Take-Home Midterm Exam: Sequential Circuits and Verilog

Scope: Sequential logic, finite state machines, registers, counters, with Verilog modeling
Duration: 48 hours
Instructions

  • Attempt all questions. Show reasoning, derivations, and clearly state assumptions.
  • Provide commented, synthesizable Verilog and a self‑checking testbench where requested.
  • Include brief timing/area reasoning (big‑O style or gate/count estimates).
  • No collaboration. Cite any external references you consulted.
  • Tutorial of Online tools for Verilog simulation can be found here.

Part A — Multiple Choice (10 × 3 pts = 30 pts)

Select the best answer.

Q1. A positive-level D latch is best described as:

  • A. Sampling on rising edges only
  • B. Transparent when clock=1 and opaque when clock=0
  • C. Triggered on both edges
  • D. Metastability-free by design

Q2. The parameter that primarily limits f_max is:

  • A. Hold time
  • B. Recovery time
  • C. t_clk-q + t_comb + t_setup
  • D. Clock duty cycle

Q3. A T flip-flop divides the clock by two because it:

  • A. Filters every other edge by delay
  • B. Toggles its output at each active edge
  • C. Samples input twice per cycle
  • D. Has J=0, K=1

Q4. A ripple counter differs from a synchronous counter because:

  • A. It uses fewer flip-flops
  • B. Its stages are clocked by preceding stage outputs
  • C. It is immune to propagation delay
  • D. It is always faster

Q5. In a Moore machine, outputs depend on:

  • A. Current input only
  • B. Current state only
  • C. Next state only
  • D. Current and previous inputs

Q6. In a Mealy machine:

  • A. Outputs change only at clock edges
  • B. Outputs depend on state and inputs
  • C. It needs more states than Moore always
  • D. It cannot be coded in Verilog

Q7. A 4-bit shift register with serial input 1101 after four clocks contains (MSB..LSB):

  • A. 1011
  • B. 1101
  • C. 0110
  • D. 1110

Q8. A hold-time violation can be mitigated by:

  • A. Adding delay to data path
  • B. Increasing clock frequency
  • C. Reducing setup time
  • D. Removing all registers

Q9. One-hot encoding of an N-state FSM uses:

  • A. log2(N) flip-flops
  • B. N flip-flops
  • C. N-1 flip-flops
  • D. 2N flip-flops

Q10. Pipeline registers primarily:

  • A. Reduce combinational delay per stage
  • B. Store only final outputs
  • C. Remove all hazards
  • D. Reduce setup time of FFs

Part B — Design & Analysis (10 × 7 = 70 pts)

For each problem, complete the Verilog template in the zip and verify using the provided self‑checking testbench. Name your top‑level modules exactly as specified.

Files provided (in the questions zip):

  • Templates: *.v (one per problem)
  • Testbenches: tb_*.v (one per problem)
  • Timescale: 1ns/1ps

Problems:

Problem 1 — Synchronizer + Edge Detect (sync_edge)
Synchronize asynchronous btn_async into clk with a two‑FF synchronizer; output btn_sync level and one‑cycle btn_pulse on rising edges. Active‑low rst_n.

Problem 2 — Dual‑Edge Capture (ddr_reg)
Capture D on posedge into Q_pos and on negedge into Q_neg. Active‑low rst_n.

Problem 3 — Mealy Sequence Detector “11010” (seq_11010_mealy)
Detect the overlapping pattern and assert Z on the final bit. Use a minimal FSM.

Problem 4 — Mod‑6 Up/Down Counter with Enable (mod6_counter)
3‑bit counter over 0..5. En gates counting; Dir=1 up, 0 down. Synchronous reset to 0.

Problem 5 — 4×4 Serial Multiplier Controller (mul4_ctrl)
Shift‑add controller with signals LdA,LdB,ClrP,Add,Shift,Done. Start with start=1. Iterate 4 times.

Problem 6 — 2‑Stage Pipeline (A+B)*C with Valid/Ready (pipe_add_mul)
Implement a two‑stage pipeline (add then multiply) with back‑pressure (in_valid/in_ready, out_valid/out_ready).

Problem 7 — CDC Bridge 1 MHz → 100 MHz (cdc_bridge)
Use a req/ack handshake and 2FF synchronizers both directions to transfer an 8‑bit word reliably.

Problem 8 — Moore FSM with Registered Output (moore_safe)
Provide both combinational output Zc and registered Zr (hazard‑free).

Problem 9 — Sequential 4‑bit ALU (seq_alu4)
Opcode: 00=ADD, 01=AND, 10=XOR, 11=SHL. Registered outputs Y and Cout with synchronous reset.

Problem 10 — Traffic Lights with Pedestrian Preempt (traffic_ped)
Main: G×3, Y×1; Side: G×2, Y×1. Insert WALK×4 at a safe point when ped_req=1; resume correctly.

Deliverables:

  • PDF with answers to Section A and brief design notes for Section B.
  • Verilog sources for all 10 designs.
  • Simulation logs/screenshots demonstrating passing testbenches.