include/safety.h

@@ -0,0 +1,51 @@
+#ifndef SAFETY_H
+#define SAFETY_H
+
+#include <stdint.h>
+#include <stdbool.h>
+
+/*
+ * SaltyLab Safety Systems
+ *
+ * Covers:
+ *  - IWDG hardware watchdog (MCU reset if main loop hangs)
+ *  - RC signal timeout monitoring
+ *  - Tilt fault alert via buzzer
+ *  - Arm hold interlock (must hold arm for ARMING_HOLD_MS)
+ */
+
+/*
+ * safety_init() — call once in main() after HAL_Init().
+ * Starts IWDG with WATCHDOG_TIMEOUT_MS timeout from config.h.
+ * Starts ARMING_HOLD_MS countdown from config.h.
+ */
+void safety_init(void);
+
+/*
+ * safety_refresh() — call every main loop iteration.
+ * Resets IWDG counter. If not called within WATCHDOG_TIMEOUT_MS,
+ * the MCU will reset (independent of software — cannot be disabled).
+ */
+void safety_refresh(void);
+
+/*
+ * safety_rc_alive() — returns true if RC receiver has sent a frame
+ * within RC_TIMEOUT_MS. Call from the balance loop.
+ */
+bool safety_rc_alive(uint32_t now);
+
+/*
+ * safety_alert_tilt_fault() — one-shot buzzer beep for tilt fault.
+ * Safe to call repeatedly; only fires once per fault.
+ */
+void safety_alert_tilt_fault(bool faulted);
+
+/*
+ * safety_arm_interlock() — returns true once arm button has been
+ * held for ARMING_HOLD_MS from the moment safety_arm_start() was called.
+ */
+void safety_arm_start(uint32_t now);  /* Call when arm requested */
+bool safety_arm_ready(uint32_t now);  /* Poll until true, then arm */
+void safety_arm_cancel(void);         /* Cancel pending arm */
+
+#endif /* SAFETY_H */

src/main.c

@@ -8,6 +8,9 @@
 #include "hoverboard.h"
 #include "config.h"
 #include "status.h"
+#include "safety.h"
+#include "crsf.h"
+#include <math.h>
 #include <string.h>
 #include <stdio.h>
 
@@ -101,6 +104,14 @@ int main(void) {
     status_init();
     HAL_Delay(3000);  /* Wait for USB host to enumerate */
 
+    /*
+     * IWDG starts after the USB enumeration delay — avoids spurious reset
+     * during the 3s startup hold. Once started, safety_refresh() MUST be
+     * called at least every WATCHDOG_TIMEOUT_MS (50ms) or MCU resets.
+     * IWDG cannot be stopped once started (hardware enforced).
+     */
+    safety_init();
+
     /* Init IMU (MPU6000 via SPI1 — mpu6000.c wraps icm42688 + complementary filter) */
     int imu_ret = mpu6000_init() ? 0 : -1;
 
@@ -125,13 +136,34 @@ int main(void) {
 
         uint32_t now = HAL_GetTick();
 
-        /* Handle arm/disarm requests from USB */
+        /* Feed hardware watchdog — must happen every WATCHDOG_TIMEOUT_MS */
+        safety_refresh();
+
+        /* RC timeout: disarm if signal lost while armed */
+        if (bal.state == BALANCE_ARMED && !safety_rc_alive(now)) {
+            /* USB-only mode: no RC receiver expected yet — skip RC timeout.
+             * Uncomment when CRSF is wired: balance_disarm(&bal); */
+        }
+
+        /* Tilt fault buzzer alert (one-shot on fault edge) */
+        safety_alert_tilt_fault(bal.state == BALANCE_TILT_FAULT);
+
+        /* Handle arm/disarm requests from USB with arm-hold interlock */
         if (cdc_arm_request) {
             cdc_arm_request = 0;
+            /* Start the arm hold timer — motors don't enable until ARMING_HOLD_MS */
+            if (bal.state == BALANCE_DISARMED && fabsf(bal.pitch_deg) < 10.0f) {
+                safety_arm_start(now);
+            }
+        }
+        /* Complete arming once hold timer expires */
+        if (safety_arm_ready(now) && bal.state == BALANCE_DISARMED) {
+            safety_arm_cancel();
             balance_arm(&bal);
         }
         if (cdc_disarm_request) {
             cdc_disarm_request = 0;
+            safety_arm_cancel();
             balance_disarm(&bal);
         }
 

src/safety.c

@@ -0,0 +1,77 @@
+/*
+ * safety.c — SaltyLab Safety Systems
+ *
+ * IWDG: 40kHz LSI, prescaler 32 → tick = 0.8ms.
+ * WATCHDOG_TIMEOUT_MS from config.h (default 50ms → reload = 62).
+ * Formula: reload = (timeout_ms / (prescaler / 40000)) - 1
+ *          reload = (50 * 40000 / 32) - 1 = 62499 → clamp to 4095 (12-bit max)
+ * At PSC=256: tick = 6.4ms, reload = ceil(50/6.4)-1 = 7 → ~57.6ms
+ * Using PSC=32: tick = 0.8ms, reload = ceil(50/0.8)-1 = 62 → 50ms ✓
+ */
+
+#include "safety.h"
+#include "config.h"
+#include "crsf.h"
+#include "stm32f7xx_hal.h"
+
+/* IWDG prescaler 32 → LSI(40kHz)/32 = 1250 ticks/sec → 0.8ms/tick */
+#define IWDG_PRESCALER   IWDG_PRESCALER_32
+#define IWDG_TICK_MS     (32.0f / 40.0f)  /* 0.8ms per tick */
+#define IWDG_RELOAD      ((uint32_t)(WATCHDOG_TIMEOUT_MS / IWDG_TICK_MS))
+
+#if IWDG_RELOAD > 4095
+#  error "WATCHDOG_TIMEOUT_MS too large for IWDG_PRESCALER_32 — increase prescaler"
+#endif
+
+static IWDG_HandleTypeDef hiwdg;
+
+/* Arm interlock */
+static uint32_t s_arm_start_ms = 0;
+static bool     s_arm_pending  = false;
+
+/* Tilt fault alert state — edge-detect to fire buzzer once */
+static bool s_was_faulted = false;
+
+void safety_init(void) {
+    hiwdg.Instance       = IWDG;
+    hiwdg.Init.Prescaler = IWDG_PRESCALER;
+    hiwdg.Init.Reload    = IWDG_RELOAD;
+    hiwdg.Init.Window    = IWDG_WINDOW_DISABLE;
+    HAL_IWDG_Init(&hiwdg);  /* Starts watchdog immediately */
+}
+
+void safety_refresh(void) {
+    HAL_IWDG_Refresh(&hiwdg);
+}
+
+bool safety_rc_alive(uint32_t now) {
+    /* If crsf_state has never received a frame, last_rx_ms == 0 */
+    if (crsf_state.last_rx_ms == 0) return false;
+    return (now - crsf_state.last_rx_ms) < RC_TIMEOUT_MS;
+}
+
+void safety_alert_tilt_fault(bool faulted) {
+    if (faulted && !s_was_faulted) {
+        /* Rising edge: single buzzer burst to alert rider */
+        HAL_GPIO_WritePin(BEEPER_PORT, BEEPER_PIN, GPIO_PIN_SET);
+        HAL_Delay(200);
+        HAL_GPIO_WritePin(BEEPER_PORT, BEEPER_PIN, GPIO_PIN_RESET);
+    }
+    s_was_faulted = faulted;
+}
+
+void safety_arm_start(uint32_t now) {
+    if (!s_arm_pending) {
+        s_arm_start_ms = now;
+        s_arm_pending  = true;
+    }
+}
+
+bool safety_arm_ready(uint32_t now) {
+    if (!s_arm_pending) return false;
+    return (now - s_arm_start_ms) >= ARMING_HOLD_MS;
+}
+
+void safety_arm_cancel(void) {
+    s_arm_pending = false;
+}