include/balance.h

@@ -3,6 +3,7 @@
 
 #include <stdint.h>
 #include "mpu6000.h"
+#include "slope_estimator.h"
 
 /*
  * SaltyLab Balance Controller
@@ -36,6 +37,9 @@ typedef struct {
     /* Safety */
     float max_tilt;         /* Cutoff angle (degrees) */
     int16_t max_speed;      /* Speed limit */
+
+    /* Slope compensation (Issue #600) */
+    slope_estimator_t slope;
 } balance_t;
 
 void balance_init(balance_t *b);

include/jlink.h

@@ -87,6 +87,7 @@
 #define JLINK_TLM_SCHED         0x85u  /* jlink_tlm_sched_t (1+N*16 bytes, Issue #550) */
 #define JLINK_TLM_MOTOR_CURRENT 0x86u  /* jlink_tlm_motor_current_t (8 bytes, Issue #584) */
 #define JLINK_TLM_FAULT_LOG     0x87u  /* jlink_tlm_fault_log_t (20 bytes, Issue #565) */
+#define JLINK_TLM_SLOPE         0x88u  /* jlink_tlm_slope_t (4 bytes, Issue #600) */
 
 /* ---- Telemetry STATUS payload (20 bytes, packed) ---- */
 typedef struct __attribute__((packed)) {
@@ -164,6 +165,14 @@ typedef struct __attribute__((packed)) {
     uint8_t  _pad;         /* reserved                                               */
 } jlink_tlm_motor_current_t;  /* 8 bytes */
 
+/* ---- Telemetry SLOPE payload (4 bytes, packed) Issue #600 ---- */
+/* Published at SLOPE_TLM_HZ (1 Hz); reports slow-adapting terrain slope estimate. */
+typedef struct __attribute__((packed)) {
+    int16_t  slope_x100;  /* slope estimate degrees x100 (0.01° resolution) */
+    uint8_t  active;      /* 1 = slope compensation enabled                 */
+    uint8_t  _pad;        /* reserved                                        */
+} jlink_tlm_slope_t;  /* 4 bytes */
+
 /* ---- Telemetry FAULT_LOG payload (20 bytes, packed) Issue #565 ---- */
 /* Sent on boot (if last fault != NONE) and in response to FAULT_LOG_GET. */
 typedef struct __attribute__((packed)) {
@@ -272,6 +281,13 @@ JLinkSchedSetBuf *jlink_get_sched_set(void);
  */
 void jlink_send_motor_current_tlm(const jlink_tlm_motor_current_t *tlm);
 
+/*
+ * jlink_send_slope_tlm(tlm) - transmit JLINK_TLM_SLOPE (0x88) frame
+ * (10 bytes total) to Jetson.  Issue #600.
+ * Rate-limiting is handled by slope_estimator_send_tlm(); call from there only.
+ */
+void jlink_send_slope_tlm(const jlink_tlm_slope_t *tlm);
+
 /*
  * jlink_send_fault_log(fl) - transmit JLINK_TLM_FAULT_LOG (0x87) frame
  * (26 bytes) on boot (if fault log non-empty) and in response to

include/slope_estimator.h

@@ -0,0 +1,101 @@
+#ifndef SLOPE_ESTIMATOR_H
+#define SLOPE_ESTIMATOR_H
+
+#include <stdint.h>
+#include <stdbool.h>
+
+/*
+ * slope_estimator — slow-adapting terrain slope estimator for Issue #600.
+ *
+ * On a slope the robot must lean slightly into the hill to stay balanced.
+ * The IMU pitch reading therefore includes both the robot's balance offset
+ * and the ground incline.  This module decouples the two by tracking the
+ * slowly-changing DC component of the pitch signal with a first-order IIR
+ * low-pass filter (time constant SLOPE_TAU_S = 5 s).
+ *
+ * HOW IT WORKS:
+ *   Every call to slope_estimator_update(pitch_deg, dt):
+ *
+ *     alpha    = dt / (SLOPE_TAU_S + dt)       // ≈ 0.0002 at 1 kHz
+ *     raw      = slope * (1 - alpha) + pitch * alpha
+ *     slope    = clamp(raw, -SLOPE_MAX_DEG, +SLOPE_MAX_DEG)
+ *
+ *   The IIR converges to the steady-state pitch in ~5 s.  Fast tilt
+ *   transients (balance corrections, steps, bumps) are attenuated by
+ *   the long time constant and do not corrupt the estimate.
+ *
+ * INTEGRATION IN BALANCE PID:
+ *   Subtract slope_estimate from the measured pitch before computing
+ *   the PID error so the controller balances around the slope surface
+ *   rather than absolute vertical:
+ *
+ *     tilt_corrected = pitch_deg - slope_estimate_deg
+ *     error = setpoint - tilt_corrected
+ *
+ *   This is equivalent to continuously adjusting the balance setpoint
+ *   to track the incline.
+ *
+ * SAFETY:
+ *   - Estimate is clamped to ±SLOPE_MAX_DEG (15°) to prevent drift from
+ *     extreme falls being mistaken for genuine slopes.
+ *   - slope_estimator_reset() zeroes the state; call on disarm or after
+ *     a tilt fault so re-arming starts fresh.
+ *
+ * TELEMETRY:
+ *   JLINK_TLM_SLOPE (0x88) published at SLOPE_TLM_HZ (1 Hz):
+ *     jlink_tlm_slope_t { int16 slope_x100, uint8 active, uint8 _pad }
+ */
+
+/* ---- Configuration ---- */
+#define SLOPE_TAU_S         5.0f    /* IIR time constant (seconds)          */
+#define SLOPE_MAX_DEG      15.0f    /* Maximum estimate magnitude (degrees) */
+#define SLOPE_TLM_HZ        1u      /* JLINK_TLM_SLOPE publish rate (Hz)    */
+
+/* ---- State ---- */
+typedef struct {
+    float    estimate_deg;   /* current slope estimate (degrees, + = nose-up) */
+    bool     enabled;        /* compensation on/off; off = estimate frozen     */
+    uint32_t last_tlm_ms;   /* timestamp of last TLM transmission             */
+} slope_estimator_t;
+
+/* ---- API ---- */
+
+/*
+ * slope_estimator_init(se) — zero state, enable estimation.
+ * Call once during system init.
+ */
+void slope_estimator_init(slope_estimator_t *se);
+
+/*
+ * slope_estimator_reset(se) — zero estimate without changing enabled flag.
+ * Call on disarm or after BALANCE_TILT_FAULT to avoid stale state on rearm.
+ */
+void slope_estimator_reset(slope_estimator_t *se);
+
+/*
+ * slope_estimator_update(se, pitch_deg, dt) — advance the IIR filter.
+ *   pitch_deg : current fused pitch angle from IMU (degrees)
+ *   dt        : loop interval (seconds)
+ * No-op if se->enabled == false or dt <= 0.
+ */
+void slope_estimator_update(slope_estimator_t *se, float pitch_deg, float dt);
+
+/*
+ * slope_estimator_get_deg(se) — return current estimate (degrees).
+ * Returns 0 if disabled.
+ */
+float slope_estimator_get_deg(const slope_estimator_t *se);
+
+/*
+ * slope_estimator_set_enabled(se, en) — enable or disable compensation.
+ * Disabling freezes the estimate at its current value.
+ */
+void slope_estimator_set_enabled(slope_estimator_t *se, bool en);
+
+/*
+ * slope_estimator_send_tlm(se, now_ms) — transmit JLINK_TLM_SLOPE (0x88)
+ * frame to Jetson.  Rate-limited to SLOPE_TLM_HZ; safe to call every tick.
+ */
+void slope_estimator_send_tlm(const slope_estimator_t *se, uint32_t now_ms);
+
+#endif /* SLOPE_ESTIMATOR_H */

src/balance.c

@@ -1,4 +1,5 @@
 #include "balance.h"
+#include "slope_estimator.h"
 #include "config.h"
 #include <math.h>
 
@@ -19,6 +20,9 @@ void balance_init(balance_t *b) {
     /* Safety defaults from config */
     b->max_tilt = MAX_TILT_DEG;
     b->max_speed = MAX_SPEED_LIMIT;
+
+    /* Slope estimator */
+    slope_estimator_init(&b->slope);
 }
 
 void balance_update(balance_t *b, const IMUData *imu, float dt) {
@@ -28,12 +32,16 @@ void balance_update(balance_t *b, const IMUData *imu, float dt) {
     b->pitch_deg  = imu->pitch;
     b->pitch_rate = imu->pitch_rate;
 
+    /* Advance slope estimator (no-op when not armed) */
+    slope_estimator_update(&b->slope, b->pitch_deg, dt);
+
     /* Safety: tilt cutoff */
     if (b->state == BALANCE_ARMED) {
         if (fabsf(b->pitch_deg) > b->max_tilt) {
             b->state = BALANCE_TILT_FAULT;
             b->motor_cmd = 0;
             b->integral = 0.0f;
+            slope_estimator_reset(&b->slope);
             return;
         }
     }
@@ -45,8 +53,9 @@ void balance_update(balance_t *b, const IMUData *imu, float dt) {
         return;
     }
 
-    /* PID */
+    /* PID — subtract slope estimate so controller balances on incline */
-    float error = b->setpoint - b->pitch_deg;
+    float tilt_corrected = b->pitch_deg - slope_estimator_get_deg(&b->slope);
+    float error = b->setpoint - tilt_corrected;
 
     /* Proportional */
     float p_term = b->kp * error;
@@ -85,4 +94,5 @@ void balance_disarm(balance_t *b) {
     b->state = BALANCE_DISARMED;
     b->motor_cmd = 0;
     b->integral = 0.0f;
+    slope_estimator_reset(&b->slope);
 }

src/jlink.c

@@ -541,6 +541,30 @@ void jlink_send_sched_telemetry(const jlink_tlm_sched_t *tlm)
     jlink_tx_locked(frame, (uint16_t)(3u + plen + 3u));
 }
 
+/* ---- jlink_send_slope_tlm() -- Issue #600 ---- */
+void jlink_send_slope_tlm(const jlink_tlm_slope_t *tlm)
+{
+    /*
+     * Frame: [STX][LEN][0x88][4 bytes SLOPE][CRC_hi][CRC_lo][ETX]
+     * Total: 1+1+1+4+2+1 = 10 bytes
+     */
+    static uint8_t frame[10];
+    const uint8_t  plen = (uint8_t)sizeof(jlink_tlm_slope_t);  /* 4 */
+    const uint8_t  len  = 1u + plen;  /* CMD byte + payload */
+
+    frame[0] = JLINK_STX;
+    frame[1] = len;
+    frame[2] = JLINK_TLM_SLOPE;
+    memcpy(&frame[3], tlm, plen);
+
+    uint16_t crc = crc16_xmodem(&frame[2], len);
+    frame[3 + plen]     = (uint8_t)(crc >> 8);
+    frame[3 + plen + 1] = (uint8_t)(crc & 0xFFu);
+    frame[3 + plen + 2] = JLINK_ETX;
+
+    jlink_tx_locked(frame, sizeof(frame));
+}
+
 /* ---- jlink_send_fault_log() -- Issue #565 ---- */
 void jlink_send_fault_log(const jlink_tlm_fault_log_t *fl)
 {

src/slope_estimator.c

@@ -0,0 +1,82 @@
+/*
+ * slope_estimator.c — slow-adapting terrain slope estimator (Issue #600).
+ *
+ * First-order IIR low-pass filter on fused IMU pitch with tau = SLOPE_TAU_S (5 s).
+ * The long time constant means fast balance corrections and transients are
+ * ignored; only sustained pitch bias (i.e., genuine slope) accumulates.
+ *
+ * Estimate is clamped to ±SLOPE_MAX_DEG (15°) per the safety requirement.
+ */
+
+#include "slope_estimator.h"
+#include "jlink.h"
+
+/* ---- slope_estimator_init() ---- */
+void slope_estimator_init(slope_estimator_t *se)
+{
+    se->estimate_deg  = 0.0f;
+    se->enabled       = true;
+    /* Initialize so first send_tlm call fires immediately */
+    se->last_tlm_ms   = (uint32_t)(-(uint32_t)(1000u / (SLOPE_TLM_HZ > 0u ? SLOPE_TLM_HZ : 1u)));
+}
+
+/* ---- slope_estimator_reset() ---- */
+void slope_estimator_reset(slope_estimator_t *se)
+{
+    se->estimate_deg = 0.0f;
+}
+
+/* ---- slope_estimator_update() ---- */
+void slope_estimator_update(slope_estimator_t *se, float pitch_deg, float dt)
+{
+    if (!se->enabled || dt <= 0.0f) return;
+
+    /*
+     * First-order IIR: alpha = dt / (tau + dt)
+     * At 1 kHz (dt=0.001): alpha ≈ 0.0002  → time to ~63% ≈ 5 s
+     * At 100 Hz (dt=0.01):  alpha ≈ 0.002   → time to ~63% ≈ 5 s (same tau)
+     */
+    float alpha = dt / (SLOPE_TAU_S + dt);
+    float raw   = se->estimate_deg * (1.0f - alpha) + pitch_deg * alpha;
+
+    /* Clamp to ±SLOPE_MAX_DEG */
+    if (raw >  SLOPE_MAX_DEG) raw =  SLOPE_MAX_DEG;
+    if (raw < -SLOPE_MAX_DEG) raw = -SLOPE_MAX_DEG;
+
+    se->estimate_deg = raw;
+}
+
+/* ---- slope_estimator_get_deg() ---- */
+float slope_estimator_get_deg(const slope_estimator_t *se)
+{
+    if (!se->enabled) return 0.0f;
+    return se->estimate_deg;
+}
+
+/* ---- slope_estimator_set_enabled() ---- */
+void slope_estimator_set_enabled(slope_estimator_t *se, bool en)
+{
+    se->enabled = en;
+}
+
+/* ---- slope_estimator_send_tlm() ---- */
+void slope_estimator_send_tlm(const slope_estimator_t *se, uint32_t now_ms)
+{
+    if (SLOPE_TLM_HZ == 0u) return;
+
+    uint32_t interval_ms = 1000u / SLOPE_TLM_HZ;
+    if ((now_ms - se->last_tlm_ms) < interval_ms) return;
+    /* Cast away const for timestamp update -- only mutable field */
+    ((slope_estimator_t *)se)->last_tlm_ms = now_ms;
+
+    jlink_tlm_slope_t tlm;
+    /* Scale to ×100 for 0.01° resolution, clamped to int16 range */
+    float scaled = se->estimate_deg * 100.0f;
+    if (scaled >  32767.0f) scaled =  32767.0f;
+    if (scaled < -32768.0f) scaled = -32768.0f;
+    tlm.slope_x100 = (int16_t)scaled;
+    tlm.active     = se->enabled ? 1u : 0u;
+    tlm._pad       = 0u;
+
+    jlink_send_slope_tlm(&tlm);
+}

test/test_slope_estimator.c

@@ -0,0 +1,421 @@
+/*
+ * test_slope_estimator.c — Unit tests for slope estimator (Issue #600).
+ *
+ * Build (host, no hardware):
+ *   gcc -I include -I test/stubs -DTEST_HOST -lm \
+ *       -o /tmp/test_slope src/slope_estimator.c test/test_slope_estimator.c
+ *
+ * All tests are self-contained; no HAL, no ADC, no UART required.
+ * slope_estimator.c calls jlink_send_slope_tlm() which is stubbed below.
+ */
+
+/* ---- Stubs ---- */
+
+/* Prevent jlink.h from pulling in HAL / pid_flash types */
+#define JLINK_H
+#include <stdint.h>
+#include <stdbool.h>
+
+/* Minimal jlink_tlm_slope_t matching the real definition */
+typedef struct __attribute__((packed)) {
+    int16_t  slope_x100;
+    uint8_t  active;
+    uint8_t  _pad;
+} jlink_tlm_slope_t;
+
+/* Capture last transmitted tlm for inspection */
+static jlink_tlm_slope_t g_last_tlm;
+static int g_tlm_count = 0;
+
+void jlink_send_slope_tlm(const jlink_tlm_slope_t *tlm)
+{
+    g_last_tlm = *tlm;
+    g_tlm_count++;
+}
+
+/* ---- Include implementation directly ---- */
+#include "../src/slope_estimator.c"
+
+/* ---- Test framework ---- */
+#include <stdio.h>
+#include <math.h>
+#include <string.h>
+
+static int g_pass = 0;
+static int g_fail = 0;
+
+#define ASSERT(cond, msg) do { \
+    if (cond) { g_pass++; } \
+    else { g_fail++; printf("FAIL [%s:%d] %s\n", __FILE__, __LINE__, msg); } \
+} while(0)
+
+#define ASSERT_NEAR(a, b, tol, msg) \
+    ASSERT(fabsf((a)-(b)) <= (tol), msg)
+
+/* ---- Tests ---- */
+
+static void test_init_zeroes_state(void)
+{
+    slope_estimator_t se;
+    /* dirty the memory first */
+    memset(&se, 0xAB, sizeof(se));
+    slope_estimator_init(&se);
+    ASSERT(se.estimate_deg == 0.0f,  "init: estimate_deg == 0");
+    ASSERT(se.enabled == true,        "init: enabled == true");
+}
+
+static void test_get_when_disabled_returns_zero(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    /* Force a non-zero estimate by running a few ticks, then disable */
+    slope_estimator_update(&se, 10.0f, 0.1f);
+    slope_estimator_set_enabled(&se, false);
+    ASSERT(slope_estimator_get_deg(&se) == 0.0f,
+           "get while disabled must return 0");
+}
+
+static void test_update_when_disabled_no_change(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    slope_estimator_set_enabled(&se, false);
+    slope_estimator_update(&se, 10.0f, 0.01f);
+    ASSERT(se.estimate_deg == 0.0f,
+           "update while disabled must not change estimate");
+}
+
+static void test_update_zero_dt_no_change(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    slope_estimator_update(&se, 5.0f, 0.0f);
+    ASSERT(se.estimate_deg == 0.0f,
+           "update with dt=0 must not change estimate");
+}
+
+static void test_update_negative_dt_no_change(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    slope_estimator_update(&se, 5.0f, -0.001f);
+    ASSERT(se.estimate_deg == 0.0f,
+           "update with dt<0 must not change estimate");
+}
+
+static void test_single_step_converges_toward_input(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    /* One update with dt=0.01s, pitch=10 deg */
+    slope_estimator_update(&se, 10.0f, 0.01f);
+    /* alpha = 0.01 / (5.0 + 0.01) ≈ 0.002; new estimate ≈ 0 + 0.002*10 = 0.02 */
+    ASSERT(se.estimate_deg > 0.0f,
+           "estimate should move toward positive pitch");
+    ASSERT(se.estimate_deg < 10.0f,
+           "estimate should not jump to full pitch in one step");
+}
+
+static void test_iir_reaches_63pct_in_one_tau(void)
+{
+    /* Run estimator at 1 kHz for exactly SLOPE_TAU_S seconds */
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    const float pitch   = 10.0f;
+    const float dt      = 0.001f;
+    const int   steps   = (int)(SLOPE_TAU_S / dt);  /* 5000 steps */
+    for (int i = 0; i < steps; i++) {
+        slope_estimator_update(&se, pitch, dt);
+    }
+    /* IIR should be at ≈63.2% of the step */
+    float expected_lo = pitch * 0.60f;
+    float expected_hi = pitch * 0.66f;
+    ASSERT(se.estimate_deg >= expected_lo && se.estimate_deg <= expected_hi,
+           "IIR should reach ~63% of step at t=tau");
+}
+
+static void test_iir_reaches_63pct_at_100hz(void)
+{
+    /* Same tau, but at 100 Hz */
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    const float pitch   = 8.0f;
+    const float dt      = 0.01f;
+    const int   steps   = (int)(SLOPE_TAU_S / dt);  /* 500 steps */
+    for (int i = 0; i < steps; i++) {
+        slope_estimator_update(&se, pitch, dt);
+    }
+    float expected_lo = pitch * 0.60f;
+    float expected_hi = pitch * 0.66f;
+    ASSERT(se.estimate_deg >= expected_lo && se.estimate_deg <= expected_hi,
+           "IIR 100 Hz: should reach ~63% at t=tau");
+}
+
+static void test_positive_clamp(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    /* Drive with a huge pitch for a long time — estimate must clamp at SLOPE_MAX_DEG */
+    for (int i = 0; i < 100000; i++) {
+        slope_estimator_update(&se, 90.0f, 0.001f);
+    }
+    ASSERT_NEAR(se.estimate_deg, SLOPE_MAX_DEG, 0.001f,
+                "estimate must clamp at +SLOPE_MAX_DEG");
+}
+
+static void test_negative_clamp(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    for (int i = 0; i < 100000; i++) {
+        slope_estimator_update(&se, -90.0f, 0.001f);
+    }
+    ASSERT_NEAR(se.estimate_deg, -SLOPE_MAX_DEG, 0.001f,
+                "estimate must clamp at -SLOPE_MAX_DEG");
+}
+
+static void test_reset_zeroes_estimate(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    /* Build up some estimate */
+    for (int i = 0; i < 1000; i++) {
+        slope_estimator_update(&se, 12.0f, 0.001f);
+    }
+    ASSERT(se.estimate_deg > 0.1f, "pre-reset: estimate should be non-zero");
+    slope_estimator_reset(&se);
+    ASSERT(se.estimate_deg == 0.0f, "post-reset: estimate should be zero");
+    /* enabled state must be preserved */
+    ASSERT(se.enabled == true, "reset must not change enabled flag");
+}
+
+static void test_reset_preserves_disabled_state(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    slope_estimator_set_enabled(&se, false);
+    slope_estimator_reset(&se);
+    ASSERT(se.enabled == false, "reset must not re-enable disabled estimator");
+}
+
+static void test_balance_setpoint_compensation(void)
+{
+    /*
+     * Simulate: robot on 5-deg slope, pitch = 5 deg, slope estimate converges.
+     * After convergence, tilt_corrected = pitch - slope ≈ 0.
+     * Verify that the effective PID error (setpoint=0, tilt_corrected≈0) is near 0.
+     */
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    const float slope_deg = 5.0f;
+    const float dt        = 0.001f;
+    const int   steps     = (int)(3.0f * SLOPE_TAU_S / dt);  /* 3 tau = ~95% converged */
+
+    for (int i = 0; i < steps; i++) {
+        /* Robot holds steady on slope — pitch stays constant */
+        slope_estimator_update(&se, slope_deg, dt);
+    }
+
+    float est   = slope_estimator_get_deg(&se);
+    float tilt_corrected = slope_deg - est;
+
+    /* After 3 tau, estimate should be within 5% of slope */
+    ASSERT(est >= slope_deg * 0.90f, "3-tau estimate >= 90% of slope");
+    /* tilt_corrected should be close to zero — minimal PID error */
+    ASSERT(fabsf(tilt_corrected) < slope_deg * 0.10f,
+           "corrected tilt error < 10% of slope after 3 tau");
+}
+
+static void test_fast_tilt_not_tracked(void)
+{
+    /*
+     * Simulate: robot starts balanced (pitch≈0), then tips to 20 deg suddenly
+     * for 100 ms (balance intervention), then returns to 0.
+     * The slope estimate should not move significantly.
+     */
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    const float dt = 0.001f;
+
+    /* 2 s of stable balancing at pitch ≈ 0 */
+    for (int i = 0; i < 2000; i++) {
+        slope_estimator_update(&se, 0.0f, dt);
+    }
+    float before = se.estimate_deg;
+
+    /* 100 ms spike to 20 deg (fast tilt) */
+    for (int i = 0; i < 100; i++) {
+        slope_estimator_update(&se, 20.0f, dt);
+    }
+    /* Return to 0 for another 100 ms */
+    for (int i = 0; i < 100; i++) {
+        slope_estimator_update(&se, 0.0f, dt);
+    }
+    float after = se.estimate_deg;
+
+    /* 200ms / 5000ms = 4% of tau — estimate should move < 1 deg */
+    ASSERT(fabsf(after - before) < 1.0f,
+           "fast tilt transient should not significantly move slope estimate");
+}
+
+static void test_slope_change_tracks_slowly(void)
+{
+    /* Robot transitions from flat (0°) to 10° slope at t=0.
+     * After 1 tau, estimate should be ~63% of 10 = ~6.3 deg. */
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    const float dt     = 0.001f;
+    const int   steps  = (int)(SLOPE_TAU_S / dt);
+
+    for (int i = 0; i < steps; i++) {
+        slope_estimator_update(&se, 10.0f, dt);
+    }
+    ASSERT(se.estimate_deg >= 6.0f && se.estimate_deg <= 6.5f,
+           "slope change: 1-tau estimate should be 6.0-6.5 deg");
+}
+
+static void test_symmetry_negative_slope(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    const float dt    = 0.001f;
+    const int   steps = (int)(SLOPE_TAU_S / dt);
+
+    for (int i = 0; i < steps; i++) {
+        slope_estimator_update(&se, -8.0f, dt);
+    }
+    /* Should be ~63% of -8 */
+    ASSERT(se.estimate_deg <= -4.0f && se.estimate_deg >= -5.5f,
+           "negative slope: estimate should converge symmetrically");
+}
+
+static void test_enable_disable_toggle(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+
+    /* Build estimate */
+    for (int i = 0; i < 2000; i++) {
+        slope_estimator_update(&se, 10.0f, 0.001f);
+    }
+    float val = se.estimate_deg;
+    ASSERT(val > 0.1f, "estimate built before disable");
+
+    /* Disable: estimate frozen */
+    slope_estimator_set_enabled(&se, false);
+    slope_estimator_update(&se, 10.0f, 0.001f);
+    ASSERT(se.estimate_deg == val, "estimate frozen while disabled");
+
+    /* get returns 0 when disabled */
+    ASSERT(slope_estimator_get_deg(&se) == 0.0f, "get returns 0 while disabled");
+
+    /* Re-enable: estimate resumes */
+    slope_estimator_set_enabled(&se, true);
+    slope_estimator_update(&se, 10.0f, 0.001f);
+    ASSERT(se.estimate_deg > val, "estimate resumes after re-enable");
+}
+
+static void test_tlm_rate_limiting(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+
+    g_tlm_count = 0;
+
+    /* Call at 1000 Hz for 3 seconds → should send 3 TLMs (at 1 Hz rate limit) */
+    for (uint32_t ms = 0; ms < 3000; ms++) {
+        slope_estimator_send_tlm(&se, ms);
+    }
+    /* First call at ms=0 sends, then 1000ms, 2000ms → 3 total */
+    ASSERT(g_tlm_count == 3, "TLM rate-limited to SLOPE_TLM_HZ (1 Hz)");
+}
+
+static void test_tlm_payload_encoding(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+
+    /* Force a known estimate */
+    se.estimate_deg = 7.5f;
+    se.enabled = true;
+
+    g_tlm_count = 0;
+    g_last_tlm.slope_x100 = 0;
+
+    /* Trigger a TLM by advancing time enough */
+    slope_estimator_send_tlm(&se, 0u);
+
+    ASSERT(g_tlm_count == 1, "TLM sent");
+    ASSERT(g_last_tlm.slope_x100 == 750, "7.5 deg -> slope_x100 = 750");
+    ASSERT(g_last_tlm.active == 1u, "active flag set when enabled");
+}
+
+static void test_tlm_disabled_active_flag(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    slope_estimator_set_enabled(&se, false);
+    se.estimate_deg = 3.0f;
+
+    g_tlm_count = 0;
+    slope_estimator_send_tlm(&se, 0u);
+
+    ASSERT(g_tlm_count == 1, "TLM sent when disabled");
+    ASSERT(g_last_tlm.active == 0u, "active flag clear when disabled");
+    /* slope_x100 reflects stored estimate but get() returns 0 */
+    ASSERT(g_last_tlm.slope_x100 == 300, "slope_x100 encodes stored estimate");
+}
+
+static void test_tlm_clamped_to_int16(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    /* Manually set beyond int16 range for test */
+    se.estimate_deg = 400.0f;  /* 400 * 100 = 40000, within int16 */
+    se.enabled = true;
+    g_tlm_count = 0;
+    slope_estimator_send_tlm(&se, 0u);
+    ASSERT(g_last_tlm.slope_x100 == 32767, "large estimate clamped to INT16_MAX");
+}
+
+static void test_multiple_resets_idempotent(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    slope_estimator_reset(&se);
+    slope_estimator_reset(&se);
+    ASSERT(se.estimate_deg == 0.0f, "multiple resets: still zero");
+    ASSERT(se.enabled == true, "multiple resets: still enabled");
+}
+
+/* ---- main ---- */
+int main(void)
+{
+    printf("=== test_slope_estimator ===\n");
+
+    test_init_zeroes_state();
+    test_get_when_disabled_returns_zero();
+    test_update_when_disabled_no_change();
+    test_update_zero_dt_no_change();
+    test_update_negative_dt_no_change();
+    test_single_step_converges_toward_input();
+    test_iir_reaches_63pct_in_one_tau();
+    test_iir_reaches_63pct_at_100hz();
+    test_positive_clamp();
+    test_negative_clamp();
+    test_reset_zeroes_estimate();
+    test_reset_preserves_disabled_state();
+    test_balance_setpoint_compensation();
+    test_fast_tilt_not_tracked();
+    test_slope_change_tracks_slowly();
+    test_symmetry_negative_slope();
+    test_enable_disable_toggle();
+    test_tlm_rate_limiting();
+    test_tlm_payload_encoding();
+    test_tlm_disabled_active_flag();
+    test_tlm_clamped_to_int16();
+    test_multiple_resets_idempotent();
+
+    printf("\nResults: %d passed, %d failed\n", g_pass, g_fail);
+    return g_fail ? 1 : 0;
+}