diff --git a/frontend/src/simulation/AVRSimulator.ts b/frontend/src/simulation/AVRSimulator.ts
index 97c22084..d08c39c4 100644
--- a/frontend/src/simulation/AVRSimulator.ts
+++ b/frontend/src/simulation/AVRSimulator.ts
@@ -295,6 +295,15 @@ const MEGA_PORT_CONFIGS = [
 ];
 
 export class AVRSimulator {
+  // Digital input pins are driven from the SPICE solve
+  // (connectDigitalInputsToMcu) for nets backed by a real source/element, so
+  // `digitalRead()` reflects the real wiring (a pin wired to 5V reads HIGH, a
+  // button to 5V reads HIGH when pressed) instead of a hardcoded part seed.
+  // The connector skips floating nets, so event-driven parts with no SPICE
+  // model (rotary encoder, keypad, dialer, dip-switch, stepper) keep driving
+  // their pins via the part layer. Input-control parts (button / slide-switch)
+  // check this flag and skip their direct seed — see BasicParts.spiceDriven().
+  readonly spiceDrivenInputs = true;
   private cpu: CPU | null = null;
   /** Peripherals kept alive by reference so GC doesn't collect their CPU hooks */
   private peripherals: unknown[] = [];
diff --git a/frontend/src/simulation/spice/CircuitSimulationService.ts b/frontend/src/simulation/spice/CircuitSimulationService.ts
index 1fb3e8d7..3988e51b 100644
--- a/frontend/src/simulation/spice/CircuitSimulationService.ts
+++ b/frontend/src/simulation/spice/CircuitSimulationService.ts
@@ -67,6 +67,9 @@ export interface ElectricalSnapshot {
   timeWaveforms?: TimeWaveforms;
   /** Convergence warnings from the solver. */
   warnings: string[];
+  /** Nets backed by a real source/element (see NetlistBuilder). Gates which
+   *  MCU input pins connectDigitalInputsToMcu may drive from the solve. */
+  sourcedNets: Set<string>;
 }
 
 /** What the service needs from the scheduler. */
@@ -130,6 +133,7 @@ export class CircuitSimulationService {
     nets: string[];
     voltageSources: string[];
     analysisKind: 'op' | 'tran' | 'ac';
+    sourcedNets: Set<string>;
   } | null = null;
 
   /** Set by `stop()`. Once true, `tick()` and `handleMcuEdge()`
@@ -302,7 +306,7 @@ export class CircuitSimulationService {
       })),
     };
     const input = buildInputFromStore(snap as Parameters<typeof buildInputFromStore>[0]);
-    const { netlist, pinNetMap, nets, voltageSources } = buildNetlist(input);
+    const { netlist, pinNetMap, nets, voltageSources, sourcedNets } = buildNetlist(input);
 
     // Tell the scheduler exactly which vectors we want — every net
     // voltage + every branch current.
@@ -326,6 +330,7 @@ export class CircuitSimulationService {
       nets,
       voltageSources,
       analysisKind: input.analysis.kind,
+      sourcedNets,
     };
     this.publishFromLastResult();
   }
@@ -380,6 +385,7 @@ export class CircuitSimulationService {
       analysisMode: ctx.analysisKind,
       timeWaveforms,
       warnings: result.warnings,
+      sourcedNets: ctx.sourcedNets,
     });
   }
 
diff --git a/frontend/src/simulation/spice/NetlistBuilder.ts b/frontend/src/simulation/spice/NetlistBuilder.ts
index be20a913..3e632c0b 100644
--- a/frontend/src/simulation/spice/NetlistBuilder.ts
+++ b/frontend/src/simulation/spice/NetlistBuilder.ts
@@ -67,6 +67,18 @@ export interface BuildNetlistResult {
    * request branch currents (`i(v_<name>)`).
    */
   voltageSources: string[];
+  /**
+   * Nets that are backed by a real electrical source/element — a power rail
+   * ('0' / 'vcc_rail'), a board GPIO V-source, an internal pull resistor, or
+   * any net a componentToSpice mapper emitted a card for (resistor, button
+   * switch, divider, etc.). Excludes purely-floating nets (which only get the
+   * step-7 auto-pull-down). `connectDigitalInputsToMcu` drives an MCU input pin
+   * from the solve ONLY when its net is in here, so event-driven parts with no
+   * SPICE model (rotary encoder, keypad, dialer, dip-switch, stepper) keep
+   * driving their own pins via the part layer instead of being forced LOW by a
+   * floating-net read.
+   */
+  sourcedNets: Set<string>;
 }
 
 export function buildNetlist(input: BuildNetlistInput): BuildNetlistResult {
@@ -138,6 +150,11 @@ export function buildNetlist(input: BuildNetlistInput): BuildNetlistResult {
   const cards: string[] = [];
   const modelLines = new Set<string>();
   const dominantVcc = boards[0]?.vcc ?? 5;
+  // Nets backed by a real source/element (see BuildNetlistResult.sourcedNets).
+  // Rails are always sourced; component + GPIO-source + pull nets are added
+  // below. Deliberately NOT populated from the step-7 auto-pull-down cards,
+  // since those mark FLOATING nets.
+  const sourcedNets = new Set<string>(['0', 'vcc_rail']);
 
   for (const comp of components) {
     const localLookup = (pinName: string) => netLookup(comp.id, pinName);
@@ -145,6 +162,11 @@ export function buildNetlist(input: BuildNetlistInput): BuildNetlistResult {
     if (!emission) continue;
     cards.push(...emission.cards);
     for (const m of emission.modelsUsed) modelLines.add(m);
+    // Every net this component connects to now has a real SPICE element on it.
+    for (const pinName of pinsReferencedByWires(comp.id, wires)) {
+      const n = netLookup(comp.id, pinName);
+      if (n) sourcedNets.add(n);
+    }
   }
 
   // ── 5. Board GPIO sources ─────────────────────────────────────────────────
@@ -174,6 +196,7 @@ export function buildNetlist(input: BuildNetlistInput): BuildNetlistResult {
           cards.push(
             `R_pull_${sanitizeSpiceId(board.id)}_${sanitizeSpiceId(pinName)} ${net} ${rail} 45000`,
           );
+          sourcedNets.add(net); // weak pull to a rail → determinate idle level
         }
         continue;
       }
@@ -181,6 +204,7 @@ export function buildNetlist(input: BuildNetlistInput): BuildNetlistResult {
       if (net === '0' || net === 'vcc_rail') continue; // already served
       const v = state.type === 'digital' ? state.v : state.duty * board.vcc;
       cards.push(`V_${sanitizeSpiceId(board.id)}_${sanitizeSpiceId(pinName)} ${net} 0 DC ${v}`);
+      sourcedNets.add(net); // board GPIO V-source drives this net (e.g. cross-board input)
     }
   }
 
@@ -203,6 +227,8 @@ export function buildNetlist(input: BuildNetlistInput): BuildNetlistResult {
     const b = netLookup(w.end.componentId, w.end.pinName);
     if (!a || !b) continue;
     cards.push(`R_wire_${w.id} ${a} ${b} ${ohms}`);
+    sourcedNets.add(a);
+    sourcedNets.add(b);
   }
 
   // ── 7. Auto pull-downs for floating nets ─────────────────────────────────
@@ -279,6 +305,7 @@ export function buildNetlist(input: BuildNetlistInput): BuildNetlistResult {
     pinNetMap,
     nets,
     voltageSources,
+    sourcedNets,
   };
 }
 
diff --git a/frontend/src/simulation/spice/connectDigitalInputsToMcu.ts b/frontend/src/simulation/spice/connectDigitalInputsToMcu.ts
index fb2effee..cd8f5f84 100644
--- a/frontend/src/simulation/spice/connectDigitalInputsToMcu.ts
+++ b/frontend/src/simulation/spice/connectDigitalInputsToMcu.ts
@@ -43,7 +43,7 @@ export function connectDigitalInputsToMcu(): () => void {
   const lastLevel = new Map<string, boolean>();
 
   function injectDigitalInputs() {
-    const { nodeVoltages, pinNetMap } = useElectricalStore.getState();
+    const { nodeVoltages, pinNetMap, sourcedNets } = useElectricalStore.getState();
     const { boards } = useSimulatorStore.getState();
     for (const board of boards) {
       const sim = getBoardSimulator(board.id) as
@@ -58,6 +58,14 @@ export function connectDigitalInputsToMcu(): () => void {
         const gpio = gpioFromPinName(key.slice(prefix.length));
         if (gpio < 0) continue;
         if (driven.has(gpio)) continue; // the MCU drives this pin (digitalWrite)
+        // Only drive pins whose net is backed by a real source/element (rail,
+        // pull, button switch, divider, cross-board output, …). A net that is
+        // only floating (an event-driven part like a rotary encoder / keypad
+        // that has no SPICE model) is left to the part layer, which seeds the
+        // pin directly — otherwise its ~0 V floating read would force it LOW
+        // and fight the part. This is what makes it safe to enable
+        // spiceDrivenInputs on the AVR (which has many such part-driven pins).
+        if (!sourcedNets.has(net)) continue;
         const v = nodeVoltages[net];
         if (v == null) continue;
         const stateKey = `${board.id}:${gpio}`;
diff --git a/frontend/src/simulation/spice/start.ts b/frontend/src/simulation/spice/start.ts
index 0979c4ad..f583bc55 100644
--- a/frontend/src/simulation/spice/start.ts
+++ b/frontend/src/simulation/spice/start.ts
@@ -50,6 +50,7 @@ function createElectricalStorePort(): ElectricalStorePort {
         error: snapshot.warnings[0] ?? null,
         lastSolveMs: 0,
         submittedNetlist: '',
+        sourcedNets: snapshot.sourcedNets,
       });
     },
   };
diff --git a/frontend/src/store/useElectricalStore.ts b/frontend/src/store/useElectricalStore.ts
index b8e47f6b..d2a7a46b 100644
--- a/frontend/src/store/useElectricalStore.ts
+++ b/frontend/src/store/useElectricalStore.ts
@@ -25,6 +25,10 @@ export interface ElectricalSnapshot {
   error: string | null;
   lastSolveMs: number;
   submittedNetlist: string;
+  /** Nets backed by a real source/element (rail, GPIO V-source, pull, or any
+   *  component card). connectDigitalInputsToMcu only drives MCU input pins
+   *  whose net is here, so floating event-part pins aren't forced LOW. */
+  sourcedNets: Set<string>;
 }
 
 interface ElectricalState extends ElectricalSnapshot {
@@ -51,6 +55,7 @@ const EMPTY: ElectricalSnapshot = {
   error: null,
   lastSolveMs: 0,
   submittedNetlist: '',
+  sourcedNets: new Set(),
 };
 
 export const useElectricalStore = create<ElectricalState>((set) => ({