Mercurial > iotbalkon / file revision

# Driver for the ams CCS811 digital gas sensor for monitoring indoor air quality

The driver is for the usage with the ESP-IDF.

## About the sensor

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
---- | ------------- | ------ |:--------:|:----------:
Idle, Low Current Mode | `CCS811_MODE_IDLE` | -  | - | -
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*
(`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.

**Please 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.

```C
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);
...
if (ccs811_init(&sensor) == ESP_OK)
{
   ...
   // start periodic measurement with one measurement per second
   ESP_ERROR_CHECK(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 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.

## 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.

```C
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.

**Please 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.

### 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.

```C
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);
...
```

### NTC

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:

```text
          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:

```C
...
#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
....
```

### Interrupts

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()`.

```C
...
// enable the data ready interrupt
ESP_ERROR_CHECK(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.

```C
...
// set threshold parameters and enable threshold interrupt mode
ESP_ERROR_CHECK(ccs811_set_eco2_thresholds(&sensor, 600, 1100, 40));
...
```

### Baseline

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.

## Usage

First, the hardware configuration has to be established.

### Communication interface settings

Dependent on the hardware configuration, the communication interface settings
have to be defined.

```C
// 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      5
```

### Main program

Before using the CCS811 driver, function `i2cdev_init()` needs to be called.

**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`.

```C
static ccs811_dev_t sensor;    // pointer to sensor device data structure
...
memset(&sensor, 0, sizeof(ccs811_dev_t));
i2cdev_init();    // Init i2cdev library
i2c_set_timeout(I2C_PORT, CCS811_I2C_CLOCK_STRETCH);
...
ccs811_init_desc(&sensor, I2C_PORT, CCS811_I2C_ADDRESS_1, I2C_SDA_PIN, I2C_SCL_PIN);
```

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.

```C
...
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.

```C
...
// 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.

```C
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.

```C
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.

**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.

```C
...
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).

```C
...
// 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.

```C
...
// 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);
...
```