drivers: sensor: ccs811: support update of environmental conditions

Accurate estimate of gas presence requires temperature and humidity
data.  Add API to update these values.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>

drivers/sensor/ccs811/ccs811.c

@@ -86,6 +86,7 @@ int ccs811_baseline_fetch(struct device *dev)
 	if (rc <= 0) {
 		rc = baseline;
 	}
+
 	return rc;
 }
 
@@ -104,6 +105,74 @@ int ccs811_baseline_update(struct device *dev,
 	return rc;
 }
 
+int ccs811_envdata_update(struct device *dev,
+			  const struct sensor_value *temperature,
+			  const struct sensor_value *humidity)
+{
+	struct ccs811_data *drv_data = dev->driver_data;
+	int rc;
+	u8_t buf[5] = { CCS811_REG_ENV_DATA };
+
+	/*
+	 * Environment data are represented in a broken whole/fraction
+	 * system that specified a 9-bit fractional part to represent
+	 * milli-units.  Since 1000 is greater than 512, the device
+	 * actually only pays attention to the top bit, treating it as
+	 * indicating 0.5.  So we only write the first octet (7-bit
+	 * while plus 1-bit half).
+	 *
+	 * Humidity is simple: scale it by two and round to the
+	 * nearest half.  Assume the fractional part is not
+	 * negative.
+	 */
+	if (humidity) {
+		int value = 2 * humidity->val1;
+
+		value += (250000 + humidity->val2) / 500000;
+		if (value < 0) {
+			value = 0;
+		} else if (value > (2 * 100)) {
+			value = 2 * 100;
+		}
+		LOG_DBG("HUM %d.%06d becomes %d",
+			humidity->val1, humidity->val2, value);
+		buf[1] = value;
+	} else {
+		buf[1] = 2 * 50;
+	}
+
+	/*
+	 * Temperature is offset from -25 Cel.  Values below minimum
+	 * store as zero.  Default is 25 Cel.  Again we round to the
+	 * nearest half, complicated by Zephyr's signed representation
+	 * of the fractional part.
+	 */
+	if (temperature) {
+		int value = 2 * temperature->val1;
+
+		if (temperature->val2 < 0) {
+			value += (250000 + temperature->val2) / 500000;
+		} else {
+			value += (-250000 + temperature->val2) / 500000;
+		}
+		if (value < (2 * -25)) {
+			value = 0;
+		} else {
+			value += 2 * 25;
+		}
+		LOG_DBG("TEMP %d.%06d becomes %d",
+			temperature->val1, temperature->val2, value);
+		buf[3] = value;
+	} else {
+		buf[3] = 2 * (25 + 25);
+	}
+
+	set_wake(drv_data, true);
+	rc = i2c_write(drv_data->i2c, buf, sizeof(buf), DT_INST_0_AMS_CCS811_BASE_ADDRESS);
+	set_wake(drv_data, false);
+	return rc;
+}
+
 static int ccs811_sample_fetch(struct device *dev, enum sensor_channel chan)
 {
 	struct ccs811_data *drv_data = dev->driver_data;

drivers/sensor/ccs811/ccs811.h

@@ -17,6 +17,7 @@
 #define CCS811_REG_MEAS_MODE            0x01
 #define CCS811_REG_ALG_RESULT_DATA      0x02
 #define CCS811_REG_RAW_DATA             0x03
+#define CCS811_REG_ENV_DATA             0x05
 #define CCS811_REG_THRESHOLDS           0x10
 #define CCS811_REG_BASELINE             0x11
 #define CCS811_REG_HW_ID                0x20

include/drivers/sensor/ccs811.h

@@ -20,6 +20,7 @@ extern "C" {
 #endif
 
 #include <device.h>
+#include <drivers/sensor.h>
 
 /**
  * @brief Fetch the current value of the BASELINE register.
@@ -51,6 +52,24 @@ int ccs811_baseline_fetch(struct device *dev);
  */
 int ccs811_baseline_update(struct device *dev, u16_t baseline);
 
+/**
+ * @brief Update the ENV_DATA register.
+ *
+ * Accurate calculation of gas levels requires accurate environment
+ * data.  Measurements are only accurate to 0.5 Cel and 0.5 %RH.
+ *
+ * @param dev Pointer to the sensor device
+ *
+ * @param temperature the current temperature at the sensor
+ *
+ * @param humidity the current humidity at the sensor
+ *
+ * @return 0 if successful, negative errno code if failure.
+ */
+int ccs811_envdata_update(struct device *dev,
+			  const struct sensor_value *temperature,
+			  const struct sensor_value *humidity);
+
 #ifdef __cplusplus
 }
 #endif