Understanding BSEC 2 Sample Rates

[marsfan]

I'm working on a small personal environmental monitoring project that uses the BME688 alongside the BSEC2 library, and I am confused about some of the behvaior related to the sample rate of virtual sensors, as documented in section 1.2.4 of the integration guide PDF, and in the example code provided in the library.

1. Looking through the integration guide PDF and the examples provided by Bosch, I'm confused as to how the sample rate for the virtual sensors are supposed to be configured. The array that is passed into bsec_update_subscription allows for a different sample rate to be set for each virtual sensor, but the documentation, as well as the provided examples make it look as if only one sample rate should be used for all of the different virtual sensors. Should I be using the same sampel rate for all sensors, or can I mix which sensors are used (i.e. some sensors in ULP mode, and others in SCAN mode)?
2. I'm confused as to what sample rates the different virtual sensors support. In the bsec_iot_example.c file that is provided with the BSEC library, the sample rate of the gas index and estimate sensors are set to BSEC_SAMPLE_RATE_LP, however the integration guide PDF that is provided with the library states (in section 1.2.4) that these sensors only support BSEC_SAMPLE_RATE_SCAN. Is this an error in the integration guide, or in the example code?
3. Looking at the integration guide (in section 1.2.4), it seems that the gas index features and the IAQ/VOC/eCO2 measurements are not able to operate in the same sample modes, and thus likely cannot be operated at the same time. This is further indicated by the statements that the sensor shall not perform both at once, and shall not switch between different modes during the lifetime of the sensor. Does this mean that to be able to perform both types of measurements, multiple sensors must be used?
4. In the integration guide (in section 1.2.4), it states that " the user shall not switch between different modes during the lifetime of
   a given sensor." What is the meaning of "lifetime"? Does this mean for an operational purpose, or that once I start using the sensor in one mode, I should never change it again, even if I intend to use the sensor in an entirely different prototype project?

[BSTRobin · Accepted solution]

Hi marsfan,

Virtual sensors cannot be mixed in BSEC_SAMPLE_RATE_SCAN mode and BSEC_SAMPLE_RATE_LP mode.
BSEC software does not yet support the mixing of virtual sensors in BSEC_SAMPLE_RATE_SCAN mode and BSEC_SAMPLE_RATE_LP mode. It is not yet supported to use both functions simultaneously.
BSEC_OUTPUT_RAW_GAS_INDEX, BSEC_OUTPUT_RAW_GAS_ESTIMATE only support BSEC_SAMPLE_RATE_SCAN mode, setting BSEC_OUTPUT_RAW_GAS_INDEX, BSEC_OUTPUT_RAW_GAS_ESTIMATE to BSEC_SAMPLE_RATE_LP mode in reference code bsec_iot_example.c is incorrect.

[marsfan]

Thanks for the quick reply. So SCAN and LP modes cannot be mixed at all. Is it possible to mix modes such as CONT, and LP, so that some sensors update faster than others.

Also, regarding lifetime (question #4 that I had), does that mean that once I have used the sensor in LP mode, I should never use it in SCAN mode, even for a new project I may be prototyping, or does that just mean in the active application?

[BSTRobin]

Hi marsfan,

CONT, and LP cannot be mixed.
The current BSEC software architecture does not support simultaneous operation of SCAN mode and LP mode, and the BME688 sensor hardware itself does not limit the software's use of which mode.

[marsfan]

Awesome. Thank you.