Hardware

• adafruit feather HUZZAH board
• BME680 temperature, humidity, barometric pressure and VOC gas sensor

Software

• micropython/ (https://github.com/micropython/micropython.git)
• modules/bsec/
  • https://docs.micropython.org/en/latest/develop/cmodules.html
  • https://www.bosch-sensortec.com/software-tools/software/bsec/
  • https://github.com/BoschSensortec/BME68x-Sensor-API

Build

https://github.com/micropython/micropython/blob/master/ports/esp8266/README.md#build-instructions

Dependencies

Use esp-open-sdk in docker, from https://github.com/larsks/docker-image-esp-open-sdk

Use our own patch on top:

make -C esp-open-sdk-for-micropython

# install 'esp-sdk' wrapper that forwards calls to docker, it mounts full $HOME
docker run --rm -u $UID -v $HOME/bin:/target esp-open-sdk install-wrapper
# usage
https://github.com/larsks/docker-image-esp-open-sdk#wrapper-script

Prepare

cd micropython
# get submodules for esp8266 port
esp-sdk make -C ports/esp8266 submodules
# build micropython cross compiler (<1min)
esp-sdk make -C mpy-cross
# patch micropython, cf https://github.com/devbis/st7789_mpy/#overflow-of-iram1_0_seg
git apply ../0001-fix-iram1-overflow-move-bsec-text-to-irom0.patch

Build

# build port (~30s)
cd micropython/ports/esp8266
#esp-sdk make clean
esp-sdk make USER_C_MODULES=../../../modules

Output (example):

Use make V=1 or set BUILD_VERBOSE in your environment to increase build verbosity.
Including User C Module from ../../../modules/bsec
...
LINK build-GENERIC/firmware.elf
   text    data     bss     dec     hex filename
 661645    1016   67640  730301   b24bd build-GENERIC/firmware.elf
Create build-GENERIC/firmware-combined.bin
esptool.py v1.2
flash     34832
 .text    31444 at 0x40100000
 .data    1016 at 0x3ffe8000
 .rodata  2332 at 0x3ffe8400
padding   2032
irom0text 627872
total     664736
md5       72bb9964e63979e5d79471410fb143b1

More build options:

• append MICROPY_BSEC_DEBUG_VERBOSE=1 to the make command to get debug logs of the BSEC module

Flash

Follow flash doc: https://github.com/micropython/micropython/blob/master/ports/esp8266/README.md#build-instructions

Usage

import machine
import time
import bsec

i2c = machine.I2C(scl=machine.Pin(5), sda=machine.Pin(4), freq=100000)
bme680 = bsec.BME680_I2C(i2c)
bme680.init()

while True:
    measurement_timestamp_ns = time.time_ns()
    bme680.force_measurement(measurement_timestamp_ns)
    delay_us = bme680.get_read_data_delay_us()
    time.sleep_us(delay_us)
    data = bme680.read_data(measurement_timestamp_ns)
    print(data)
    next_call_timestamp_ns = bme680.get_next_call_timestamp_ns()
    delay_us = max(next_call_timestamp_ns - time.time_ns(), 0) // 1000
    print("sleep (in s)", delay_us/1000000)
    time.sleep_us(delay_us)

# example output:
# {'iaq': 144.758, 'staticIaqAccuracy': 3, 'gasResistance': 171795.0, 'rawHumidity': 49.9882, 'co2Accuracy': 3, 'breathVocAccuracy': 3, 'staticIaq': 103.249, 'iaqAccuracy': 3, 'breathVocEquivalent': 1.63182, 'co2Equivalent': 1032.49, 'humidity': 65.8691, 'pressure': 100328.0, 'temperature': 24.1201, 'rawTemperature': 28.7868}


# After iaqAccuracy reaches 3, i.e. calibrated:

# Save state
calibrated_state = bme680.get_state()
# Save to persistent memory
state_filename = 'bsec_bme680_calibrated_state.bin'
with open(state_filename, 'wb') as f:
    wrote = f.write(calibrated_state)
print('wrote', wrote, 'bytes on file', state_filename)

# Load from persistent memory
with open(state_filename, 'rb') as f:
    calibrated_state = f.read()
print('read', len(calibrated_state), 'bytes on file', state_filename)
# Restore
bme680.set_state(calibrated_state)

Calibrate the BME680 and BSEC

(see BSEC_1.4.8.0_Generic_Release/integration_guide/BST-BME680-Integration-Guide-AN008-48.pdf)

• expose the sensor once to good air (e.g. outdoor air) and bad air (e.g. box with exhaled breath) for auto-trimming.
• wait for accuracy to reach 3
• save calibration state:
  • connect with screen
  • CTRL-C
  • sensors_bme680_save_state(sensor_bme680)
  • reset board

Development

Some resources that helped:

• https://docs.micropython.org/en/latest/develop/cmodules.html
• https://micropython-usermod.readthedocs.io/en/latest/index.html
• https://forum.micropython.org/, various posts to better know external C modules, also notably https://forum.micropython.org/viewtopic.php?t=9703 to better understand the libmath linking issue
• https://github.com/devbis/st7789_mpy/ real example, notably with the iram1 overflow
• https://github.com/BoschSensortec/BSEC-Arduino-library
• https://www.bosch-sensortec.com/software-tools/software/bsec/ Integration Guide pdf
• https://community.bosch-sensortec.com/t5/Bosch-Sensortec-Community/ct-p/bst_community, notably https://community.bosch-sensortec.com/t5/MEMS-sensors-forum/BME680-on-LPC845-using-Bosch-s-BSEC-library/m-p/17717 to better understand the libmath linking issue