BME280 Micropython driver for the BME280 sensor

This is a driver for the Bosch BME280 temperature/pressure/humidity sensor, for use with MicroPython on Pycom of ESP8266 boards. It is also compatible with the BMP280 which provides the same interface but temperature + pressure only.

Two different variants of the library are supplied. bme20_int.py uses integer arithmetic, bme280_float.py uses float arithmetic for the compensation of the raw values. The results are (almost) the identical, but the format of the returned values differs.

About the BME280

The Bosch BME280 Environmental Sensor is a combined temperature, pressure and humidity sensor. It can communicate via I2C or SPI; this driver uses I2C.

See the datasheet at https://ae-bst.resource.bosch.com/media/_tech/media/datasheets/BST-BME280_DS002.pdf for details.

Class

bme= BME280(i2c=i2c, mode=BME280_OSAMPLE_8, address=BME280_I2CADDR)

mode is the setting for oversampling of the humidity value, address the i2c address used.

Properties

values = BME280.values

The values property is a convenience function that provides a tuple of human-readable string values to quickly check that the sensor is working. In practice, the method to use is read_compensated_data() which returns a (temperature, pressure, humidity)-tuple

altitude = bmp.altitude

Altitude in m. The altitude is calculated based on the value given to the property sealevel (see below). Obviously, this value does not have to be the sealevel pressure, but any pressure you may select, for instance to measure altitude difference in general.

bmp.sealevel = sealevel

sealevel = bmp.sealevel

Setting and getting the pressure for altitude calculation.
The default is 101325 Pa, but you can use your local QNH in Pa, or set a local pressure to determine altitude difference.

dew_point = bmp.dew_point

Returns the dew_point temperature (°C) calculated from the actual temperature and humidity.

Methods

values = read_compensated_data(result = None)

Values is an array of either integers (bme280_int.py) of floats (bme280_float.py), holding the values of temperature, pressure and humidity. The format differs for integers and floats:

Integer formats:

• temperature: the temperature in hundredths of a degree Celsius. For example, the value 2534 indicates a temperature of 25.34 degrees.
• pressure: the atmospheric pressure. This 32-bit value consists of 24 bits indicating the integer value, and 8 bits indicating the fractional value. To get a value in Pascals, divide the return value by 256. For example, a value of 24674867 indicates 96386.2Pa, or 963.862hPa.
• humidity: the relative humidity. This 32-bit value consists of 22 bits indicating the integer value, and 10 bits indicating the fractional value. To get a value in %RH, divide the return value by 1024. For example, a value of 47445 indicates 46.333%RH.

Float formats

• temperature: the temperature in degree Celsius.
• pressure: the atmospheric pressure in Pascal.
• humidity: the relative humidity in percent.

If the parameter result is supplied as an array of the appropriate type, The return values will in addition be stored in that array, and the array will be returned.

read_raw_data(result)

Store the raw sensor data into the array result, which must provide space for three 32 bit integers, as provided for instance by array("i", [0, 0, 0]). This method is used internally.

Example

Copy bme280_float.py onto the board. Then:

#
# this script assumes the default connection of the I2C bus
# On pycom devuces that is P9 = SDA, P10 = scl
#
import machine
import bme280_float

i2c = machine.I2C()
bme = bme280_float.BME280(i2c=i2c)

print(bme.values)