BME280_math module

Since	Origin / Contributor	Maintainer	Source
2016-02-21	vsky279	vsky279	bme280_math.c

This module provides calculation routines for BME280/BMP280 temperature/air presssure/humidity sensors (Bosch Sensortec). Communication with the sensor is ensured by Lua code through I2C or SPI interface. Read registers are passed to the module to calculate measured values.

See bme280 Lua module for examples.

bme280_math.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

bme280_math.altitude([self], P, QNH)

Parameters

• (optional) self userdata or table structure so that the function can be directly called as object method, parameter is ignored in the calculation
• P measured pressure
• QNH current sea level pressure

Returns

altitude in meters of measurement point

bme280_math.dewpoint()

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

Syntax

bme280_math.dewpoint([self], H, T)

Parameters

• (optional) self userdata or table structure so that the function can be directly called as object method, parameter is ignored in the calculation
• H relative humidity in percent (100 means 100%)
• T temperate in celsius

Returns

dew point in celsisus

bme280_math.qfe2qnh()

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

Syntax

bme280_math.qfe2qnh([self], P, altitude)

Parameters

• (optional) self userdata or table structure so that the function can be directly called as object method, parameter is ignored in the calculation
• P measured pressure
• altitude altitude in meters of measurement point

Returns

sea level pressure

bme280_math.read()

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

Syntax

bme280_math.read(bme280sensor, registers, [altitude])

Parameters

• bme280sensor - BME280 sensor user data returned by bme280_math.setup()
• registers - string of 8 bytes (chars) registers read from BME280_REGISTER_PRESS
• (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

Returns nil if the conversion is not successful.

bme280_math.setup()

Initializes module. Initialization is mandatory before read values.

Syntax

bme280_math.setup(registers, [temp_oss, press_oss, humi_oss, power_mode, inactive_duration, IIR_filter])

Parameters

• registers - String of configuration registers read from the BME280 sensor. It consists of 6 bytes (chars) of BME280_REGISTER_DIG_T, 18 bytes (chars) BME280_REGISTER_DIG_P and optional (not present for BMP280 sensor) 8 bytes (chars) of BME280_REGISTER_DIG_H1 (1 byte) and BME280_REGISTER_DIG_H2 (7 bytes)
• (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.

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

• bme280sensor user data (nil if initialization has failed)
• config 3 (2 for BME280) field table with configuration parameters to be written to registers BME280_REGISTER_CONFIG, BME280_REGISTER_CONTROL_HUM, BME280_REGISTER_CONTROL consecutively

Example

See bme280 Lua module documentation.

BME280 (selected) registers

name	address
BME280_REGISTER_CONTROL	0xF4
BME280_REGISTER_CONTROL_HUM	0xF2
BME280_REGISTER_CONFIG	0xF5
BME280_REGISTER_CHIPID	0xD0
BME280_REGISTER_DIG_T	0x88 (0x88-0x8D (6))
BME280_REGISTER_DIG_P	0x8E (0x8E-0x9F (18))
BME280_REGISTER_DIG_H1	0xA1
BME280_REGISTER_DIG_H2	0xE1 (0xE1-0xE7 (7))
BME280_REGISTER_PRESS	0xF7 (0xF7-0xF9 (8))