kittles / ft232-bme280

logging temp and humidity

Reading temperature, pressure and relative humidity with a BME280 and an FT232H

Date:	2018-04-28
tags:	BME280, FT232H, Python3
Author:	Roland Smith

Introduction

This code was written to get the Adafruit BME280 breakout board to work with my computer, using an Adafruit FT232H breakout board as a USB ↔ SPI or USB ↔ I²C bridge.

The bme280.py module supports both SPI and I²C connections between the FT232H and the BME280. If uses pyftdi to handle those connections. This module in turn requires pyserial and pyusb. The advantage of pyftdi is that it is a pure python solution. It does not require native libraries which makes installing it easier.

Additionally, there is a program called bme280-monitor-spi.py that can query the data from the sensor at a configurable interval and write it to a file.

This code is based on similar code I wrote for the BMP280. I have used the datasheet for the sensor, the Bosch code on github and the Adafruit CircuitPython as references in writing these.

This software has been written for Python 3 on the FreeBSD operating system. I expect it will work on other POSIX systems, and maybe even on ms-windows. But I haven't tested that.

Wiring the BME280

Both the BME280 and the FT232H breakout boards were placed on a small breadboard. I connected the breakout boards via SPI to run bme280-monitor-spi.py:

• 5V to VIN
• GND to GND
• D0 to SCK
• D1 to SDI
• D2 to SDO
• D3 to CS

Note that for this to work, any native driver for FTDI chips needs to be unloaded and disabled. On FreeBSD the first is achieved by running kldunload uftdi.ko as root. The second step is accomplished by commenting out the nomatch statement in /etc/devd/usb.conf that loads uftdi driver and restarting devd by running service devd restart as root.

The module

Assuming you are in the directory where the bme280.py module lives, and you have installed pyftdi, you can use it as follows. First copy the following into an IPython 3 session or a Python 3 interpreter.

from time import sleep
from pyftdi.spi import SpiController
from bme280 import Bme280spi
ctrl = SpiController()
ctrl.configure('ftdi://ftdi:232h/1')  # Assuming there is only one FT232H.
spi = ctrl.get_port(0)  # Assuming D3 is used for chip select.
spi.set_frequency(1000000)
bme280 = Bme280spi(spi)

Now you are ready to measure.

while True:
    print(bme280.read())
    sleep(5)

This should print a (temperature, pressure, relative humidity) tuple every five seconds.

The monitoring program

The bme280-monitor-spi.py program is designed to be started from the command line, where it should probably be started so as to run in the background. Run ./bme280-monitor-spi.py -h to see the optional and required parameters.

An example:

./bme280-monitor-spi.py -i 900 /tmp/bme280-{}.d

This will write data to a file in /tmp every fifteen minutes. The data will look like this:

# BME280 data.
# Started monitoring at 2018-04-28T19:38:01Z.
# Per line, the data items are:
# * UTC date and time in ISO8601 format
# * Temperature in °C
# * Pressure in Pa
# * Relative humidity in %.
2018-04-28T19:38:02Z 20.72 68476 78.04
2018-04-28T19:53:02Z 20.41 100980 56.52
2018-04-28T20:08:02Z 20.38 100992 56.93

The first measurement should be ignored since it was taken shortly after the chip was reset.