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Digital Logic SC1005 Final Exam
Raw
fsm.v
/*
Key Idea: next state, current state
use the observer pattern to do actions on state change
*/
module FSM_Q4b (
input CLK,
input RESET, // Asynchronous active high reset
input T,
output reg Y
);
// Define state encodings for readability
parameter STATE_A = 2'b00,
STATE_B = 2'b01,
STATE_C = 2'b11,
STATE_D = 2'b10;
// State registers
reg [1:0] current_state;
reg [1:0] next_state;
// --- State Transition Logic (Sequential Block) ---
// Updates current_state on CLK edge, handles asynchronous RESET
always @(posedge CLK or posedge RESET) begin
if (RESET) begin
// Asynchronous Reset to initial state A
current_state <= STATE_A;
end else begin
// Synchronous update on CLK
current_state <= next_state;
end
end
// --- Next State Logic (Combinational Block) ---
// Determines the next state based on current_state and input T
always @(*) begin
// Default next_state to current_state to avoid latch inference
next_state = current_state;
case (current_state)
STATE_A: begin
if (T == 1'b0)
next_state = STATE_A;
else // T == 1'b1
next_state = STATE_B;
end
STATE_B: begin
if (T == 1'b0)
next_state = STATE_A;
else // T == 1'b1
next_state = STATE_C;
end
STATE_C: begin
if (T == 1'b0)
next_state = STATE_B;
else // T == 1'b1
next_state = STATE_D;
end
STATE_D: begin
if (T == 1'b0)
next_state = STATE_C;
else // T == 1'b1
next_state = STATE_D;
end
default: begin
// Should not happen, but reset to a safe state
next_state = STATE_A;
end
endcase
end
// --- Output Logic (Combinational Block for Moore Machine) ---
// Output Y is solely determined by the current_state
always @(current_state) begin
case (current_state)
STATE_A, STATE_B: begin
// A (Y=0), B (Y=0)
Y = 1'b0;
end
STATE_C, STATE_D: begin
// C (Y=1), D (Y=1)
Y = 1'b1;
end
default: begin
Y = 1'b0; // Safe default
end
endcase
end
endmodule
@liang799
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liang799 commented Nov 20, 2025 •
edited
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module circuit(
  input wire clk, start, stop, pedestrian, vehicle, timeout,
  output reg green, red
);

  // --- DEFINITIONS ---
  parameter IDLE = 2'b00, DETECT_PED = 2'b01, DETECT_VEH = 2'b10, TIMER = 2'b11;
  reg [1:0] state, next_state;

  // --- 1. STATE MEMORY (Synchronous - The "Anchor") ---
  // This is the only part that remembers history.
  always @(posedge clk) begin
    if (stop) state <= IDLE;
    else      state <= next_state;
  end

  // --- 2. NEXT STATE LOGIC (Combinational - The "Brain") ---
  // Pure function: (current_state, inputs) -> next_state
  always @(*) begin
    next_state = state; // Default to stay (prevents latch creation)
    case (state)
      IDLE:       if (start) next_state = DETECT_PED;
      DETECT_PED: if (!pedestrian) next_state = DETECT_VEH;
      DETECT_VEH: if (vehicle) next_state = TIMER; 
                  else next_state = DETECT_PED; // The missing link from before
      TIMER:      if (timeout) next_state = DETECT_PED;
      default:    next_state = IDLE;
    endcase
  end

  // --- 3. OUTPUT LOGIC (Combinational - The "Display") ---
  // PURE MOORE: Depends ONLY on state. No side effects from inputs.
  always @(*) begin
    case (state)
      IDLE:       begin green = 0; red = 0; end
      DETECT_PED: begin green = 0; red = 1; end
      DETECT_VEH: begin green = 0; red = 1; end
      TIMER:      begin green = 1; red = 0; end
      default:    begin green = 0; red = 0; end
    endcase
  end

endmodule

@liang799
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liang799 commented Nov 21, 2025 •
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@liang799
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liang799 commented Nov 21, 2025

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