Learning to Pi

Python bmx280_spi

The bmx280_spi python3 library uses SPI to communicate with a BME280 or BMP280 (BMx280) sensor. After testing a few different SPI modules that I wasn’t able to get working I decided to write a new module to configure and query the sensor.

This python class will query the sensor for the model (currently supports BME280 and BP280) and will facilitate programming and reading from the sensor.

Table of Contents
Related LearningToPi Posts
Git: https://github.com/LearningToPi/dht11_spi
PyPi: https://pypi.org/project/dht11-spi/

Wiring Diagram

Diagram Notes:

How It Works

  • SPI bus is configured for a speed compatible with the sensor (1kHz to 1MHz).
  • The sensor is configured with no filter and no oversampling and set to run in FORCED mode
  • Alternate values can be passed during initialization or later using the .init(...) function to reprogram all settings, or individual configuration functions to update a single setting.
  • Readings can be pulled from the sensor using the update_readings() function. This will return an instance of the Bmx280Readings class containing all the returned values as well as unit converion functions.

bmx280_spi – Install from PyPi

pip3 install -U bmx280_spi

Usage Example – CLI

The dht11_spi library can be called from the command line using some basic command line options to query the DHT11 or DHT22 sensor and print the temperature and humidity data to the console.

$ python3 -m bmx280_spi --help
usage: __main__.py [-h] [--gpio-chip GPIO_CHIP] [--time TIME] [--interval INTERVAL] [--spi SPI_BUS] [--spi-freq SPI_FREQ] [--temp-f] [--pressure PRES_FORMAT] gpio

Start a bme280/bmp280 test run using the bmx_spi library or the default RPi based library. Exact model is polled from the SPI device.

positional arguments:
  gpio                  GPIO to use for signaling the DHT sensor. If using GPIO_CHIP other than 0, set the "--gpio-chip x" option.

optional arguments:
  -h, --help            show this help message and exit
  --gpio-chip GPIO_CHIP
                        (default 0) GPIO chip for the GPIO provided. (0 typical for Pi4)
  --time TIME           (default 120) Time in seconds to run the test
  --interval INTERVAL   (default 1) Interval between reads. 1sec min for DHT11, 2sec min for DHT22 (per spec)
  --spi SPI_BUS         (default 0) SPI Bus number to use (assumes kernel driver loaded and accessible by spidev)
  --spi-freq SPI_FREQ   (default 500000Hz) Frequence to run on the SPI Bus.
  --temp-f              (default False) Print temps in F rather than C
  --pressure PRES_FORMAT
                        (default psi) Prints the pressure reading in psi|bar|pa

(venv) $ python3 -m bmx280_spi --spi 1 13 --time 10 --pressure psi --temp-f

BME280:
2023-03-03 23:10:15,263 - root - INFO - 10/10 (100.0%): Temps (min/avg/max): 75.74/75.77/75.79 deg F, Humidity (min/avg/max): 24.94/24.95/24.98 %, Pressure (min/avg/max): 14.05/14.06/14.06 psi

BMP280:

Usage Example – Importing into Code

The dht11_spi library can be imported into your code and executed using the following example:

from ammeter_logger import AmmeterRecvSerial

# Import DHT SPI class
from dht11_spi import DHT11_Spi, DHT22_Spi

# initialize device, use (DHT11_Spi or DHT22_Spi)
# cs_chip and cs_pin from "gpioinfo".  gpiod used for platform compat
dht = DHT22_Spi(spiBus=0, cs_chip=0, cs_pin=26)
reading = dht22.read() # returns instance of DhtReadings
print(reading)
# --or--
print(reading.temp_c, reading.temp_f, reading.humidity)

class BMX280_Exception(Exception)

Represents an error with the DHT11 class.

class Bmx280Readings

Represents a reading returend from the dht11_spi class. An object of this class will be returned when a read is called.

(property) temp_c()

Returns the temperature reading in Celcius

(property) temp_f()

Returns the temperature reading in Fahrenheit

(property) humidity()

Returns the humidity reading as a percentage (%)

(property) pressure_pa()

Returns the pressure reading in Pascal (Pa)

(property) pressure_psi()

Returns the pressure in pounds per square inch (psi)

(property) pressure_bar()

Returns the pressure reading in Bar

class Bmx280Spi

__init__(spiBus:int, cs_pin:int, cs_chip=None, set_spi_hz=True, logger=None,temp_enable=True, pressure_enable=True, humidity_enable=True, filter=0,temp_oversample=1, pressure_oversample=1, humidity_oversample=1,mode=MODE_FORCED, sleep_duration=0)

Initalizes BME280 or BMP280 sensor.

  • spiBus (int) – SPI bus number the class should listen on.
    • NOTE: For RPI4B systems, only SPI Bus 0 is operable
  • cs_pin (int) – GPIO pin used as a CS for the BMx280
  • cs_chip (int) – GPIO chip number for the cs_pin. Use “gpioinfo” to check for the proper chip number. If “None” uses chip 0
    • NOTE: For RPI4B systems, all GPIO’s are on chip 0
  • set_spi_hz (bool) – Updates the SPI bus speed to match the BMx280 sensor. If the current SPI bus speed is higher than supported, the bus speed is lowered to the maximum allowed, if the current SPI bus speed is lower than supported, the bus speed is raised to the minimum allowed.
    • NOTE: If you disable setting the SPI bus speed, be sure to set the speed to a supported frequency
  • logger (Python logger) – Python logger to use for messages. See Python logging_handler for a function to create loggers.
  • temp_enable (bool: True) – Enables temperature monitor
  • pressure_enable (bool:True) – Enables the pressure monitor
  • humidity_enable (bool:True) – Enables the humidity monitor
  • filter (int:0) – Enables the filter, see set_filter() function below for more details
  • temp_oversample (int: 0) – Sets oversampling for the temp sensor. See set_temp_oversample() below for more details
  • pressure_oversample – Sets oversampling for the pressure sensor. See set_pressure_oversample() below for more details
  • humidity_oversample – Sets oversampling for the humidity sensor. See set_humidity_oversample() below for more details
  • mode (int:1) – Sets the mode of the sensor. See set_power_mode() for more details
  • sleep_duration (int:0) – Sets the sleep duration when running in NORMAL mode. See set_sleep_duration_value() for more details.

init(temp_enable=True, pressure_enable=True, humidity_enable=True, filter=0,temp_oversample=1, pressure_oversample=1, humidity_oversample=1,mode=MODE_FORCED, sleep_duration=0)

Initializes the sensor with the provided values. See the __init__() function for details.

get_filter() -> int

Get the current filter value. See set_filter() for details

set_filter(vaule:int) -> bool

Set the filter value. The filter is used limit transient changes in the temperature and pressure readings. Readings from one time period are carried forward and averaged with subsequent readings. The BMx280 datasheet refers to this as Infinite Impulse Response (IIR) filtering.

The BMx280 sensor supports values of 0, 2, 4, 8, and 16. At each reading the value stored in the data register is updated using the following formula:

See the Bosch datasheet for more details.

Returns True if the change was successful.

get_sleep_duration_value() -> int

Get the current sleep duration value. See set_sleep_duration_value() for details

get_sleep_duration_ms() -> int

Get the current sleep duration in milliseconds. See set_sleep_duration_value() for details

set_sleep_duration_value(vault:int) -> bool

Set the sleep duration between readings when running in NORMAL mode. (see set_power_mode() for details on setting the mode). The integer values match up to the following times (in milliseconds):

Sleep ValueTime (ms)
00.5
162.5
2125
3250
4500
51000
62000
74000

Returns True if the change was successful.

set_sleep_duration_ms(vaule:int) -> bool

Sets the sleep duration to a number of milliseconds. The value provided is rounded DOWN to the nearest matching value. See set_sleep_duration_value() for detailed values supported by the BMx280 sensor.

Returns True if the change was successful.

get_temp_oversample() -> int

Get the current temperature sensor oversampling value. See set_temp_oversample() for details.

set_temp_oversample(value:int) -> bool

Set the temperature sensor oversampling value. Oversampling utilizes the digital signal processor on the BMx280 to take multiple samples in rapid succession. The returned value is averaged to remove random noise. The BMx280 supports the following oversampling values: 1, 2, 4, 8, 16

Returns True if the change was successful.

get_pressure_oversample() -> int

Get the current pressure sensor oversampling value. See set_pressure_oversample() for details.

set_pressure_oversample(value:int) -> bool

Set the pressure sensor oversampling value. Oversampling utilizes the digital signal processor on the BMx280 to take multiple samples in rapid succession. The returned value is averaged to remove random noise. The BMx280 supports the following oversampling values: 1, 2, 4, 8, 16

Returns True if the change was successful.

get_humidity_oversample() -> int

Get the current humidity sensor oversampling value. See set_temp_oversample() for details.

set_humidity_oversample(value:int) -> bool

Set the humidity sensor oversampling value. Oversampling utilizes the digital signal processor on the BMx280 to take multiple samples in rapid succession. The returned value is averaged to remove random noise. The BMx280 supports the following oversampling values: 1, 2, 4, 8, 16

Returns True if the change was successful.

get_power_mode() -> int

Get the current power mode. 0 = Standby, 1 or 2 = Forced, 3 = Normal. Forced mode is only applicable while a forced read is in progress and is changed to 0 (Standby) once the read is complete.

set_power_mode(value:int) -> bool

Sets the power mode. 0 = Standby, 1 or 2 = Forced, 3 = Normal. Setting the power mode to Forced will trigger a reading then the mode will change to Standby.

Returns True if the change was successful.

(property) measuring() -> bool

Returns True if a measurement is in progress.

(property) updating() -> bool

Returns True if the sensor is in process of udating radings in NVRAM

reset_device()

Resets the sensor. Be sure to update settings after a reset.

update_readings(timeout:int) -> Bmx280Readings

Triggers a read. If the device is in Forced mode, this will trigger a read and return the results. If the device is in Normal mode, this will read the latest available data from the sensor. Be sure that reads are far enough apart based on the sleep timout set in set_sleep_timeout_value()