The driver is for the usage with the ESP-IDF.
The CCS811 is an ultra-low power digital sensor which detects Volatile Organic Compounds (VOC) for Indoor Air Quality (IAQ) monitoring that. The sensor allows to
- convert raw sensor data to Total Volatile Organic Compound (TVOC) and equivalent CO₂ (eCO2),
- compensate gas readings due to temperature and humidity using an external sensor,
- trigger interrupts when new measurement results are available or eCO2 value exceeds thresholds,
- correct baseline automatically or manually
- connect a NTC thermistor to provide means of calculating the local ambient temperature.
The sensor uses an I2C interface and supports clock stretching. See the notes on clock stretching during I2C interface intialization.
The CCS811 can operate in 5 different modes:
| Mode | Driver symbol | Period | RAW data | IAQ values |
|---|---|---|---|---|
| Idle, Low Current Mode | CCS811_MODE_IDLE | n/a | n/a | n/a |
| Constant Power Mode | CCS811_MODE_1S | 1 s | X | X |
| Pulse Heating Mode | CCS811_MODE_10S | 10 s | X | X |
| Low Power Pulse Heating Mode | CCS811_MODE_60S | 60 s | X | X |
| Constant Power Mode | CCS811_MODE_250MS | 250 ms | X | n/a |
After power up, the sensor starts automatically in Idle, Low Current Mode (CCS811_MODE_IDLE). To start periodic measurements, the mode of the sensor has to be changed to any measurement mode. Measurement modes with with different rates of periodic measurements are available, see table above.
Note
In Constant Power Mode with measurements every 250 ms (CCS811_MODE_250MS) only raw data are available. In all other measurement modes, the Indoor Air Quality (IAQ) values are available additionally.
The Constant Power Mode with measurements every 250 ms (CCS811_MODE_250ms) is only intended for systems where an external host system wants to run an algorithm with raw data.
Once the sensor is initialized with function ccs811_init(), function ccs811_set_mode() can be used to start periodic measurements with a given period.
ESP_ERROR_CHECK(i2cdev_init());
...
static ccs811_dev_t *sensor;
ESP_ERROR_CHECK(ccs811_init_desc(&sensor, 0, CCS811_I2C_ADDRESS_1, 5, 4);
if (ccs811_init(&sensor) == ESP_OK)
{
...
// start periodic measurement with one measurement per second
ESP_ERROR_CHECK(ccs811_set_mode(&sensor, CCS811_MODE_1S));
}
...Note
1. After setting the mode, the sensor is in conditioning period that needs up to 20 minutes, before accurate readings are generated, see the data sheet for more details.
2. During the early-live (burn-in) period, the CCS811 sensor should run for 48 hours in the selected mode of operation to ensure sensor performance is stable, see the datasheet for more details.
3. When the sensor operating mode is changed to a new mode with a lower sample rate, e.g., from Pulse Heating Mode (CCS811_MODE_10S) to Low Power Pulse Heating Mode (CCS811_MODE_60S), it should be placed in Idle, Low Current Mode (CCS811_MODE_IDLE) for at least 10 minutes before enabling the new mode.
When a sensor operating mode is changed to a new mode with a higher sample rate, e.g., from Low Power Pulse Heating Mode (CCS811_MODE_60S) to Pulse Heating Mode (CCS811_MODE_10S), there is no requirement to wait before enabling the new mode.
Once the measurement mode is set, the user task can use function ccs811_get_results() with same rate as the measurement rate to fetch the results. The function returns raw data as well as Indoor Air Quality (IAQ) values.
While raw data represents simply the current through the sensor and the voltage across the sensor with the selected current, IAQ values are the results of the processing these raw data by the sensor. IAQ values consist of the equivalent CO2 (eCO2) with a range from 400 ppm to 8192 ppm and Total Volatile Organic Compound (TVOC) with a range from 0 ppb to 1187 ppb.
uint16_t iaq_tvoc;
uint16_t iaq_eco2;
uint8_t raw_i;
uint16_t raw_v;
...
// get the results and do something with them
if (ccs811_get_results(&sensor, &tvoc, &eco2, &raw_i, &raw_v) == ESP_OK)
{
...
}
...If some of the results are not needed, the corresponding pointer parameters can be set to NULL.
If the function ccs811_get_results() is called and no new data are available, e.g., due to the sensor mode time tolerance of 2%, the function still returns successfully. In this case, the results of the last measurement are returned and the error code CCS811_ERR_NO_NEW_DATA.
Note
1. In Constant Power Mode with measurements every 250 ms (CCS811_MODE_250MS) only raw data are available.
2. The rate of fetching data must not be greater than the rate of measurement. Due to the sensor mode timing tolerance of 2 %, the rate of fetching data should be lower than the measurement rate.
3. If the function is called and no new data are available, the results of the latest measurement are returned and error code CCS811_ERR_NO_NEW_DATA is set.
If information about the environment like temperature and humidity are available from another sensor, they can be used by CCS811 to compensate gas readings due to temperature and humidity changes. Function ccs811_set_environmental_data() can be used to set these environmental data.
float temperature;
float humidity;
...
if (sht3x_get_results(sht3x, &temperature, &humidity) == ESP_OK)
// set CCS811 environmental data with values fetched from SHT3x
ccs811_set_environmental_data(ccs811, temperature, humidity);
...CCS811 supports an external interface for connecting a negative thermal coefficient thermistor (R_NTC) to provide a cost effective and power efficient means of calculating the local ambient temperature. The sensor measures the voltage V_NTC across R_NTC as well as the voltage V_REF across a connected reference resistor (R_REF). Function ccs811_get_ntc_resistance() can be used to fetch the current resistance of R_NTC. It uses the resistance of R_REF and measured voltages V_REF and V_NTV with the following equation:
R_NTC = R_REF / V_REF * V_NTC
Using the data sheet of the NTC, the ambient temperature can be calculated. See application note ams AN000372 for more details. For example, with Adafruit CCS811 Air Quality Sensor Breakout the ambienttemperature can be determined as following:
#define CCS811_R_REF 100000 // resistance of the reference resistor
#define CCS811_R_NTC 10000 // resistance of NTC at a reference temperature
#define CCS811_R_NTC_TEMP 25 // reference temperature for NTC
#define CCS811_BCONSTANT 3380 // B constant
// get NTC resistance
uint32_t r_ntc;
ccs811_get_ntc_resistance(&sensor, CCS811_R_REF, &r_ntc);
// calculation of temperature from application note ams AN000372
double ntc_temp;
ntc_temp = log((double)r_ntc / CCS811_R_NTC); // 1
ntc_temp /= CCS811_BCONSTANT; // 2
ntc_temp += 1.0 / (CCS811_R_NTC_TEMP + 273.15); // 3
ntc_temp = 1.0 / ntc_temp; // 4
ntc_temp -= 273.15; // 5
...CCS811 supports two types of interrupts that can be used to fetch data:
- data ready interrupt (INT_DATA_RDY)
- threshold interrupt (INT_THRESHOLD)
At the end of each measurement cycle (every 250 ms, 1 second, 10 seconds, or 60 seconds), CCS811 can optionally trigger an interrupt. The signal nINT is driven low as soon as new sensor values are ready to read. It will stop being driven low when sensor data are read with function ccs811_get_results().
The interrupt is disabled by default. It can be enabled with function ccs811_enable_interrupt().
...
// enable the data ready interrupt
ESP_ERROR_CHECK(ccs811_enable_interrupt(&sensor, true));
...The user task can choose that the data ready interrupt is not generated every time when new sensor values become ready but only if the eCO2 value moves from the current range (LOW, MEDIUM, or HIGH) into another range by more than a hysteresis value. Hysteresis is used to prevent multiple interrupts close to a threshold.
The interrupt is disabled by default and can be enabled with function ccs811_set_eco2_thresholds(). The ranges are defined by parameters low and high as following
- LOW - below parameter value low
- MEDIUM - between parameter values low and high
- HIGH - above parameter value high is range HIGH.
If all parameters have valid values, the function sets the thresholds and enables the data ready interrupt. Using 0 for all parameters disables the interrupt.
...
// set threshold parameters and enable threshold interrupt mode
ESP_ERROR_CHECK(ccs811_set_eco2_thresholds(&sensor, 600, 1100, 40));
...CCS81 supports automatic baseline correction over a minimum time of 24 hours. Using function ccs811_get_baseline(), the current baseline value can be saved before the sensor is powered down. This baseline can then be restored with function ccs811_set_baseline() after sensor is powered up again to continue the automatic baseline process.
First, the hardware configuration has to be established.
Dependent on the hardware configuration, the communication interface settings have to be defined.
// define I2C interfaces at which CCS811 sensors can be connected
#define I2C_PORT 0
#define I2C_SCL_PIN 14
#define I2C_SDA_PIN 13
// define GPIO for interrupt
#define INT_GPIO 5Before using the CCS811 driver, function i2cdev_init() needs to be called.
Note
CCS811 uses clock streching that can be longer than the default I2C clock stretching. Therefore the clock stretching parameter of I2C has to be set to at least CCS811_I2C_CLOCK_STRETCH.
ESP_ERROR_CHECK(i2cdev_init());
...
static ccs811_dev_t sensor;
memset(&sensor, 0, sizeof(ccs811_dev_t)); // Zero descriptor
ESP_ERROR_CHECK(ccs811_init_desc(&sensor, 0, CCS811_I2C_ADDRESS_1, 5, 4);
...Once I2C library initialized, function ccs811_init() has to be called for each CCS811 sensor to initialize the sensor and to check its availability as well as its error state.
...
if (ccs811_init(&sensor) == ESP_OK)
{
...
}
...If initialization of the sensor was successful, the sensor mode has be set to start periodic measurement. The sensor mode can be changed anytime later.
...
// start periodic measurement with one measurement per second
ccs811_set_mode(&sensor, CCS811_MODE_1S);
...Finally, a user task that uses the sensor has to be created.
xTaskCreate(user_task, "user_task", 256, NULL, 2, 0);The user task can use different approaches to fetch new data. Either new data are fetched periodically or the interrupt signal nINT is used when new data are available or eCO2 value exceeds defined thresholds.
If new data are fetched periodically the implementation of the user task is quite simply and could look like following.
void user_task(void *pvParameters)
{
uint16_t tvoc;
uint16_t eco2;
TickType_t last_wakeup = xTaskGetTickCount();
while (1)
{
// get the results and do something with them
if (ccs811_get_results(&sensor, &tvoc, &eco2, 0, 0) == ESP_OK)
...
// passive waiting until 1 second is over
vTaskDelayUntil(&last_wakeup, 1000 / portTICK_PERIOD_MS);
}
}
...The user task simply fetches new data with the same rate as the measurements are performed.
Note
The rate of fetching the measurement results must be not greater than the rate of periodic measurements of the sensor, however, it should be less to avoid conflicts caused by the timing tolerance of the sensor.
A different approach is to use the interrupt nINT. This interrupt signal is either triggered every time when new data are available (INT_DATA_RDY) or only whenever eCO2 value exceeds defined thresholds (INT_THRESHOLD). In both cases, the user has to implement an interrupt handler that either fetches the data directly or triggers a task, that is waiting to fetch the data.
...
TaskHandle_t nINT_task;
// Interrupt handler which resumes user_task_interrupt on interrupt
void nINT_handler(uint8_t gpio)
{
xTaskResumeFromISR(nINT_task);
}
// User task that fetches the sensor values.
void user_task_interrupt(void *pvParameters)
{
uint16_t tvoc;
uint16_t eco2;
while (1)
{
// task suspends itself and waits to be resumed by interrupt handler
vTaskSuspend(NULL);
// after resume get the results and do something with them
if (ccs811_get_results(&sensor, &tvoc, &eco2, 0, 0) == ESP_OK)
...
}
}
...
xTaskCreate(user_task_interrupt, "user_task_interrupt", 256, NULL, 2, &nINT_task);
...In this example, a task is defined which suspends itself in each cycle to wait for fetching the data. The task is resumed by the interrupt handler.
Finally, the interrupt handler has to be activated for the GPIO which is connected to the interrupt signal. Furthermore, the interrupt has to be enabled in the CCS811 sensor.
Function ccs811_enable_interrupt() enables the interrupt that is triggered whenever new data are available (INT_DATA_RDY).
...
// activate the interrupt for INT_GPIO and set the interrupt handler
gpio_set_interrupt(INT_GPIO, GPIO_INTTYPE_EDGE_NEG, nINT_handler);
// enable the data ready interrupt INT_DATA_RDY
ccs811_enable_interrupt(&sensor, true);
...Function ccs811_set_eco2_thresholds() enables the interrupt that is triggered whenever eCO2 value exceeds the thresholds (INT_THRESHOLD) defined by parameters.
...
// activate the interrupt for INT_GPIO and set the interrupt handler
gpio_set_interrupt(INT_GPIO, GPIO_INTTYPE_EDGE_NEG, nINT_handler);
// set threshold parameters and enable threshold interrupt mode INT_THRESHOLD
ccs811_set_eco2_thresholds(&sensor, 600, 1100, 40);
...ccs811