The driver supports multiple BME680 sensors which are either connected to the SPI or to the same or different I2C interfaces with different addresses.
It is for the usage with the ESP8266 and [esp-open-rtos](https://github.com/SuperHouse/esp-open-rtos).
The driver 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.
BME680 is an ultra-low-power environmental sensor that integrates temperature, pressure, humidity and gas sensors in only one unit.
## Communication interfaces
The BME680 sensor can be connected using I2C or SPI.
The I2C interface supports data rates up to 3.4 Mbps. It is possible to connect multiple BME680 sensors with different I2C slave addresses to the same I2C bus or to different I2C buses. Possible I2C slave addresses are 0x76 and 0x77.
The SPI interface allows clock rates up to 10 MHz and the SPI modes '00' (CPOL=CPHA=0) and '11' (CPOL=CPHA=1).
Interface selection is done automatically by the sensor using the SPI CS signal. As long as the CS signal keeps high after power-on reset, the I2C interface is used. Once the CS signal has been pulled down, SPI interface is used until next power-on reset.
Once the BME680 has been initialized, it can be used for measurements. The BME680 operates in two different modes, the **sleep mode** and the **forced mode**.
The sensor starts after power-up automatically in the *sleep mode* where it does not perform any measurement and consumes only 0.15 μA. Measurements are only done in *forced mode*.
**Please note:** There are two further undocumented modes, the *parallel* and the *sequential* mode. They can't be supported by the driver, since it is not clear what they do and how to use them.
#### Measurement cylce
To perform measurements, the BME680 sensor has to be triggered to switch to the **forced mode**. In this mode, it performs exactly one measurement of temperature, pressure, humidity, and gas in that order, the so-called **TPHG measurement cycle**. After the execution of this TPHG measurement cycle, **raw sensor data** become available and the sensor returns automatically back to sleep mode.
Each of the individual measurements can be configured or skipped separately via the sensor settings, see section **Measurement settings**. Dependent on the configuration, the **duration of a TPHG measurement cycle** can vary from some milliseconds up to about 4.5 seconds, especially if gas measurement is enabled.
To avoid the blocking of the user task during measurements, the measurement process is therefore separated into the following steps:
1. Trigger the sensor with function ```bme680_force_measurement``` to switch to *forced mode* in which it performs exactly one THPG measurement cycle.
2. Wait the measurement duration using function ```vTaskDelay``` and the value returned from function ```bme680_get_measurement_duration``` or wait as long as function ```bme680_is_measuring``` returns true.
3. Fetch the results as fixed point values with function ```bme680_get_results_fixed``` or as floating point values with function ```bme680_get_results_float```.
```
...
// as long as sensor configuration isn't changed, the duration can be considered as constant
// STEP 2: passive waiting until measurement results are available
vTaskDelay (duration);
// STEP 3: get the results and do something with them
if (bme680_get_results_float (sensor, &values))
...
}
...
```
Alternatively, busy waiting can be realized using function ```bme680_is_measuring```.
```
...
if (bme680_force_measurement (sensor)) // STEP 1
{
// STEP 2: busy waiting until measurement results are available
while (bme680_is_measuring (sensor)) ;
// STEP 3: get the results and do something with them
if (bme680_get_results_float (sensor, &values))
...
}
...
```
For convenience, it is also possible to use the high-level functions ```bme680_measure_float``` or ```bme680_measure_fixed```. These functions combine all 3 steps above within a single function and are therefore very easy to use. **Please note** that these functions must not be used when they are called from a software timer callback function since the calling task is delayed using function *vTaskDelay*.
```
...
// ONE STEP: measure, wait, get the results and do something with them
Once the sensor has finished the measurement raw data are available at the sensor. Either function ```bme680_get_results_fixed``` or function ```bme680_get_results_float``` can be used to fetch the results. Both functions read raw data from the sensor and converts them into utilizable fixed point or floating point sensor values.
**Please note:** Conversion of raw sensor data into the final sensor values is based on very complex calculations that use a large number of calibration parameters. Therefore, the driver does not provide functions that only return the raw sensor data.
Dependent on sensor value representation, measurement results contain different dimensions:
| Value | Fixed Point | Floating Point | Conversion
To increase the resolution of raw sensor data, the sensor supports oversampling for temperature, pressure, and humidity measurements. Using function ```bme680_set_oversampling_rates```, individual **oversampling rates** can be defined for these measurements. With an oversampling rate *osr*, the resolution of the according raw sensor data can be increased from 16 bit to 16+ld(*osr*) bit.
Possible oversampling rates are 1x (default by the driver) 2x, 4x, 8x and 16x. It is also possible to define an oversampling rate of 0. This **deactivates** the corresponding measurement and the output values become invalid.
```
...
// Changes the oversampling rate for temperature to 4x and for pressure to 2x. Humidity measurement is skipped.
The sensor also integrates an internal IIR filter (low pass filter) to reduce short-term changes in sensor output values caused by external disturbances. It effectively reduces the bandwidth of the sensor output values.
The filter can optionally be used for pressure and temperature data that are subject to many short-term changes. With the IIR filter the resolution of pressure and temperature data increases to 20 bit. Humidity and gas inside the sensor does not fluctuate rapidly and does not require such a low pass filtering.
Using function ```bme680_set_filter_size```, the user task can change the **size of the filter**. The default size is 3. If the size of the filter becomes 0, the filter is **not used**.
```
...
// Change the IIR filter size for temperature and pressure to 7.
bme680_set_filter_size(sensor, iir_size_7);
...
// Don't use IIR filter
bme680_set_filter_size(sensor, iir_size_0);
...
```
#### Heater profile
For the gas measurement, the sensor integrates a heater. Parameters for this heater are defined by **heater profiles**. The sensor supports up to 10 such heater profiles, which are numbered from 0 to 9. Each profile consists of a temperature set-point (the target temperature) and a heating duration. By default, only the heater profile 0 with 320 degree Celsius as target temperature and 150 ms heating duration is defined.
**Please note:** According to the data sheet, target temperatures between 200 and 400 degrees Celsius are typical and about 20 to 30 ms are necessary for the heater to reach the desired target temperature.
Function ```bme680_set_heater_profile``` can be used to set the parameters for one of the heater profiles 0 ... 9. Once the parameters of a heater profile are defined, the gas measurement can be activated with that heater profile using function ```bme680_use_heater_profile```. If -1 or ```BME680_HEATER_NOT_USED``` is used as heater profile, gas measurement is deactivated completely.
```
...
// Change the heater profile 1 to 300 degree Celsius for 100 ms and activate it
If several heater profiles have been defined with function ```bme680_set_heater_profile```, a sequence of gas measurements with different heater parameters can be realized by a sequence of activations of different heater profiles for successive TPHG measurements using function ```bme680_use_heater_profile```.
For example, if there were 5 heater profiles defined with following code during the setup
```
bme680_set_heater_profile (sensor, 0, 200, 100);
bme680_set_heater_profile (sensor, 1, 250, 120);
bme680_set_heater_profile (sensor, 2, 300, 140);
bme680_set_heater_profile (sensor, 3, 350, 160);
bme680_set_heater_profile (sensor, 4, 400, 180);
```
the user task could use them as a sequence like following:
```
...
while (1)
{
switch (count++ % 5)
{
case 0: bme680_use_heater_profile (sensor, 0); break;
case 1: bme680_use_heater_profile (sensor, 1); break;
case 2: bme680_use_heater_profile (sensor, 2); break;
case 3: bme680_use_heater_profile (sensor, 3); break;
case 4: bme680_use_heater_profile (sensor, 4); break;
The heater resistance calculation algorithm takes into account the ambient temperature of the sensor. Using function ```bme680_set_ambient_temperature```, the ambient temperature either determined from the sensor itself or from another temperature sensor can be set.
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 or SPI communication and errors of the BME680 sensor itself. To test for a certain error, first you can AND the *error_code* with one of the error masks, ```BME680_INT_ERROR_MASK``` for I2C or SPI errors and ```BME680_DRV_ERROR_MASK``` for other errors. Then you can test the result for a certain error code.
For example, error handling for ```bme680_get_results_float``` could look like:
First, the hardware configuration has to be established. This can differ dependent on the communication interface and the number of sensors used.
### Hardware configurations
The driver supports multiple BME680 sensors at the same time that are connected either to I2C or SPI. Following figures show some possible hardware configurations.
First figure shows the configuration with only one sensor at I2C bus 0.
2. GPIO15 which is used as CS signal of SPI bus 1 on ESP8266 does not work correctly together with the BME680. Therefore, the user has to specify another GPIO pin as CS signal, e.g., GPIO2.
SPI interface has not to be initialized explicitly.
Once the I2C interfaces are initialized, function ```bme680_init_sensor``` has to be called for each BME680 sensor in order to initialize the sensor and to check its availability as well as its error state. This function returns a pointer to a sensor device data structure or NULL in case of error.
The parameter *bus* specifies the ID of the I2C or SPI bus to which the sensor is connected.
```
static bme680_sensor_t* sensor;
```
For sensors connected to an I2C interface, a valid I2C slave address has to be defined as parameter *addr*. In that case parameter *cs* is ignored.
The remaining part of the program is independent on the communication interface.
Optionally, you could wish to set some measurement parameters. For details see the section **Measurement settings** above, the header file of the driver ```bme680.h```, and of course the data sheet of the sensor.
Finally, a user task that uses the sensor has to be created.
**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.
BME680 supports only the *forced mode* that performs exactly one measurement. Therefore, the measurement has to be triggered in each cycle. The waiting for measurement results is also required in each cycle, before the results can be fetched.
Thus the user task could look like the following:
```
void user_task(void *pvParameters)
{
bme680_values_float_t values;
TickType_t last_wakeup = xTaskGetTickCount();
// as long as sensor configuration isn't changed, duration is constant
Function ```bme680_force_measurement``` is called inside the task loop to perform exactly one measurement in each cycle.
The task is then delayed using function ```vTaskDelay``` and the value returned from function ```bme680_get_measurement_duration``` or as long as function ```bme680_is_measuring``` returns true.
Since the measurement duration only depends on the current sensor configuration, it changes only when sensor configuration is changed. Therefore, it can be considered as constant as long as the sensor configuration isn't changed and can be determined with function ```bme680_get_measurement_duration``` outside the task loop. If the sensor configuration changes, the function has to be executed again.
Once the measurement results are available, they can be fetched as fixed point oder floating point sensor values using function ```bme680_get_results_float``` and ```bme680_get_results_fixed```, respectively.
