The driver is for the usage with the ESP8266 and [esp-open-rtos](https://github.com/SuperHouse/esp-open-rtos).
It is also working with ESP32 and [ESP-IDF](https://github.com/espressif/esp-idf.git) using a wrapper component for ESP8266 functions, see folder ```components/esp8266_wrapper```, as well as Linux based systems using a wrapper library.
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.
## Measurement Process
### Sensor modes
The CCS811 can operate in 5 different modes:
Mode | Driver symbol | Period | RAW data | IAQ values
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 | -
After power up, the sensor starts automatically in *Idle, Low Current Mode* (```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.
**Please note:** In *Constant Power Mode* with measurements every 250 ms (```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 (```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_sensor```, function ```ccs811_set_mode``` can be used to start periodic measurements with a given period.
if ((sensor = ccs811_init_sensor (I2C_BUS, CCS811_I2C_ADDRESS_1)))
{
...
// start periodic measurement with one measurement per second
ccs811_set_mode (sensor, ccs811_mode_1s);
}
...
```
**Please 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 data sheet 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* (```mode_10s```) to *Low Power Pulse Heating Mode* (```mode_60s```), it should be placed in *Idle, Low Current Mode* (```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* (```mode_60s```) to *Pulse Heating Mode* (```mode_10s```), there is no requirement to wait before enabling the new mode.
## Measurement results
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))
{
...
}
...
```
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_DRV_NO_NEW_DATA is set.
**Please note:**
1. In *Constant Power Mode* with measurements every 250 ms (```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_DRV_NO_NEW_DATA is set.
### Compensation
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))
// set CCS811 environmental data with values fetched from SHT3x
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
CCS811 supports two types of interrupts that can be used to fetch data:
- data ready interrupt (INT_DATA_RDY)
- threshold interrupt (INT_THRESHOLD)
#### Data ready interrupt
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
ccs811_enable_interrupt (sensor, true);
...
```
#### Threshold interrupt
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
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.
## Error Handling
Most driver functions return a simple boolean value to indicate whether its execution was successful or an error happened. In the latter case, the member ```error_code``` of the sensor device data structure is set which indicates what error happened.
There are two different error levels that are ORed into one single *error_code*, errors in the I2C communication and errors of the CCS811 sensor itself. To test for a certain error, first you can AND the *error_code* with one of the error masks, ```CCS811_I2C_ERROR_MASK``` for I2C errors and ```CCS811_DRV_ERROR_MASK``` for other errors. Then you can test the result for a certain error code.
For example, error handling for ```ccs811_get_results``` could look like:
```
if (ccs811_get_results (sensor, &tvoc, &eco2, &raw_i, &raw_v))
Before using the CCS811 driver, function ```i2c_init``` needs to be called for each I2C interface to setup them.
**Please 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```.
Once I2C interfaces to be used are initialized, function ```ccs811_init_sensor``` has to be called for each CCS811 sensor to initialize the sensor and to check its availability as well as its error state. The parameters specify the I2C bus to which it is connected and its I2C slave address.
```
static ccs811_sensor_t* sensor; // pointer to sensor device data structure
...
if ((sensor = ccs811_init_sensor (I2C_BUS, CCS811_I2C_ADDRESS_1)))
{
...
}
...
```
Function ```ccs811_init_sensor``` returns a pointer to the sensor device data structure or NULL in case of error.
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
**Please note:** To avoid concurrency situations when driver functions are used to access the sensor, for example to read data, the user task must not be created until the sensor configuration is completed.
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))
The user task simply fetches new data with the same rate as the measurements are performed.
**Please 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))
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
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
