In our previous post “USITWIX – Using UART as TWI / I2C” we looked at the USITWIX library that implements TWI / I2C communication between а  ATtiny85 micro-controller and peripherals. Let’s see now how we can use that library to work with the BOSCH BMP180 atmospheric pressure sensor and a ATtiny85/Tinusaur boards.

The BMP180tiny Library

So, we wrote a simple library (called it BMP180tiny) that uses USITWIX to read and write from/to BMP180 registers, retrieve the measurements, do some additional calculations and produce result suitable for use in an application.

Setup

Here’s the setup:

NOTE: We need the USB-to-Serial just for debugging – it isn’t essential part of the setup.

Here is a short fragment of initialization code

uint8_t bmp180_result;
if ((bmp180_result = bmp180_init()) != BMP180_RESULT_SUCCESS)
{
return -1;
}

And here is now to use the functions …

// Read raw temperature
uint16_t temp_rawdata = bmp180_read_temp_raw();

uint16_t temp_10x = bmp180_read_temp10x();
// temp_10x holds the result dC, i.e. 123 means 12.3 Celsius

// Read raw pressure
int32_t pres_rawdata = bmp180_read_pres_raw();

// Read pressure
int32_t pres = bmp180_read_pres();
int16_t pres_hpa = pres / 100;
// pres_hpa holds the result in hPa (hectopascals)

// Read altitude
int32_t alt_x = bmp180_read_alt_x();
int16_t alt_dm = alt_x / 100;
// alt_dm holds the result in dm (decimeters)

The debugging output would look something like this …

t:raw=28622; t(dC)=253; p:raw/hi=5; p:raw/lo=4940; p(Pa)/hi=1; p(Pa)/lo=33948; p(hPa)=994; a/lo=24222; a(dm)=1552;
t:raw=28624; t(dC)=253; p:raw/hi=5; p:raw/lo=4964; p(Pa)/hi=1; p(Pa)/lo=33933; p(hPa)=994; a/lo=24307; a(dm)=1553;
t:raw=28623; t(dC)=253; p:raw/hi=5; p:raw/lo=4951; p(Pa)/hi=1; p(Pa)/lo=33939; p(hPa)=994; a/lo=22619; a(dm)=1536;
t:raw=28624; t(dC)=253; p:raw/hi=5; p:raw/lo=4942; p(Pa)/hi=1; p(Pa)/lo=33940; p(hPa)=994; a/lo=24560; a(dm)=1556;

NOTE: This is generated using OWOWOD library and hardware (not required by the library itself to work)

References

The source code is available at https://bitbucket.org/tinusaur/bmp180tiny.
— The library is in the “bmp180tiny” folder.
— A sample code could found in the “bmp180tiny_test1” folder.

The page about BMP180tiny and articles about BMP180tiny.

The page about USITWIX and articles about USITWIX.