Due to the way the BSEC library is designed, it is mandatory to call the library every 3s if you want to benefit from the ULP+ feature. Also note that the ULP+ feature is expected to be used sporadically, therefore there is limit of one measurement per minute enforced even to the ULP+ feature.
If you would need your MCU to sleep for the whole 300s period of ULP mode, you would need to use the strict ULP mode without ULP+ feature.
Alternatively it is possible to subscribe to the gas outputs in ULP mode (300s sampling period), while subscribing to temperature/pressure/humidity outputs in LP mode (3s sampling period). That way would still need to wake-up your CPU every 3s, but you would benefit from the faster sampling rate of temperature/humidity outputs, while maintaining the lower current consumption of the gas sensor from ULP mode.
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