Attribute usage for synthesis control in VHDL - Step-by-Step Execution

Concept Flow - Attribute usage for synthesis control

Declare signal/entity

↓

Define attribute

↓

Assign attribute value

↓

Synthesis tool reads attribute

↓

Control synthesis behavior

↓

Generate hardware accordingly

This flow shows how attributes are declared and assigned to signals or entities to guide synthesis tools in hardware generation.

Execution Sample

VHDL

signal clk : std_logic;
attribute keep : boolean;
attribute keep of clk : signal is true;

This code declares a signal clk and uses the 'keep' attribute to tell synthesis to preserve this signal.

Execution Table

Step	Action	Target	Attribute	Value	Effect
1	Declare signal	clk	-	-	Signal clk created
2	Declare attribute	keep	-	-	Attribute 'keep' defined as boolean
3	Assign attribute	clk	keep	true	clk marked to be preserved during synthesis
4	Synthesis reads attribute	clk	keep	true	Synthesis tool keeps clk signal
5	Hardware generated	-	-	-	clk signal preserved in hardware netlist

💡 All steps completed; synthesis control attribute applied successfully

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
clk	undefined	std_logic signal declared	std_logic signal declared	std_logic signal with keep=true	std_logic signal with keep=true
keep attribute	undefined	undefined	boolean attribute defined	boolean attribute assigned to clk	boolean attribute assigned to clk

Key Moments - 3 Insights

Why do we declare the attribute separately before assigning it?

Attributes must be declared first to define their type before assigning values, as shown in steps 2 and 3 in the execution_table.

What happens if the attribute is not assigned to the signal?

If not assigned, the synthesis tool ignores the attribute for that signal, so the signal may be optimized away, as seen by the absence of assignment in step 3.

Does the attribute change the signal's value during simulation?

No, attributes only affect synthesis behavior, not simulation values, as the signal clk remains std_logic throughout.

Visual Quiz - 3 Questions

Look at the execution_table, what is the attribute value assigned to signal clk at step 3?

A true

B false

C undefined

D boolean

💡 Hint

Check the 'Value' column in row for step 3 in execution_table

At which step does the synthesis tool read the attribute to control hardware generation?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look for 'Synthesis reads attribute' action in execution_table

If the attribute 'keep' was assigned false instead of true, what would change in the execution_table?

A clk would be preserved in hardware

B Attribute declaration would fail

C clk might be optimized away by synthesis

D Signal clk would change type

💡 Hint

Refer to the 'Effect' column in execution_table rows 3 and 4

Concept Snapshot

Attribute usage for synthesis control in VHDL:
- Declare attribute with 'attribute name : type;'
- Assign attribute to signal/entity with 'attribute name of target : type is value;'
- Synthesis tools read attributes to guide hardware generation
- Attributes do not affect simulation behavior
- Common use: preserve signals, control optimization

Full Transcript

This visual execution shows how attributes in VHDL are used to control synthesis. First, a signal clk is declared. Then, an attribute named 'keep' of boolean type is declared. Next, the attribute 'keep' is assigned the value true for the signal clk. The synthesis tool reads this attribute and preserves the clk signal in the hardware netlist. Variables clk and keep attribute change state accordingly. Key points include the need to declare attributes before assignment, that attributes affect synthesis but not simulation, and that missing assignment means no effect. The quiz tests understanding of attribute assignment, synthesis reading, and effects of attribute values.

Practice

1. What is the primary purpose of using attributes in VHDL synthesis control?

easy

A. To declare new signals in the design

B. To define the clock frequency of the design

C. To provide extra instructions to synthesis tools about signals or components

D. To write testbench stimulus

Solution

1. Step 1: Understand the role of attributes in VHDL

   Attributes are special metadata that give extra information to synthesis tools about how to treat signals or components.

2. Step 2: Differentiate attributes from other VHDL constructs

   Attributes do not declare signals or define clock frequencies; they provide instructions for synthesis optimization or constraints.

3. Final Answer:

   To provide extra instructions to synthesis tools about signals or components -> Option C

4. Quick Check:

   Attributes = extra synthesis instructions [OK]

Hint: Attributes give hints to synthesis tools, not declare signals [OK]

Common Mistakes:

• Confusing attributes with signal declarations
• Thinking attributes set clock frequencies
• Using attributes to write testbench code

2. Which of the following is the correct syntax to declare an attribute named keep_signal of type boolean in VHDL?

easy

A. attribute keep_signal : boolean is true;

B. attribute keep_signal : boolean;

C. attribute keep_signal : boolean := true;

D. attribute keep_signal : boolean := '1';

Solution

1. Step 1: Recall attribute declaration syntax

   Attributes are declared with the keyword attribute, followed by the name, colon, and type, ending with a semicolon.

2. Step 2: Identify correct syntax

   attribute keep_signal : boolean; matches the correct syntax: attribute keep_signal : boolean;. The other options incorrectly assign values or use invalid syntax.

3. Final Answer:

   attribute keep_signal : boolean; -> Option B

4. Quick Check:

   Declare attribute with name and type only [OK]

Hint: Declare attribute with name and type only, no value assigned [OK]

Common Mistakes:

• Assigning values during attribute declaration
• Using := instead of : for type declaration
• Using quotes for boolean type

3. Given the following VHDL code snippet, what is the effect of the attribute assignment?

attribute keep : boolean;
attribute keep of signal_a : signal is true;

medium

A. The signal signal_a will be preserved and not optimized away by synthesis

B. The signal signal_a will be removed during synthesis

C. The attribute has no effect on synthesis

D. The signal signal_a will be forced to zero

Solution

1. Step 1: Understand the attribute declaration and assignment

   The attribute keep is declared as boolean and assigned true to signal_a.

2. Step 2: Interpret the meaning of keep = true

   This instructs synthesis tools to preserve signal_a and not optimize it away, even if it seems unused.

3. Final Answer:

   The signal signal_a will be preserved and not optimized away by synthesis -> Option A

4. Quick Check:

   keep = true means preserve signal [OK]

Hint: Attribute keep=true means keep signal during synthesis [OK]

Common Mistakes:

• Assuming attribute removes the signal
• Thinking attribute forces signal value
• Ignoring attribute effect on synthesis

4. Identify the error in the following VHDL code related to attribute usage:

attribute keep : boolean;
attribute keep of signal_b : signal is 'true';

medium

A. The attribute value should not be in single quotes for boolean type

B. The attribute name cannot be the same as the signal name

C. The attribute must be declared after the signal

D. The attribute type should be integer, not boolean

Solution

1. Step 1: Check attribute value syntax

   The attribute keep is boolean, so its value should be true or false without quotes.

2. Step 2: Identify the error in value assignment

   Using single quotes around true makes it a character literal, which is invalid for boolean type.

3. Final Answer:

   The attribute value should not be in single quotes for boolean type -> Option A

4. Quick Check:

   Boolean attribute values are unquoted true/false [OK]

Hint: Boolean attribute values must be unquoted true or false [OK]

Common Mistakes:

• Putting boolean values in quotes
• Declaring attribute after signal
• Using wrong attribute type

5. You want to ensure a signal data_reg is not optimized away and also specify a maximum delay constraint using attributes. Which of the following is the correct way to declare and assign these attributes in VHDL?

hard

A. attribute keep : boolean;\nattribute max_delay : time;\nattribute keep of data_reg : signal is 'true';\nattribute max_delay of data_reg : signal is '10 ns';

B. attribute keep : boolean := true;\nattribute max_delay : time := 10 ns;\nattribute keep of data_reg : signal;\nattribute max_delay of data_reg : signal;

C. attribute keep : boolean;\nattribute max_delay : integer;\nattribute keep of data_reg : signal is 'true';\nattribute max_delay of data_reg : signal is 10;

D. attribute keep : boolean;\nattribute max_delay : time;\nattribute keep of data_reg : signal is true;\nattribute max_delay of data_reg : signal is 10 ns;

Solution

1. Step 1: Declare attributes with correct types

   Declare keep as boolean and max_delay as time type without assigning values during declaration.

2. Step 2: Assign attribute values correctly

   Assign keep as true (unquoted boolean) and max_delay as 10 ns (time literal) to data_reg.

3. Step 3: Verify syntax correctness

   attribute keep : boolean;\nattribute max_delay : time;\nattribute keep of data_reg : signal is true;\nattribute max_delay of data_reg : signal is 10 ns; matches correct syntax and value formats. Other options have errors like assigning values during declaration, wrong types, or quoting boolean/time literals.

4. Final Answer:

   attribute keep : boolean; attribute max_delay : time; attribute keep of data_reg : signal is true; attribute max_delay of data_reg : signal is 10 ns; -> Option D

5. Quick Check:

   Declare attributes then assign correct typed values [OK]

Hint: Declare attributes first, assign typed values without quotes [OK]

Common Mistakes:

• Assigning values during attribute declaration
• Using quotes around boolean or time values
• Using wrong attribute types