Attribute usage for synthesis control in VHDL - Mini Project: Build & Apply

Attribute usage for synthesis control

📖 Scenario: You are designing a simple digital circuit in VHDL. Sometimes, you want to give hints to the synthesis tool to control how it builds the hardware. This is done using attributes. Attributes can tell the tool to keep certain signals or to optimize in a specific way.

🎯 Goal: You will write a VHDL code that declares a signal and uses an attribute to tell the synthesis tool to keep that signal during optimization. This helps you understand how to use attributes for synthesis control.

📋 What You'll Learn

Declare a signal named my_signal of type std_logic.

Declare an attribute named keep of type boolean.

Apply the keep attribute to my_signal and set it to true.

Write a simple process that assigns a value to my_signal.

Print or output the VHDL code with the attribute usage.

💡 Why This Matters

🌍 Real World

In real hardware design, synthesis attributes help control how tools optimize your circuit. This is important when you want to keep certain signals for debugging or special hardware behavior.

💼 Career

Hardware engineers and FPGA designers use synthesis attributes regularly to ensure their designs meet timing and functional requirements without losing important signals during optimization.

1

Declare the signal

Write a VHDL signal declaration for a signal named my_signal of type std_logic inside an architecture.

VHDL

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity AttributeExample is
    Port ( clk : in std_logic );
end AttributeExample;

architecture Behavioral of AttributeExample is
    -- Declare signal my_signal of type std_logic
    -- Your code here
begin

end Behavioral;

Hint

Use the keyword signal followed by the name my_signal and type std_logic.

2

Declare the attribute

Declare an attribute named keep of type boolean inside the architecture before the begin keyword.

VHDL

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity AttributeExample is
    Port ( clk : in std_logic );
end AttributeExample;

architecture Behavioral of AttributeExample is
    signal my_signal : std_logic;
    -- Declare attribute keep of type boolean
    -- Your code here
begin

end Behavioral;

Hint

Use the keyword attribute followed by the name keep and type boolean.

3

Apply the attribute to the signal

Apply the attribute keep to the signal my_signal and set its value to true inside the architecture before the begin keyword.

VHDL

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity AttributeExample is
    Port ( clk : in std_logic );
end AttributeExample;

architecture Behavioral of AttributeExample is
    signal my_signal : std_logic;
    attribute keep : boolean;
    -- Apply attribute keep to my_signal and set it to true
    -- Your code here
begin

end Behavioral;

Hint

Use the syntax attribute keep of my_signal : signal is true; to apply the attribute.

4

Assign a value and complete the architecture

Write a process triggered by clk that assigns '1' to my_signal on the rising edge of clk. Then, print the complete VHDL code.

VHDL

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity AttributeExample is
    Port ( clk : in std_logic );
end AttributeExample;

architecture Behavioral of AttributeExample is
    signal my_signal : std_logic;
    attribute keep : boolean;
    attribute keep of my_signal : signal is true;

    -- Write a process triggered by clk
    -- Your code here
end Behavioral;

Hint

Use a process with if rising_edge(clk) then to assign '1' to my_signal.

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