MH-Z19 sensor: disable auto-calibration and some odd fixes

src/esphome/sensor/mhz19_component.cpp

@@ -13,6 +13,7 @@ static const char *TAG = "sensor.mhz19";
 static const uint8_t MHZ19_REQUEST_LENGTH = 8;
 static const uint8_t MHZ19_RESPONSE_LENGTH = 9;
 static const uint8_t MHZ19_COMMAND_GET_PPM[] = {0xFF, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00};
+static const uint8_t MHZ19_COMMAND_ABC_DISABLE[] = {0xFF, 0x01, 0x79, 0x00, 0x00, 0x00, 0x00, 0x00, 0x86};
 
 MHZ19Component::MHZ19Component(UARTComponent *parent, const std::string &co2_name, uint32_t update_interval)
     : PollingComponent(update_interval), UARTDevice(parent), co2_sensor_(new MHZ19CO2Sensor(co2_name, this)) {}
@@ -26,18 +27,59 @@ uint8_t mhz19_checksum(const uint8_t *command) {
 
 void MHZ19Component::update() {
   uint8_t response[MHZ19_RESPONSE_LENGTH];
+repeat:
   if (!this->mhz19_write_command_(MHZ19_COMMAND_GET_PPM, response)) {
     ESP_LOGW(TAG, "Reading data from MHZ19 failed!");
     this->status_set_warning();
     return;
   }
 
   if (response[0] != 0xFF || response[1] != 0x86) {
-    ESP_LOGW(TAG, "Invalid preamble from MHZ19!");
+    ESP_LOGW(TAG, "Invalid preamble from MHZ19! [%0x %0x %0x %0x %0x %0x %0x %0x %0x]",
+      response[0], response[1], response[2], response[3], response[4],
+      response[5], response[6], response[7], response[8]);
     this->status_set_warning();
+    if (++this->cleanup_rx_buffer > 2) {
+        /* try to recover and give up on it if several attemprs were no successful until the next update cycle */
+        this->cleanup_rx_buffer = 0;
+        return;
+    }
+    goto repeat;
+  }
+  this->cleanup_rx_buffer = 0;
+
+  /* Sensor reports U(15000) during boot, ingnore reported CO2 until it boots */
+  uint16_t u = (response[6] << 8) + response[7];
+  if (u == 15000) {
+    ESP_LOGD(TAG, "Sensor is booting");
     return;
   }
 
+  /* MH-Z19B(s == 0) and MH-Z19(s != 0) */
+  uint8_t s = response[5];
+  if (response[5] == 0 && this->model_b == false) {
+    ESP_LOGD(TAG, "MH-Z19B detected");
+    this->model_b = true;
+  }
+
+  if (this->model_b && this->abc_disabled == false) {
+    uint8_t abc_ack[MHZ19_RESPONSE_LENGTH];
+    /*
+     * MH-Z19B allows to enable/disable 'automatic baseline calibration' (datasheet MH-Z19B v1.2),
+     * disable it to prevent sensor baseline drift in not well ventilated area
+     */
+    ESP_LOGI(TAG, "Disabling ABC on boot");
+    if (!this->mhz19_write_command_(MHZ19_COMMAND_ABC_DISABLE, abc_ack)) {
+      ESP_LOGW(TAG, "Failed to read ABC disable ack!");
+      return;
+    }
+    this->abc_disabled = true;
+    /*
+     * TODO: implement an option to recalibrate sensor, to allow for occasional
+     * manual recalibration
+     */
+  }
+
   uint8_t checksum = mhz19_checksum(response);
   if (response[8] != checksum) {
     ESP_LOGW(TAG, "MHZ19 Checksum doesn't match: 0x%02X!=0x%02X", response[8], checksum);
@@ -58,6 +100,19 @@ void MHZ19Component::update() {
 
 bool MHZ19Component::mhz19_write_command_(const uint8_t *command, uint8_t *response) {
   this->flush();
+
+  /*
+   * UART0 in NodeMCU v2, sometimes on boot has junk in RX buffer,
+   * check for it and drain all of it before sending commands to sensor
+   */
+  if (this->cleanup_rx_buffer) {
+    for (int i = 0; this->available(); i++) {
+      uint8_t junk;
+      this->read_byte(&junk);
+      ESP_LOGD(TAG, "junk in RX[%d]: %0x", i, junk);
+    }
+  }
+
   this->write_array(command, MHZ19_REQUEST_LENGTH);
   this->write_byte(mhz19_checksum(command));
 
@@ -74,7 +129,7 @@ MHZ19TemperatureSensor *MHZ19Component::make_temperature_sensor(const std::strin
 MHZ19CO2Sensor *MHZ19Component::get_co2_sensor() const { return this->co2_sensor_; }
 float MHZ19Component::get_setup_priority() const { return setup_priority::HARDWARE_LATE; }
 void MHZ19Component::dump_config() {
-  ESP_LOGCONFIG(TAG, "MH-Z19:");
+  ESP_LOGCONFIG(TAG, "MH-Z19%s%s", this->model_b ? "B:" : "", this->abc_disabled ? " (auto calibration: disabled)": " (auto calibration: enabled)");
   LOG_SENSOR("  ", "CO2", this->co2_sensor_);
   LOG_SENSOR("  ", "Temperature", this->temperature_sensor_);
 }

src/esphome/sensor/mhz19_component.h

@@ -33,6 +33,9 @@ class MHZ19Component : public PollingComponent, public UARTDevice {
 
   MHZ19TemperatureSensor *temperature_sensor_{nullptr};
   MHZ19CO2Sensor *co2_sensor_;
+  bool model_b;
+  bool abc_disabled;
+  int cleanup_rx_buffer;
 };
 
 }  // namespace sensor