Subtractor design in Verilog - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a 1-bit input named 'a'.

Verilog

input [1];

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

Aresult

Bb

Ca

Dborrow

Attempts:

3 left

💡 Hint

Common Mistakes

Using output or wire instead of input.

Using a wrong variable name.

The input 'a' is declared as a 1-bit input to represent one bit of the number.

2fill in blank

medium

Complete the code to assign the difference output as the XOR of inputs 'a' and 'b'.

Verilog

assign diff = a [1] b;

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

A~

B&

C|

D^

Attempts:

3 left

💡 Hint

Common Mistakes

Using AND (&) or OR (|) instead of XOR (^).

Using NOT (~) which is unary.

The difference bit in a subtractor is calculated using XOR (^) of inputs 'a' and 'b'.

3fill in blank

hard

Fix the error in the borrow output assignment to correctly compute borrow in a 1-bit subtractor.

Verilog

assign borrow = (~a) [1] b;

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

A&

B|

C~

D^

Attempts:

3 left

💡 Hint

Common Mistakes

Using OR (|) instead of AND (&).

Using XOR (^) which is incorrect here.

The borrow output is true when 'a' is 0 and 'b' is 1, so use AND (&) between ~a and b.

4fill in blank

hard

Fill both blanks to complete the module header and port list for a 1-bit subtractor.

Verilog

module subtractor([1], [2], diff, borrow);

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

Ainput a

Boutput diff

Cinput b

Doutput borrow

Attempts:

3 left

💡 Hint

Common Mistakes

Putting outputs before inputs in the port list.

Using output keywords inside the port list.

The module header lists inputs first: 'input a' and 'input b' are the inputs for the subtractor.

5fill in blank

hard

Fill all three blanks to complete the always block that calculates difference and borrow using inputs 'a', 'b', and 'bin'.

Verilog

always @(*) begin
  diff = a [1] b [2] bin;
  borrow = ([3] a & b) | (b & bin) | ([3] a & bin);
end

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

A^

B&

C~

D|

Attempts:

3 left

💡 Hint

Common Mistakes

Using AND or OR instead of XOR for difference.

Not complementing 'a' in borrow calculation.

The difference is XOR of a, b, and bin. Borrow is calculated using AND and OR with complemented 'a'.

Practice

1.

What is the main function of a subtractor in digital circuits?

easy

A. To multiply two binary numbers

B. To add two binary numbers

C. To find the difference between two binary numbers

D. To store binary data

Solution

1. Step 1: Understand the subtractor's purpose

   A subtractor is designed to calculate the difference between two binary inputs.

2. Step 2: Compare with other operations

   Adders add, multipliers multiply, and storage units store data, so these are not subtractors.

3. Final Answer:

   To find the difference between two binary numbers -> Option C

4. Quick Check:

   Subtractor = Difference finder [OK]

Hint: Subtractor always calculates difference, not sum or product [OK]

Common Mistakes:

• Confusing subtractor with adder
• Thinking subtractor stores data
• Mixing subtractor with multiplier

2.

Which of the following is the correct Verilog module header for a 1-bit subtractor?

module subtractor(

easy

A. module subtractor(input a, input b, output diff, output borrow);

B. module subtractor(input a, b, output diff, borrow);

C. module subtractor(a, b, diff, borrow);

D. module subtractor(input a, input b, input diff, output borrow);

Solution

1. Step 1: Check correct input/output declaration

   Inputs must be declared with 'input' keyword and outputs with 'output'.

2. Step 2: Verify syntax correctness

   module subtractor(input a, input b, output diff, output borrow); correctly declares inputs and outputs with proper keywords and commas.

3. Final Answer:

   module subtractor(input a, input b, output diff, output borrow); -> Option A

4. Quick Check:

   Correct input/output syntax = module subtractor(input a, input b, output diff, output borrow); [OK]

Hint: Always declare inputs and outputs explicitly with keywords [OK]

Common Mistakes:

• Omitting input/output keywords
• Listing inputs and outputs without keywords
• Declaring outputs as inputs

3.

What is the output difference and borrow when inputs a=1 and b=0 are applied to this 1-bit subtractor module?

assign diff = a ^ b ^ borrow_in;
assign borrow_out = (~a & b) | (borrow_in & (~a ^ b));

Assume borrow_in = 0.

medium

A. diff = 0, borrow_out = 0

B. diff = 0, borrow_out = 1

C. diff = 1, borrow_out = 1

D. diff = 1, borrow_out = 0

Solution

1. Step 1: Calculate diff with given inputs

   diff = a ^ b ^ borrow_in = 1 ^ 0 ^ 0 = 1

2. Step 2: Calculate borrow_out with given inputs

   borrow_out = (~a & b) | (borrow_in & (~a ^ b)) = (0 & 0) | (0 & (0 ^ 0)) = 0

3. Final Answer:

   diff = 1, borrow_out = 0 -> Option D

4. Quick Check:

   1 - 0 = 1, no borrow needed [OK]

Hint: XOR inputs and borrow_in for diff; borrow if a < b [OK]

Common Mistakes:

• Forgetting borrow_in in diff calculation
• Misapplying borrow logic
• Mixing up a and b in borrow expression

4.

Identify the error in this Verilog code for a 1-bit subtractor:

module subtractor(input a, input b, output diff, output borrow);
  assign diff = a - b;
  assign borrow = (b > a);
endmodule

medium

A. Using '-' operator for bitwise subtraction is incorrect in Verilog

B. 'borrow' should be declared as 'output reg' instead of 'output'

C. The module lacks a clock input for synchronous operation

D. The borrow calculation logic is incorrect

Solution

1. Step 1: Check arithmetic operator usage

   Verilog '-' operator performs arithmetic subtraction but does not produce borrow output directly.

2. Step 2: Understand borrow signal generation

   Borrow must be calculated using logic expressions, not by comparing b and a with '>' operator.

3. Final Answer:

   Using '-' operator for bitwise subtraction is incorrect in Verilog -> Option A

4. Quick Check:

   Subtractor needs logic for borrow, not '-' operator [OK]

Hint: Use logic expressions for borrow, not '-' operator [OK]

Common Mistakes:

• Using arithmetic '-' expecting borrow output
• Ignoring borrow logic
• Assuming '>' operator works for borrow

5.

Design a 2-bit subtractor using two 1-bit subtractor modules connected in series. Which of the following correctly describes the borrow connection?

hard

A. The borrow input of the least significant bit subtractor connects to the borrow output of the most significant bit subtractor

B. The borrow output of the least significant bit subtractor connects to the borrow input of the most significant bit subtractor

C. The borrow outputs of both subtractors are connected together to form a single borrow output

D. The borrow inputs of both subtractors are tied to 0

Solution

1. Step 1: Understand borrow chaining in multi-bit subtractors

   Borrow from the lower bit subtractor must feed into the higher bit subtractor's borrow input.

2. Step 2: Verify connection direction

   The least significant bit's borrow output connects to the most significant bit's borrow input to propagate borrow correctly.

3. Final Answer:

   The borrow output of the least significant bit subtractor connects to the borrow input of the most significant bit subtractor -> Option B

4. Quick Check:

   Borrow chains from LSB to MSB [OK]

Hint: Borrow flows from lower to higher bits in subtractors [OK]

Common Mistakes:

• Connecting borrow inputs to zero for all bits
• Connecting borrow outputs together incorrectly
• Reversing borrow input/output connections