Attribute usage for synthesis control in VHDL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an attribute named 'keep' for a signal.

VHDL

attribute [1] : string;

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

Akeep_signal

Bkeep

Cpreserve

Dsynthesis

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect attribute names like 'preserve' or 'synthesis'.

The attribute 'keep' is used to instruct synthesis tools to preserve the signal.

2fill in blank

medium

Complete the code to apply the 'keep' attribute with value 'true' to the signal 'data_sig'.

VHDL

attribute keep of data_sig : signal is [1];

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

A'true'

B'false'

C"true"

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using boolean true without quotes or double quotes instead of single quotes.

The attribute value must be a string literal 'true' to enable the keep behavior.

3fill in blank

hard

Fix the error in the attribute declaration to correctly specify the attribute type as STRING.

VHDL

attribute keep : [1];

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

AINTEGER

BBOOLEAN

CSTRING

DSTD_LOGIC

Attempts:

3 left

💡 Hint

Common Mistakes

Using BOOLEAN or STD_LOGIC as attribute type instead of STRING.

The attribute type for synthesis control attributes like 'keep' must be STRING.

4fill in blank

hard

Fill both blanks to declare and apply the 'keep' attribute with value 'true' to signal 'clk'.

VHDL

attribute [1] : [2];
attribute keep of clk : signal is 'true';

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

Akeep

Bpreserve

CSTRING

DBOOLEAN

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'preserve' as attribute name or BOOLEAN as type.

The attribute name is 'keep' and its type must be STRING to apply the value 'true'.

5fill in blank

hard

Fill all three blanks to declare attribute 'dont_touch' of type STRING and apply it with value 'true' to signal 'rst'.

VHDL

attribute [1] : [2];
attribute [3] of rst : signal is 'true';

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

Adont_touch

BSTRING

DBOOLEAN

Attempts:

3 left

💡 Hint

Common Mistakes

Using BOOLEAN type or mismatching attribute names.

The attribute name is 'dont_touch', its type is STRING, and it is applied to 'rst' with value 'true'.

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