BME280 module

Since Origin / Contributor Maintainer Source
2020-10-04 vsky279 vsky279 bme280.c

This module communicates with BME280/BMP280 temperature/air presssure/humidity sensors (Bosch Sensortec) through I2C interface.

Note: the module works only with the bme280_math module.

bme280.setup()

Creates bme280sensor object and initializes module. Initialization is mandatory before read values. Note that there has to be a delay between some tens to hundreds of milliseconds between calling setup() and reading measurements.

Functions supported by bme280sensor object: - setup() - read() - startreadout() - qfe2qnh - altitude - dewpoint

Syntax

bme280.setup(id, [address, temp_oss, press_oss, humi_oss, power_mode, inactive_duration, IIR_filter])

Parameters

  • id - I2C bus number
  • (optional)address - BME280 sensor address. 1 for BME280_I2C_ADDRESS1 = 0x76, 2 for BME280_I2C_ADDRESS2 = 0x77. Default sensor address is BME280_I2C_ADDRESS1.
  • (optional) temp_oss - Controls oversampling of temperature data. Default oversampling is 16x.
  • (optional) press_oss - Controls oversampling of pressure data. Default oversampling is 16x.
  • (optional) humi_oss - Controls oversampling of humidity data. Default oversampling is 16x
  • (optional) sensor_mode - Controls the sensor mode of the device. Default sensor more is normal.
  • (optional) inactive_duration - Controls inactive duration in normal mode. Default inactive duration is 20ms.
  • (optional) IIR_filter - Controls the time constant of the IIR filter. Default filter coefficient is 16.
  • (optional) cold_start - If 0 then the BME280 chip is not initialised. Useful in a battery operated setup when the ESP deep sleeps and on wakeup needs to initialise the driver (the module) but not the chip itself. The chip was kept powered (sleeping too) and is holding the latest reading that should be fetched quickly before another reading starts (bme280sensor:startreadout()). By default the chip is initialised.
temp_oss, press_oss, humi_oss Data oversampling
0 Skipped (output set to 0x80000)
1 oversampling ×1
2 oversampling ×2
3 oversampling ×4
4 oversampling ×8
5 oversampling ×16
sensor_mode Sensor mode
0 Sleep mode
1 and 2 Forced mode
3 Normal mode

Using forced mode is recommended for applications which require low sampling rate or hostbased synchronization. The sensor enters into sleep mode after a forced readout. Please refer to BME280 Final Datasheet for more details.

inactive_duration t standby (ms)
0 0.5
1 62.5
2 125
3 250
4 500
5 1000
6 10
7 20
IIR_filter Filter coefficient
0 Filter off
1 2
2 4
3 8
4 16

Returns

sobj - BME280 Sensor Object (nil if initialization has failed)

BME280 Sensor Object Methods

sobj:setup()

Re-initializes the sensor.

Parameters

Parameters are the same as for the bme280.setup function.

Return

Returned values are the same as for the bme280.setup function.

sobj:altitude()

For given air pressure (called QFE in aviation - see wiki QNH article) and sea level air pressure returns the altitude in meters, i.e. altimeter function.

Syntax

sobj:altitude(P, QNH)

Parameters

  • P measured pressure
  • QNH current sea level pressure

Returns

altitude in meters of measurement point

sobj:dewpoint()

For given temperature and relative humidity returns the dew point in celsius.

Syntax

sobj:dewpoint(H, T)

Parameters

  • H relative humidity in percent (100 means 100%)
  • T temperate in celsius

Returns

dew point in celsisus

sobj:qfe2qnh()

For given altitude converts the air pressure to sea level air pressure (QNH).

Syntax

sobj:qfe2qnh(P, altitude)

Parameters

  • P measured pressure
  • altitude altitude in meters of measurement point

Returns

sea level pressure

sobj:read()

Reads the sensor and returns the temperature, the air pressure, the air relative humidity and see level pressure when altitude is specified.

Syntax

sobj:read([altitude])

Parameters

  • (optional) altitude- altitude in meters of measurement point. If provided also the air pressure converted to sea level air pressure is returned.

Returns

  • T temperature in celsius
  • P air pressure in hectopascals
  • H relative humidity in percent
  • (optional) QNH air pressure in hectopascals (when altitude is specified)

Returns nil if the readout is not successful.

sobj:startreadout()

Starts readout (turns the sensor into forced mode). After the readout the sensor turns to sleep mode. Callback function is called with readout results.

Syntax

sobj:startreadout(delay, callback)

Parameters

  • callback if provided it will be invoked after given delay. Callback parameters are identical to sobj:read results.
  • altitude in meters of measurement point (QNH is returned when specified)
  • delay sets sensor to forced mode and calls the callback (if provided) after given number of milliseconds. For 0 the default delay is set to 113ms (sufficient time to perform reading for oversampling settings 16x). For different oversampling setting please refer to BME280 Final Datasheet - Appendix B: Measurement time and current calculation.

Returns

nil

Example

alt=320 -- altitude of the measurement place

sda, scl = 3, 4
i2c.setup(0, sda, scl, i2c.SLOW)
s = require('bme280').setup(0)

tmr.create():alarm(500, tmr.ALARM_AUTO, function()
    local T, P, H, QNH = s:read(alt)
    local D = s:dewpoint(H, T)
    print(("T=%.2f, QFE=%.3f, QNH=%.3f, humidity=%.3f, dewpoint=%.2f"):format(T, P, QNH, H, D))
end)

Example with sensor in sleep mode between readouts (asynchronous readouts)

alt=320 -- altitude of the measurement place

sda, scl = 3, 4
i2c.setup(0, sda, scl, i2c.SLOW)
s = require('bme280').setup(0, nil, nil, nil, nil, 0) -- initialize to sleep mode

tmr.create():alarm(1000, tmr.ALARM_AUTO, function()
    s:startreadout(function(T, P, H, QNH)
        local D = s:dewpoint(H, T)
        print(("T=%.2f, QFE=%.3f, QNH=%.3f, humidity=%.3f, dewpoint=%.2f"):format(T, P, QNH, H, D))
    end, alt)
end)

Altimeter function - calculate altitude based on current sea level pressure (QNH) and measure pressure

alt = 0 -- initial altitude

sda, scl = 3, 4
i2c.setup(0, sda, scl, i2c.SLOW)
s = require('bme280').setup(0)

tmr.create():alarm(100, tmr.ALARM_AUTO, function()
    local _, P, _, lQNH = s:read(alt)
    if not QNH then QNH = lQNH end
    local altitude = s:altitude(P, QNH)

    print(("altitude=%.3f m"):format(altitude))
end)