[mlx90614] pec validation (#6689)

Co-authored-by: Jonathan Swoboda <154711427+swoboda1337@users.noreply.github.com> Co-authored-by: Jesse Hills <3060199+jesserockz@users.noreply.github.com>
2026-09-11 15:27:33 +00:00 · 2026-09-10 16:14:43 +12:00
parent ddbd89dd2a
commit 05f7d5e4f1
2 changed files with 145 additions and 47 deletions
@@ -26,44 +26,129 @@ static const uint8_t MLX90614_ID4 = 0x3F;

 static const char *const TAG = "mlx90614";

+// The EEPROM cell has a limited number of write cycles, so stop retrying after a few failures
+static constexpr uint8_t EMISSIVITY_WRITE_ATTEMPTS = 3;
+
+// SMBus packet error code: CRC-8 with polynomial 0x07, MSB first
+static uint8_t crc8_pec(const uint8_t *data, uint8_t len) { return crc8(data, len, 0x00, 0x07, true); }
+
 void MLX90614Component::setup() {
-  if (!this->write_emissivity_()) {
-    ESP_LOGE(TAG, ESP_LOG_MSG_COMM_FAIL);
-    this->mark_failed();
+  if (std::isnan(this->emissivity_)) {
    return;
  }
+  this->emissivity_write_attempts_ = EMISSIVITY_WRITE_ATTEMPTS;
+  this->try_write_emissivity_();
+  if (this->emissivity_write_attempts_ != 0) {
+    this->status_set_warning(LOG_STR("Failed to write emissivity, will retry"));
+  }
+}
+
+void MLX90614Component::try_write_emissivity_() {
+  if (this->emissivity_write_attempts_ == 0) {
+    return;
+  }
+  if (this->write_emissivity_()) {
+    this->emissivity_write_attempts_ = 0;
+    return;
+  }
+  if (--this->emissivity_write_attempts_ == 0) {
+    ESP_LOGE(TAG, "Giving up on writing emissivity after %u attempts", EMISSIVITY_WRITE_ATTEMPTS);
+    this->emissivity_write_failed_ = true;
+  }
 }

 bool MLX90614Component::write_emissivity_() {
-  if (std::isnan(this->emissivity_))
+  // Skip the write when the EEPROM already holds the desired value to save write cycles
+  uint16_t current_emissivity;
+  if (this->read_register_(MLX90614_EMISSIVITY, current_emissivity) != i2c::ERROR_OK) {
+    return false;
+  }
+
+  const auto desired_emissivity = static_cast<uint16_t>(this->emissivity_ * 0xFFFF);
+  if (current_emissivity == desired_emissivity) {
    return true;
-  uint16_t value = (uint16_t) (this->emissivity_ * 65535);
-  if (!this->write_bytes_(MLX90614_EMISSIVITY, 0)) {
-    return false;
  }
-  delay(10);
-  if (!this->write_bytes_(MLX90614_EMISSIVITY, value)) {
-    return false;
-  }
-  delay(10);
-  return true;
+
+  return this->write_register_(MLX90614_EMISSIVITY, desired_emissivity);
 }

-bool MLX90614Component::write_bytes_(uint8_t reg, uint16_t data) {
+bool MLX90614Component::write_register_(uint8_t reg, uint16_t data) {
+  // The PEC covers the whole write transaction: SLA+W, command, data low, data high
  uint8_t buf[5];
  buf[0] = this->address_ << 1;
  buf[1] = reg;
-  buf[2] = data & 0xFF;
-  buf[3] = data >> 8;
-  buf[4] = crc8(buf, 4, 0x00, 0x07, true);
-  return this->write_bytes(reg, buf + 2, 3);
+
+  // See datasheet 8.3.3.1 EEPROM write sequence
+  // 1. Write 0x0000 into the cell of interest (erases the cell)
+  buf[2] = buf[3] = 0;
+  buf[4] = crc8_pec(buf, 4);
+  auto ec = this->write_register(reg, buf + 2, 3);
+  if (ec != i2c::ERROR_OK) {
+    ESP_LOGW(TAG, "Can't erase register 0x%02X, error %d", reg, ec);
+    return false;
+  }
+
+  // 2. Wait at least 5ms
+  delay(10);
+
+  // 3. Write the new value
+  if (data != 0) {
+    buf[2] = data & 0xFF;
+    buf[3] = data >> 8;
+    buf[4] = crc8_pec(buf, 4);
+    ec = this->write_register(reg, buf + 2, 3);
+    if (ec != i2c::ERROR_OK) {
+      ESP_LOGW(TAG, "Can't write register 0x%02X, error %d", reg, ec);
+      return false;
+    }
+    // 4. Wait at least 5ms
+    delay(10);
+  }
+
+  // 5. Read back to confirm the value was stored
+  uint16_t read_back;
+  ec = this->read_register_(reg, read_back);
+  if (ec != i2c::ERROR_OK) {
+    ESP_LOGW(TAG, "Can't check register 0x%02X value, error %d", reg, ec);
+    return false;
+  }
+
+  if (read_back != data) {
+    ESP_LOGW(TAG, "Read back mismatch on register 0x%02X. Expected 0x%04X, got 0x%04X", reg, data, read_back);
+    return false;
+  }
+
+  return true;
+}
+
+i2c::ErrorCode MLX90614Component::read_register_(uint8_t reg, uint16_t &data) {
+  // The PEC covers the whole read transaction: SLA+W, command, SLA+R, data low, data high
+  uint8_t buf[6];
+  buf[0] = this->address_ << 1;
+  buf[1] = reg;
+  buf[2] = (this->address_ << 1) | 0x01;
+
+  const auto ec = this->read_register(reg, buf + 3, 3);
+  if (ec != i2c::ERROR_OK) {
+    ESP_LOGW(TAG, "i2c read error %d", ec);
+    return ec;
+  }
+
+  const auto expected_pec = crc8_pec(buf, 5);
+  if (buf[5] != expected_pec) {
+    ESP_LOGW(TAG, "i2c CRC error. Expected 0x%02X, got 0x%02X", expected_pec, buf[5]);
+    return i2c::ERROR_CRC;
+  }
+
+  data = encode_uint16(buf[4], buf[3]);
+  return i2c::ERROR_OK;
 }

 void MLX90614Component::dump_config() {
  ESP_LOGCONFIG(TAG, "MLX90614:");
  LOG_I2C_DEVICE(this);
-  if (this->is_failed()) {
-    ESP_LOGE(TAG, ESP_LOG_MSG_COMM_FAIL);
+  if (this->emissivity_write_attempts_ != 0) {
+    ESP_LOGW(TAG, "  Emissivity not written yet, will retry");
  }
  LOG_UPDATE_INTERVAL(this);
  LOG_SENSOR("  ", "Ambient", this->ambient_sensor_);
@@ -71,33 +156,41 @@ void MLX90614Component::dump_config() {
 }

 void MLX90614Component::update() {
-  uint8_t emissivity[3];
-  if (this->read_register(MLX90614_EMISSIVITY, emissivity, 3) != i2c::ERROR_OK) {
-    this->status_set_warning();
-    return;
+  // Temperature reads run regardless of the emissivity state so a failure still shows up as NAN
+  this->try_write_emissivity_();
+
+  // Publishes NAN on a bus or CRC failure so a stuck reading is visible instead of silently stale
+  auto publish_sensor = [this](sensor::Sensor *sensor, uint8_t reg) {
+    if (sensor == nullptr) {
+      return i2c::ERROR_OK;
+    }
+
+    uint16_t raw;
+    const auto ec = this->read_register_(reg, raw);
+    if (ec != i2c::ERROR_OK) {
+      sensor->publish_state(NAN);
+      return ec;
+    }
+
+    // Bit 15 set means the device flagged the reading as invalid
+    const float temperature = (raw & 0x8000) ? NAN : raw * 0.02f - 273.15f;
+    ESP_LOGD(TAG, "'%s': Got temperature=%.1f°C", sensor->get_name().c_str(), temperature);
+    sensor->publish_state(temperature);
+    return ec;
+  };
+
+  const auto object_ec = publish_sensor(this->object_sensor_, MLX90614_TEMPERATURE_OBJECT_1);
+  const auto ambient_ec = publish_sensor(this->ambient_sensor_, MLX90614_TEMPERATURE_AMBIENT);
+
+  if (object_ec != i2c::ERROR_OK || ambient_ec != i2c::ERROR_OK) {
+    this->status_set_warning(LOG_STR("Failed to read some sensors"));
+  } else if (this->emissivity_write_failed_) {
+    this->status_set_warning(LOG_STR("Failed to write emissivity"));
+  } else if (this->emissivity_write_attempts_ != 0) {
+    this->status_set_warning(LOG_STR("Failed to write emissivity, will retry"));
+  } else {
+    this->status_clear_warning();
  }
-  uint8_t raw_object[3];
-  if (this->read_register(MLX90614_TEMPERATURE_OBJECT_1, raw_object, 3) != i2c::ERROR_OK) {
-    this->status_set_warning();
-    return;
-  }
-
-  uint8_t raw_ambient[3];
-  if (this->read_register(MLX90614_TEMPERATURE_AMBIENT, raw_ambient, 3) != i2c::ERROR_OK) {
-    this->status_set_warning();
-    return;
-  }
-
-  float ambient = raw_ambient[1] & 0x80 ? NAN : encode_uint16(raw_ambient[1], raw_ambient[0]) * 0.02f - 273.15f;
-  float object = raw_object[1] & 0x80 ? NAN : encode_uint16(raw_object[1], raw_object[0]) * 0.02f - 273.15f;
-
-  ESP_LOGD(TAG, "Got Temperature=%.1f°C Ambient=%.1f°C", object, ambient);
-
-  if (this->ambient_sensor_ != nullptr && !std::isnan(ambient))
-    this->ambient_sensor_->publish_state(ambient);
-  if (this->object_sensor_ != nullptr && !std::isnan(object))
-    this->object_sensor_->publish_state(object);
-  this->status_clear_warning();
 }

 }  // namespace esphome::mlx90614
@@ -18,13 +18,18 @@ class MLX90614Component final : public PollingComponent, public i2c::I2CDevice {
  void set_emissivity(float emissivity) { emissivity_ = emissivity; }

 protected:
+  void try_write_emissivity_();
  bool write_emissivity_();

-  bool write_bytes_(uint8_t reg, uint16_t data);
+  bool write_register_(uint8_t reg, uint16_t data);
+  i2c::ErrorCode read_register_(uint8_t reg, uint16_t &data);

  sensor::Sensor *ambient_sensor_{nullptr};
  sensor::Sensor *object_sensor_{nullptr};

  float emissivity_{NAN};
+  // Remaining attempts to program the emissivity EEPROM cell, bounded to limit cell wear
+  uint8_t emissivity_write_attempts_{0};
+  bool emissivity_write_failed_{false};
 };
 }  // namespace esphome::mlx90614