VHDL (Elective-II) Question Papers Andhra University
B. Tech (CSE) Degree Examination
Forth Year – Second Semester
Effective from the admitted batch of 2004-2005
Time: 3 hrs
Max Marks: 70
First Question is Compulsory
Answer any four from the remaining questions
All Questions carry equal marks
Answer all parts of any question at one place
1. (a) Give a typical design flow for designing VLSI IC circuits
(b) Distinguish between top – down and bottom – up design methodologies for digital design
(c) Define the logic value set and data types such as any nets, registers and numbers
(d) Describe the port connection rules in a module instantiation
(e) Describe fall and turn-off delays in the gate-level design
(f) Describe the continuous assignments (assign) statement and restrictions on the assign statement
(g) Explain the significance of structured procedures always and initial in behavioral modeling
2. A 4-bit ripple carry adder (Ripple Add) contains four 1-bit full adders (FA).
(a) Define the module FA Do not define the internals or the terminal list.
(b) Define the module Ripple Add. Do not define the internals or the terminal list. Istantiate four full adders of the type FA in the module Ripple Add and call them fa0,fa1,fa2, and fa3.
3. Declare the following variables in Verilog.
(a) (i) An 8-bit vector net called a_in.
(ii) A 32-bit storage register called address. Bit 31 must be the mot significant bit. Set the value of the register to a 32-bit decimal number equal to 3.
iii) A time variable called snap_shot.
(iv) A memory MEM containing 256 words of 64 bits each.
(b) What would be the output effect of the following statements?
(i) latch = 4’d12;
$display (“The current value of latch = %b\n”, latch);
(ii) in_reg = 3’d2;
$monitor ($time, “In register value = %b\n”, in_reg[2:0]);
(iii) define MEM_SIZE 1024
$display (“The maximum memory size is %h”, MEM_SIZE);
4. (a) A 4-bit parallel shaft register has 10 pins as shown in the figure below. Write the module definition for this module shift_reg. Include the list of ports and port declarations. You do not need to show the internals.
(b) Declare a top-Level module stimulus. Define REG_IN (4bit) and CLK (1 bit) as reg register variabes and REG_OUT (4bit) as wire. Instantiate the module shift_reg and call it sr1. Connect the ports by orderedlist.
5. Design a 2-to-1 multiplexer using bufif0 and bufif1 gates as shown below.
The delay specification for gates b1 and b2 are as follows.
Min Type Max Rise 1 2 3 Fall 3 4 5 Turnoff 5 6 7
6. A full subtractor has three 1-bit input x,y, and z (previous borrow) and two 1-bit outputs D(difference) and B(borrow). The logic equations forD and B are as follows:
Write the full Verilog description for the full subtractor module including I/Oports (Remembei that + in logic equations corresponds to a logical or operator ( ) in dataflow). Instantiate the subtractor inside a stimulus block and test all eight possible combinations of x,y, and z given in the following truth table.
7. (a) Design a negative edge-triggered D-flipflop (D_FF) with synchronous clear, active high (D_FF clears only at a negative edge of clock when clear is high). Use behavioral statements only. (Hint: Output q of D_FF must be declared as reg) Design a dock with a period of 10 units and test the D-FF.
(b) Design the D-flipflop in exercise 7 with asynchronous clear (D_FF clean whenever clear goes high. It does not wait for next negative edge). Test the D-FF.
8. (a) Define a task to compute the factorial of a 4-bit number. The output is a 32-bit value. The result is assigned to the out put after a delay of 10 time units.
(b) Define a task to computer even parity of a 16-bit number. The result is a 1-bit value that is assigned to the output after three positive edges of clock (Hint: Use a repeat loop in the task).