# BME280 module
| Since  | Origin / Contributor  | Maintainer  | Source  |
| :----- | :-------------------- | :---------- | :------ |
| 2016-02-21 | [vsky279](https://github.com/vsky279) | [vsky279](https://github.com/vsky279) | [bme280.c](../../../app/modules/bme280.c)|

This module provides a simple interface to [BME280/BMP280 temperature/air presssure/humidity sensors](http://www.bosch-sensortec.com/bst/products/all_products/bme280) (Bosch Sensortec).

Note that you must call [`init()`](#bme280init) before you can start reading values!

## bme280.altitude()

For given air pressure and sea level air pressure returns the altitude in meters as an integer multiplied with 100, i.e. altimeter function.

#### Syntax
`bme280.altitude(P, QNH)`

#### Parameters
- `P` measured pressure
- `QNH` current sea level pressure

#### Returns  
altitude in meters of measurement point

## bme280.baro()

Reads the sensor and returns the air pressure in hectopascals as an integer multiplied with 1000 or `nil` when readout is not successful.
Current temperature is needed to calculate the air pressure so temperature reading is performed prior reading pressure data. Second returned variable is therefore current air temperature.

#### Syntax
`bme280.baro()`

#### Parameters
none

#### Returns  
- `P` air pressure in hectopascals multiplied by 1000
- `T` temperature in celsius as an integer multiplied with 100

## bme280.dewpoint()

For given temperature and relative humidity returns the dew point in celsius as an integer multiplied with 100.

#### Syntax
`bme280.dewpoint(H, T)`

#### Parameters
- `H` relative humidity in percent multiplied by 1000.
- `T` temperate in celsius multiplied by 100.

#### Returns  
dew point in celsisus

## bme280.humi()

Reads the sensor and returns the air relative humidity in percents as an integer multiplied with 100 or `nil` when readout is not successful.
Current temperature is needed to calculate the relative humidity so temperature reading is performed prior reading pressure data. Second returned variable is therefore current temperature.

#### Syntax
`bme280.humi()`

#### Parameters
none

#### Returns  
- `H` last relative humidity reading in % times 1000
- `T` temperature in celsius as an integer multiplied with 100

## bme280.init()

Initializes module. Initialization is mandatory before read values.

#### Syntax

`bme280.init(sda, scl, [temp_oss, press_oss, humi_oss, power_mode, inactive_duration, IIR_filter])`

#### Parameters
- `sda` - SDA pin  
- `scl` - SCL pin
- (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 fitler coefficient is 16.
- (optional) `cold_start` - If 0 then the BME280 chip is not initialised. Usefull 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 (`bme280.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
`nil` if initialization has failed (no sensor connected?), `2` if sensor is BME280, `1` if sensor is BMP280

#### Example
```lua
alt=320 -- altitude of the measurement place

bme280.init(3, 4)

P, T = bme280.baro()
print(string.format("QFE=%d.%03d", P/1000, P%1000))

-- convert measure air pressure to sea level pressure
QNH = bme280.qfe2qnh(P, alt)
print(string.format("QNH=%d.%03d", QNH/1000, QNH%1000))

H, T = bme280.humi()
if T<0 then    
  print(string.format("T=-%d.%02d", -T/100, -T%100))
else
  print(string.format("T=%d.%02d", T/100, T%100))
end
print(string.format("humidity=%d.%03d%%", H/1000, H%1000))
D = bme280.dewpoint(H, T)
if D<0 then
  print(string.format("dew_point=-%d.%02d", -D/100, -D%100))
else
  print(string.format("dew_point=%d.%02d", D/100, D%100))
end

-- altimeter function - calculate altitude based on current sea level pressure (QNH) and measure pressure
P = bme280.baro()
curAlt = bme280.altitude(P, QNH)
if curAlt<0 then
  print(string.format("altitude=-%d.%02d", -curAlt/100, -curAlt%100))
else
  print(string.format("altitude=%d.%02d", curAlt/100, curAlt%100))
end
```

Or simpler and more efficient
```lua
alt=320 -- altitude of the measurement place

bme280.init(3, 4)

T, P, H, QNH = bme280.read(alt)
if T<0 then
  print(string.format("T=-%d.%02d", -T/100, -T%100))
else
  print(string.format("T=%d.%02d", T/100, T%100))
end
print(string.format("QFE=%d.%03d", P/1000, P%1000))
print(string.format("QNH=%d.%03d", QNH/1000, QNH%1000))
print(string.format("humidity=%d.%03d%%", H/1000, H%1000))
D = bme280.dewpoint(H, T)
if D<0 then
  print(string.format("dew_point=-%d.%02d", -D/100, -D%100))
else
  print(string.format("dew_point=%d.%02d", D/100, D%100))
end

-- altimeter function - calculate altitude based on current sea level pressure (QNH) and measure pressure
P = bme280.baro()
curAlt = bme280.altitude(P, QNH)
if curAlt<0 then
  print(string.format("altitude=-%d.%02d", -curAlt/100, -curAlt%100))
else
  print(string.format("altitude=%d.%02d", curAlt/100, curAlt%100))
end
```

Use `bme280.init(sda, scl, 1, 3, 0, 3, 0, 4)` for "game mode" - Oversampling settings  pressure ×4, temperature ×1, humidity ×0, sensor mode: normal mode, inactive duration  = 0.5 ms, IIR filter settings  filter coefficient 16.

Example of readout in forced mode (asynchronous)
```lua
bme280.init(3, 4, nil, nil, nil, 0) -- initialize to sleep mode
bme280.startreadout(0, function ()
  T, P = bme280.read()
  if T<0 then
    print(string.format("T=-%d.%02d", -T/100, -T%100))
  else
    print(string.format("T=%d.%02d", T/100, T%100))
  end
end)
```

## bme280.qfe2qnh()

For given altitude converts the air pressure to sea level air pressure.

#### Syntax
`bme280.qfe2qnh(P, altitude)`

#### Parameters
- `P` measured pressure
- `altitude` altitude in meters of measurement point

#### Returns  
sea level pressure


## bme280.read()

Reads the sensor and returns the temperature, the air pressure, the air relative humidity and 

#### Syntax
`bme280.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 as an integer multiplied with 100
- `P` air pressure in hectopascals multiplied by 1000
- `H` relative humidity in percent multiplied by 1000
- `QNH` air pressure in hectopascals multiplied by 1000 converted to sea level

Any of these variables is `nil` if the readout of given measure was not successful.

## bme280.startreadout()
Starts readout (turns the sensor into forced mode). After the readout the sensor turns to sleep mode.

#### Syntax
`bme280.startreadout(delay, callback)`

#### Parameters
- `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](http://ae-bst.resource.bosch.com/media/_tech/media/datasheets/BST-BME280_DS001-11.pdf#page=51).
- `callback` if provided it will be invoked after given `delay`. The sensor reading should be finalized by then so.

#### Returns  
`nil`

## bme280.temp()

Reads the sensor and returns the temperature in celsius as an integer multiplied with 100.

#### Syntax
`bme280.temp()`

#### Parameters  
none

#### Returns
- `T` temperature in celsius as an integer multiplied with 100 or `nil` when readout is not successful
- `t_fine` temperature measure used in pressure and humidity compensation formulas (generally no need to use this value)
