This example code shows how to interface the Raspberry Pi Pico to an MPL3115A2 altimeter via I2C. The MPL3115A2 has onboard pressure and temperature sensors which are used to estimate the altitude. In comparison to the BMP- family of pressure and temperature sensors, the MPL3115A2 has two interrupt pins for ultra low power operation and takes care of the sensor reading compensation on the board! It also has multiple modes of operation and impressive operating conditions.
The board used in this example comes from Adafruit, but any MPL3115A2 breakouts should work similarly.
The MPL3115A2 makes available two ways of reading its temperature and pressure data. The first is known as polling, where the Pico will continuously read data out of a set of auto-incrementing registers which are refreshed with new data every so often. The second, which this example will demonstrate, uses a 160-byte first-in-first-out (FIFO) queue and configurable interrupts to tell the Pico when to read data. More information regarding when the interrupts can be triggered available in the datasheet. This example waits for the 32 sample FIFO to overflow, detects this via an interrupt pin, and then averages the 32 samples taken. The sensor is configured to take a sample every second.
Bit math is used to convert the temperature and altitude data from the raw bits collected in the registers. Take the temperature calculation as an example: it is a 12-bit signed number with 8 integer bits and 4 fractional bits. First, we read the 2 8-bit registers and store them in a buffer. Then, we concatenate them into one unsigned 16-bit integer starting with the OUT_T_MSB register, thus making sure that the last bit of this register is aligned with the MSB in our 16 bit unsigned integer so it is correctly interpreted as the signed bit when we later cast this to a signed 16-bit integer. Finally, the entire number is converted to a float implicitly when we multiply it by 1/2^8 to shift it 8 bits to the right of the decimal point. Though only the last 4 bits of the OUT_T_LSB register hold data, this does not matter as the remaining 4 are held at zero and "disappear" when we shift the decimal point left by 8. Similar logic is applied to the altitude calculation.
|
Tip
|
Choosing the right sensor for your project among so many choices can be hard! There are multiple factors you may have to consider in addition to any constraints imposed on you. Cost, operating temperature, sensor resolution, power consumption, ease of use, communication protocols and supply voltage are all but a few factors that can play a role in sensor choice. For most hobbyist purposes though, the majority of sensors out there will do just fine! |
Wiring up the device requires 5 jumpers, to connect VCC (3.3v), GND, INT1, SDA and SCL. The example here uses I2C port 0, which is assigned to GPIO 4 (SDA) and GPIO 5 (SCL) by default. Power is supplied from the 3.3V pin.
|
Note
|
The MPL3115A2 has a 1.6-3.6V voltage supply range. This means it can work with the Pico’s 3.3v pins out of the box but our Adafruit breakout has an onboard voltage regulator for good measure. This may not always be true of other sensors, though. |
- CMakeLists.txt
-
CMake file to incorporate the example in to the examples build tree.
- mpl3115a2_i2c.c
-
The example code.
Item |
Quantity |
Details |
Breadboard |
1 |
generic part |
Raspberry Pi Pico |
1 |
|
MPL3115A2 altimeter |
1 |
|
M/M Jumper wires |
5 |
generic part |