BME280 pressure, temperature and humidity sensor with PIC MCU

This post shows how to interface Microchip PIC16F877A microcontroller with BME280 barometric pressure, temperature and humidity sensor. Values of temperature, humidity and pressure are displayed on 20×4 LCD screen connected to the microcontroller.
In this project the BME280 sensor is used in I2C mode and the compiler used is CCS PIC C.

The BME280 sensor from Bosch Sensortec is a low cost digital pressure, temperature and humidity sensor with good accuracy. Because pressure changes with altitude we can use it as an altimeter with ±1 meter accuracy (pressure accuracy = ±1 hPa). Some parameters of the sensor are listed below:

Pressure range: 300 … 1100 hPa (equivalent to +9000…-500m above/below sea level)
Pressure resolution: 0.01 hPa ( < 10 cm)
Temperature range: -40 … 85 °C
Temperature resolution: 0.01 °C
Humidity range: 0 … 100 %
Interface: I2C and SPI
Supply voltage range: 1.71 … 3.6 V

The BME280 chip works with maximum voltage of 3.6V (supply voltage range is from 1.71 to 3.6V) which means we’ve to use a 3V3 voltage regulator to supply it from a 5V source.
Also, if we’re working with a 5V system (development board, microcontroller …) like the PIC16F877A microcontroller we’ve to use a voltage level shifter (level converter) which converts the 3.3V (comes from the BME280 chip) into 5V (goes to the PIC16F877A) and vice versa. This level shifter is for the I2C bus lines (clock and data).

Some BME280 modules come with 3V3 voltage regulator and level shifter like the one provided by Adafruit Industries which is shown below.
A module like this can be used with 3.3V or 5V system without any problem.

Ardafruit BME280 sensor breakout board

In this example I’m going to use a Chinese BME280 module came with 3.3V regulator and level shifter, this means connection will be more easier!

BME280 Sensor driver for CCS C compiler:
To simplify the C code, I wrote a simple driver (library) for the BME280 sensor. With this driver the interfacing is more easier.

Driver functions:
BME280_begin(mode, T_sampling, H_sampling, P_sampling, filter, standby): this function initializes the BME280 sensor with the 6 configurations (mode, T_sampling, H_sampling, P_sampling, filter and standby), returns 1 if OK and 0 if error:
mode is sensor mode which can be: MODE_SLEEP, MODE_FORCED or MODE_NORMAL.

T_sampling is temperature oversampling, H_sampling is humidity oversampling and P_sampling is pressure oversampling, each one of them can be one of the following list (library default is SAMPLING_X1):
SAMPLING_SKIPPED    —>  skipped, output set to 0x80000
SAMPLING_X1           —>  oversampling x1
SAMPLING_X2           —>  oversampling x2
SAMPLING_X4           —>  oversampling x4
SAMPLING_X8           —>  oversampling x8
SAMPLING_X16         —>  oversampling x16

filter sets BME280 sensor IIR filter configuration, it could be (library default is FILTER_OFF):
FILTER_OFF  —>  filter off
FILTER_2     —>  filter coefficient = 2
FILTER_4     —>  filter coefficient = 4
FILTER_8     —>  filter coefficient = 8
FILTER_16    —>  filter coefficient = 16

standby is BME280 sensor standby time (library default is STANDBY_0_5):
STANDBY_0_5    —>  standby time = 0.5 ms
STANDBY_62_5   —>  standby time = 62.5 ms
STANDBY_125    —>  standby time = 125 ms
STANDBY_250    —>  standby time = 250 ms
STANDBY_500    —>  standby time = 500 ms
STANDBY_1000   —>  standby time = 1000 ms
STANDBY_10     —>  standby time = 10 ms
STANDBY_20     —>  standby time = 20 ms

Examples:
The following line initializes the BME280 sensor in normal mode, temperature oversampling = X2, humidity oversampling = X1, pressure oversampling = X16, IIR filter coefficient = 16 and standby time = 0.5 ms:
BME280_begin(MODE_NORMAL, SAMPLING_X2, SAMPLING_X1, SAMPLING_X16, FILTER_16, STANDBY_0_5);

and the following line initializes the BME280 sensor in normal mode, temperature oversampling = X1, humidity oversampling = X1, pressure oversampling = X1, IIR filter is off and standby time = 0.5 ms where default settings are used:
BME280_begin(MODE_NORMAL);

BME280_ForcedMeasurement(): this function is used to start a new measurement conversion, used in forced mode only. Returns 1 if ok and 0 if error (sensor is not in sleep mode).

BME280_readTemperature(int32_t *temp): reads temperature from BME280 sensor. Temperature is stored in hundredths °C (output value of “5123” equals 51.23 °C). Temperature value is saved to *temp, returns 1 if OK and 0 if error.

BME280_readHumidity(uint32_t *humi): reads humidity from BME280 sensor. Humidity is stored in relative humidity percent in 1024 steps (output value of “47445” represents 47445/1024 = 46.333 %RH). Humidity value is saved to *humi, returns 1 if OK and 0 if error.

BME280_readPressure(uint32_t *pres): reads pressure from BME280 sensor. Pressure is stored in Pa (output value of “96386” equals 96386 Pa = 963.86 hPa). Pressure value is saved to *pres, returns 1 if OK and 0 if error.

BME280 driver for CCS C compiler download:
Driver source file download link is below, installation of this driver is easy, just add it to project folder or CCS C drivers folder (for example C:\Program Files (x86)\CCS\Drivers). Driver name is: BME280_Lib.c.
BME280 CCS C driver

Interfacing PIC16F877A microcontroller with BME280 sensor:

PIC16F877A with BME280 sensor and 2004 LCD

Hardware Required:

  • PIC16F877A microcontroller
  • BME280 sensor module with 3.3V regulator and level shifter   —->  BME280 datasheet
  • 2004 LCD screen
  • 8 MHz crystal oscillator
  • 2 x 22pF ceramic capacitor
  • 10k ohm variable resistor (or potentiometer)
  • 330 ohm resistor
  • 10k ohm resistor
  • 5V source
  • Breadboard
  • Jumper wires

Example circuit schematic diagram is shown below.
Note that the BME280 module shown in the circuit diagram has a 3.3V regulator and level shifter.

PIC16F877A BME280 sensor and LCD circuit

Generally, the BME280 module has at least 4 pins because it can work in SPI mode or I2C mode. For the I2C mode we need 4 pins: VCC, GND, SCL and SDA where:
GND (ground) is connected to circuit ground (0V)
VCC is the supply pin, it is connected to +5V
SCL is I2C bus serial clock line, connected to PIC16F877A RC3 pin (#18)
SDA is I2C bus serial data line, connected to PIC16F877A RC4 pin (#23).

The 20×4 LCD screen is used to display temperature, humidity and pressure values where:
RS —> pin RD0
RW —> pin RD1
E   —> pin RD2
D4 —> pin RD3
D5 —> pin RD4
D6 —> pin RD5
D7 —> pin RD7
VSS, RW, D0, D1, D2, D3 and K are connected to circuit ground
VEE to the variable resistor (or potentiometer) output
VDD to +5V and A to +5V through 330 ohm resistor.

VEE pin is used to control the contrast of the LCD. A (anode) and K (cathode) are the back light LED pins.

In this project the PIC16F877A microcontroller runs with 8 MHz crystal oscillator.

Interfacing PIC16F877A with BME280 sensor C code:
The C code below is for CCS C compiler, it was tested with version 5.051.

To be able to compile project C code, a driver for the BME280 sensor is required, download link is above.
After you download the driver file which named BME280_Lib.c, add it to your project folder!

As any other I2C device, the BME280 sensor has an I2C slave address which is 0xEC or 0xEE. This address depends on the connection of the SDO pin (used for SPI mode as serial data out or MISO), if the SDO pin is connected (directly or through resistor) to VCC (3.3V) the address will be 0xEE, and if it’s connected to GND the address will be 0xEC.

The default I2C address of the library is defined as 0xEE and my device I2C address is 0xEC.
In the code the definition of the I2C slave address is as shown below:

The initialization of the BME280 sensor is done using the function BME280_begin(MODE_NORMAL) which returns 1 if OK and 0 if error. In the code the initialization of the sensor is as shown below:

The LCD displays “connection error!” if there was an error with the initialization of the device.

Reading the values of temperature, humidity and pressure is done as shown below.
Note that the library returns the temperature in hundredths °C which means we’ve to divide it by 100, it returns the humidity in relative humidity percent (RH%) in 1024 steps which means we’ve to divide it by 1024 and it returns the pressure in Pa, to get the pressure in hPa we’ve to divide it by 100.

Temperature, humidity and pressure values are displayed on 20×4 LCD screen.

1 bar = 10000 Pa = 100 hPa. ( 1 hPa = 100 Pa)
Pa: Pascal
hPa: hectoPascal

Full CCS C Code:

Related Project:
Interfacing PIC MCU with BMP280 temperature and pressure sensor

4 comments

    1. If you mean I2C library source code, it’s a built-in function (library) that comes with CCS C compiler and “I think” there’s no way to see its source code!

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