level WARNING;

D report "Warning message" with WARNING;

The correct syntax is report "message" severity WARNING;.

What effect does a report statement with severity FAILURE have during simulation?

A Ignores the message

B Prints a note only

C Logs the message silently

D Stops the simulation or flags a critical error

Severity FAILURE usually stops the simulation or signals a critical error.

Which of these is NOT a valid severity level in VHDL report statements?

A CRITICAL

B WARNING

C ERROR

D NOTE

CRITICAL is not a valid severity level in VHDL; valid levels are NOTE, WARNING, ERROR, and FAILURE.

Explain how to use the report statement in VHDL to help debug your design.

Think about how you would tell the simulator to show messages and how important those messages are.

Describe the difference between the severity levels WARNING and ERROR in a VHDL report statement.

Consider how serious each message is and what the simulator does when it sees them.

Practice

1. What is the main purpose of the report statement in VHDL?

easy

A. To assign values to variables

B. To print messages during simulation for debugging

C. To declare a new signal in the design

D. To create a new process block

Solution

1. Step 1: Understand the role of report

   The report statement is used to output messages during simulation, which helps in debugging the design.

2. Step 2: Differentiate from other statements

   Other options like declaring signals, assigning variables, or creating processes do not involve printing messages.

3. Final Answer:

   To print messages during simulation for debugging -> Option B

4. Quick Check:

   Report statement = debug messages [OK]

Hint: Report prints debug messages during simulation [OK]

Common Mistakes:

• Confusing report with signal declaration
• Thinking report assigns values
• Mixing report with process creation

2. Which of the following is the correct syntax for a simple report statement in VHDL?

easy

A. report "Simulation started" severity note;

B. report = "Simulation started";

C. print "Simulation started";

D. message("Simulation started");

Solution

1. Step 1: Recall correct report syntax

   The correct syntax uses the keyword report followed by a string in quotes and optionally a severity level, ending with a semicolon.

2. Step 2: Check each option

   report "Simulation started" severity note; matches the correct syntax. Options A, C, and D use incorrect keywords or assignment operators.

3. Final Answer:

   report "Simulation started" severity note; -> Option A

4. Quick Check:

   Correct report syntax = report "Simulation started" severity note; [OK]

Hint: Report uses keyword + string + optional severity + semicolon [OK]

Common Mistakes:

• Using assignment operator '=' with report
• Using print or message instead of report
• Omitting semicolon at the end

3. What will be the output during simulation for the following VHDL code snippet?

process
begin
  report "Value is high" severity warning;
  wait;
end process;

medium

A. An error message: Value is high

B. No output, process waits silently

C. A warning message: Value is high

D. Simulation stops immediately

Solution

1. Step 1: Analyze the report statement

   The report statement prints the message "Value is high" with severity level warning during simulation.

2. Step 2: Understand simulation behavior

   The process then waits indefinitely, but the report message is printed once before waiting.

3. Final Answer:

   A warning message: Value is high -> Option C

4. Quick Check:

   Report with severity warning prints warning message [OK]

Hint: Report prints message once before wait [OK]

Common Mistakes:

• Assuming no output because of wait
• Confusing severity warning with error
• Thinking simulation stops immediately

4. Identify the error in this VHDL report statement:

report "Check signal" severity warning

medium

A. Report keyword is misspelled

B. Severity level 'warning' is invalid

C. Quotes around message are missing

D. Missing semicolon at the end

Solution

1. Step 1: Check syntax completeness

   The report statement must end with a semicolon to be syntactically correct.

2. Step 2: Verify other parts

   The severity level 'warning' is valid, quotes are present, and 'report' is spelled correctly.

3. Final Answer:

   Missing semicolon at the end -> Option D

4. Quick Check:

   Report statement ends with semicolon [OK]

Hint: Always end report statements with semicolon [OK]

Common Mistakes:

• Forgetting semicolon
• Using invalid severity levels
• Omitting quotes around message

5. You want to report an error message only when a signal error_flag is true during simulation. Which VHDL code snippet correctly uses the report statement for this?

hard

A. if error_flag = '1' then report "Error detected" severity error; end if;

B. report "Error detected" severity error when error_flag = '1';

C. report "Error detected" severity error if error_flag = '1';

D. if error_flag then report "Error detected" severity note; end if;

Solution

1. Step 1: Understand conditional reporting

   In VHDL, to report conditionally, use an if statement checking the signal, then call report inside it.

2. Step 2: Evaluate each option

   if error_flag = '1' then report "Error detected" severity error; end if; correctly uses if with condition and report with severity error. Options B and C use invalid syntax. if error_flag then report "Error detected" severity note; end if; uses severity note instead of error.

3. Final Answer:

   if error_flag = '1' then report "Error detected" severity error; end if; -> Option A

4. Quick Check:

   Conditional report uses if + report [OK]

Hint: Use if statement to conditionally report [OK]

Common Mistakes:

• Using report with when or if inline (invalid)
• Wrong severity level for error
• Omitting condition check