Today at a Glance

▸ Phase 1 of 5  ·  ~10 min

🌍 Contextualize

State machines run the city

FSMs Are Everywhere You Looked Yesterday

An FSM is the moment HDL becomes controllable — it's how a circuit decides what to do next.

▸ Phase 2 of 5  ·  ~15 min

⚠️ Reframe

The 3-block pattern is the hardware

⚠️ An FSM Is Not a switch Statement

Three Pieces of Hardware

The 3-Block FSM Skeleton

localparam GREEN = 2'd0, YELLOW = 2'd1, RED = 2'd2;
reg [1:0] state, next;

// Block 1: state register — sequential, the only @(posedge clk)
always @(posedge clk)
    if (rst) state <= GREEN;
    else     state <= next;

// Block 2: next-state logic — combinational, depends on state + inputs
always @(*) begin
    next = state;                                    // safe default — no latch
    case (state)
        GREEN:  if (green_timer_done)  next = YELLOW;
        YELLOW: if (yellow_timer_done) next = RED;
        RED:    if (red_timer_done)    next = GREEN;
    endcase
end

// Block 3: output logic — combinational, depends on state (Moore)
always @(*) begin
    {led_g, led_y, led_r} = 3'b000;                  // default
    case (state)
        GREEN:  led_g = 1'b1;
        YELLOW: led_y = 1'b1;
        RED:    led_r = 1'b1;
    endcase
end

Memorize this layout. Every FSM you write this course follows it.

Moore vs Mealy in One Slide

▸ Phase 3 of 5  ·  ~70 min  ·  You build

🛠 Assemble

Diagram → table → code → board

Build Plan

1. Ex 1 · 30 min  Traffic light controller — 3 states, parameterized timers, LEDs on the Go Board.
2. Ex 2 · 20 min  Traffic light TB — verify the full cycle, timing, and reset behavior.
3. Ex 3 · 20 min  Pattern detector "1101" — Moore version. A state per bit of progress.

State Diagram → State Table → Code

case (state)
    S0: next = in ? S1 : S0;
    S1: next = in ? S2 : S0;
    S2: next = in ? S2 : S3;
    S3: next = in ? S4 : S0;
    S4: next = in ? S1 : S0;
endcase

Traffic Light TB — State Coverage

// Use SHORT timers in sim — match the parameter name
traffic_light #(
    .GREEN_TIME(10), .YELLOW_TIME(5), .RED_TIME(15)
) dut (.clk(clk), .rst(rst), .led_g(g), .led_y(y), .led_r(r));

initial begin
    rst = 1; #20; rst = 0;
    // Expect GREEN for 10 cycles
    repeat (10) @(posedge clk);
    if ({g,y,r} !== 3'b100) $display("FAIL: not GREEN after rst");
    // Expect YELLOW for 5 cycles
    repeat (5)  @(posedge clk);
    if ({g,y,r} !== 3'b010) $display("FAIL: did not enter YELLOW");
    // Expect RED for 15 cycles
    repeat (15) @(posedge clk);
    if ({g,y,r} !== 3'b001) $display("FAIL: did not enter RED");
    // Back to GREEN
    @(posedge clk);
    if ({g,y,r} !== 3'b100) $display("FAIL: did not return to GREEN");
    $display("PASS"); $finish;
end

Notice the parameter override — same TB stays usable when you flash with the long hardware timers.

▸ Phase 4 of 5  ·  ~45 min  ·  Verify · harden · critique AI

🛡 Fortify

Every state. Every transition. Both blocks reachable.

The FSM Verification Checklist

• State reachability: does every state get entered during the TB? If not, add stimulus.
• Transition coverage: every arrow in your diagram exercised at least once.
• Reset from every state: assert rst mid-run, confirm return to the reset state.
• Unreachable states are safe: hit default in case — output should be benign, not X.
• Timing: each state lasts exactly the expected number of cycles (use parameterized sim timers).

🤖 Check the Machine — FSM Prompt

• Did it use 3 always blocks? Or did it cram everything into one @(posedge clk)?
• Is the next-state block latch-free? Look for a next = state; default at the top.
• Does the pedestrian button get debounced and synchronized? If not, that's a real-world bug — the AI's lab works only in sim.
• Refactor what's wrong, save the prompt + before/after to your portfolio.

Hardware Verification

• Traffic light on the Go Board: LEDs cycle green → yellow → red at human-visible cadence. Phone-stopwatch the green duration — should match your parameter.
• Pattern detector: drive the input with a switch + button, press the sequence 1-1-0-1 — the "found" LED pulses for one cycle.
• False sequences do not trigger: 1-1-0-0, 1-0-1, 0-1-1-0-1 — all should stay dark.

▸ Phase 5 of 5  ·  ~10 min

🔗 Transfer

Carry it to HP this afternoon

At HP Barcelona This Afternoon

HP's Customer Center is about AI-assisted product workflows. The questions to bring:

1. Where is AI assisting, not replacing? What decisions still belong to humans? (Same question you answered for your AI TB this morning.)
2. How do they validate AI-generated outputs? Is there a "TB layer" for AI recommendations?
3. What FSMs run their products? Print job scheduling · firmware update sequencing · power-state controllers — all FSMs.

Tomorrow → Hierarchy & Parameters