[sht4x] Fix heater causing measurement jitter

Integrate heater activation into the measurement cycle instead of
running it on an independent timer. The separate heater interval
could overlap with measurement polling, causing readings to be
taken while the sensor was still hot.

Now the heater fires during update() when due, skipping that
measurement cycle. The next regular update() takes an ambient
reading after the sensor has cooled.

Also replace magic numbers with named constexpr constants for
the datasheet conversion formulas.

Closes https://github.com/esphome/esphome/issues/15011

esphome/components/sht4x/sht4x.cpp

@@ -9,14 +9,12 @@ static const char *const TAG = "sht4x";
 static const uint8_t MEASURECOMMANDS[] = {0xFD, 0xF6, 0xE0};
 static const uint8_t SERIAL_NUMBER_COMMAND = 0x89;
 
-void SHT4XComponent::start_heater_() {
-  uint8_t cmd[] = {this->heater_command_};
-
-  ESP_LOGD(TAG, "Heater turning on");
-  if (this->write(cmd, 1) != i2c::ERROR_OK) {
-    this->status_set_error(LOG_STR("Failed to turn on heater"));
-  }
-}
+// Conversion constants from SHT4x datasheet
+static constexpr float TEMPERATURE_OFFSET = -45.0f;
+static constexpr float TEMPERATURE_SPAN = 175.0f;
+static constexpr float HUMIDITY_OFFSET = -6.0f;
+static constexpr float HUMIDITY_SPAN = 125.0f;
+static constexpr float RAW_MAX = 65535.0f;
 
 void SHT4XComponent::read_serial_number_() {
   uint16_t buffer[2];
@@ -39,8 +37,8 @@ void SHT4XComponent::setup() {
   this->read_serial_number_();
 
   if (std::isfinite(this->duty_cycle_) && this->duty_cycle_ > 0.0f) {
-    uint32_t heater_interval = static_cast<uint32_t>(static_cast<uint16_t>(this->heater_time_) / this->duty_cycle_);
-    ESP_LOGD(TAG, "Heater interval: %" PRIu32, heater_interval);
+    this->heater_interval_ = static_cast<uint32_t>(static_cast<uint16_t>(this->heater_time_) / this->duty_cycle_);
+    ESP_LOGD(TAG, "Heater interval: %" PRIu32, this->heater_interval_);
 
     if (this->heater_power_ == SHT4X_HEATERPOWER_HIGH) {
       if (this->heater_time_ == SHT4X_HEATERTIME_LONG) {
@@ -62,8 +60,6 @@ void SHT4XComponent::setup() {
       }
     }
     ESP_LOGD(TAG, "Heater command: %x", this->heater_command_);
-
-    this->set_interval(heater_interval, [this]() { this->start_heater_(); });
   }
 }
 
@@ -83,43 +79,59 @@ void SHT4XComponent::dump_config() {
 }
 
 void SHT4XComponent::update() {
-  // Send command
+  bool use_heater = false;
+
+  // Check if heater is due during this measurement cycle
+  if (this->heater_interval_ > 0) {
+    uint32_t now = millis();
+    if (now - this->last_heater_millis_ >= this->heater_interval_) {
+      use_heater = true;
+      this->last_heater_millis_ = now;
+    }
+  }
+
+  if (use_heater) {
+    // Heater command heats the sensor to remove condensation, then takes a
+    // measurement (per datasheet section 4.9). The measurement is taken while
+    // the sensor is still hot, so we must discard it — it does not reflect
+    // ambient conditions. The next regular update() cycle will provide a
+    // valid reading after the sensor has cooled.
+    ESP_LOGD(TAG, "Heater turning on");
+    if (!this->write_command(this->heater_command_)) {
+      this->status_set_warning(LOG_STR("Failed to send heater command"));
+      return;
+    }
+    return;
+  }
+
   if (!this->write_command(MEASURECOMMANDS[this->precision_])) {
-    // Warning will be printed only if warning status is not set yet
     this->status_set_warning(LOG_STR("Failed to send measurement command"));
     return;
   }
 
-  this->set_timeout(10, [this]() {
-    uint16_t buffer[2];
+  this->set_timeout(10, [this]() { this->read_and_publish_(); });
+}
 
-    // Read measurement
-    if (!this->read_data(buffer, 2)) {
-      // Using ESP_LOGW to force the warning to be printed
-      ESP_LOGW(TAG, "Sensor read failed");
-      this->status_set_warning();
-      return;
-    }
+void SHT4XComponent::read_and_publish_() {
+  uint16_t buffer[2];
 
-    this->status_clear_warning();
+  if (!this->read_data(buffer, 2)) {
+    ESP_LOGW(TAG, "Sensor read failed");
+    this->status_set_warning();
+    return;
+  }
 
-    // Evaluate and publish measurements
-    if (this->temp_sensor_ != nullptr) {
-      // Temp is contained in the first result word
-      float sensor_value_temp = buffer[0];
-      float temp = -45 + 175 * sensor_value_temp / 65535;
+  this->status_clear_warning();
 
-      this->temp_sensor_->publish_state(temp);
-    }
+  if (this->temp_sensor_ != nullptr) {
+    float temp = TEMPERATURE_OFFSET + TEMPERATURE_SPAN * static_cast<float>(buffer[0]) / RAW_MAX;
+    this->temp_sensor_->publish_state(temp);
+  }
 
-    if (this->humidity_sensor_ != nullptr) {
-      // Relative humidity is in the second result word
-      float sensor_value_rh = buffer[1];
-      float rh = -6 + 125 * sensor_value_rh / 65535;
-
-      this->humidity_sensor_->publish_state(rh);
-    }
-  });
+  if (this->humidity_sensor_ != nullptr) {
+    float rh = HUMIDITY_OFFSET + HUMIDITY_SPAN * static_cast<float>(buffer[1]) / RAW_MAX;
+    this->humidity_sensor_->publish_state(rh);
+  }
 }
 
 }  // namespace sht4x

esphome/components/sht4x/sht4x.h

@@ -35,9 +35,11 @@ class SHT4XComponent : public PollingComponent, public sensirion_common::Sensiri
   SHT4XHEATERTIME heater_time_;
   float duty_cycle_;
 
-  void start_heater_();
   void read_serial_number_();
+  void read_and_publish_();
   uint8_t heater_command_;
+  uint32_t heater_interval_{0};
+  uint32_t last_heater_millis_{0};
   uint32_t serial_number_;
 
   sensor::Sensor *temp_sensor_{nullptr};