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Using the Adafruit BMP280 sensor from Python via an FT232H.
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README.rst

Reading temperature and pressure with a BMP280 and an FT232H

Date: 2018-04-28
tags:BMP280, FT232H, Python3
Author: Roland Smith

Introduction

This code was written to get the Adafruit BMP280 breakout board to work with my computer, using an Adafruit FT232H breakout board as a USB ↔ SPI or USB ↔ I²C bridge.

The bmp280.py module supports both SPI and I²C connections between the FT232H and the BMP280. 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 bmp280-monitor-spi.py that can query the data from the sensor at a configurable interval and write it to a file.

The datasheet for the sensor, the Bosch code on github and the Adafruit CircuitPython were used as references in writing this code.

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 BMP280

Both the BMP280 and the FT232H breakout boards were placed on a small breadboard. I connected the breakout boards via SPI to run bmp280-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 bmp280.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 bmp280 import Bmp280spi
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)
bmp280 = Bmp280spi(spi)

Now you are ready to measure.

while True:
    print(bmp280.read())
    sleep(5)

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

The monitoring program

The bmp280-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 ./bmp280-monitor-spi.py -h to see the optional and required parameters.

An example:

./bmp280-monitor-spi.py -i 900 /tmp/bmp280-{}.d

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

# BMP280 data.
# Started monitoring at 2018-04-24T23:46:19Z.
# Per line, the data items are:
# * UTC date and time in ISO8601 format
# * Temperature in °C
# * Pressure in Pa
2018-04-24T23:46:19Z 24.6 100809
2018-04-24T23:51:19Z 24.6 100811
2018-04-24T23:56:19Z 24.7 100787
2018-04-25T00:01:19Z 24.6 100793
2018-04-25T00:06:19Z 24.6 100806
2018-04-25T00:11:19Z 24.5 100793
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