Category: Embedded Systems

In last week’s blog post, we worked on connecting an accelerometer (BMA220) to the jetson nano thru I2C. This is just one of our steps towards the goal to eventually create a predictive maintenance model that can take an input of various different types. In this post we will continue on this path by getting the x, y, and z data from our accelerometer with python.

Using SMBus:

To communicate with our sensor, we will be using SMBus, or System Management Bus. SMBus is a two-wire bus made for working with I2C. The documentation can be found here. To use SMBus in our python file, first pip install smbus, then add the following code to the top:

from smbus import SMBus import time #this allows us to use time-related functions we will need later.

• Getting the BMA220’s address

To communicate with the accelerometer, we will need to specify the address of it, and what bus it is on. We can use the i2cdetect command below to scan a specific bus to find the accelerometer’s address. 

i2cdetect -y -r 1 #scans bus number 1

You should get an output similar to this:

This tells us that the accelerometer is located on bus 1, at the address 0x0a. We can take these pieces of data and create two variables in a python file:

i2cbus = SMBus(1)  # Create a new I2C bus i2caddress = 0x0a  # Address of BMA220

• Reading data output

We will next use SMBus to read the x, y, and z data from the corresponding register address. Below is a global memory map that shows the BMA220’s I2C register addresses and their functions. The full documentation of the BMA220 can be found here.

The useful addresses for us here are 0x4, 0x6, and 0x8. These correspond to the x, y, and z data, respectively. 

To communicate with these addresses we will use the “read_byte_data” function (which takes the parameters ‘i2c address’ and ‘register to read’) inside a loop that runs forever. We will get the x, y, and z data and assign them to a variable. We will print out the three values once every second. This is the code to implement this: 

while (True):         xdata = i2cbus.read_byte_data(             i2caddress, 0x4)  # read the value of x data         ydata = i2cbus.read_byte_data(             i2caddress, 0x6)  #read the value of y data         zdata = i2cbus.read_byte_data(             i2caddress, 0x8)  #read the value of z data         print(xdata, ydata, zdata)  # print the value of x y and z data         time.sleep(1)

After running the python file, you should get an output similar to this:

If you physically move the accelerometer, you can see the values change as they are updated every second. 

Potential problems:

• Strange addresses on buses

If you run i2cdetect on bus 0, or bus 2, you may see addresses there although you don’t have any other I2C devices connected to said bus. These are actually internal I2C devices that the jetson nano uses for internal communication. This was confusing for me when locating my i2c device, as I wasn’t sure if it was on the bus I was thinking of. A full discussion of the problem can be found here.

Other useful resources:

• Eclipse UPM sensor repository

There exists a repository created by the Eclipse foundation which is supposed to provide drivers for a variety of sensors, including the BMA220. Personally, I could not figure out how to use it, but there is valuable information in the /src/bma220 folder pertaining to the BMA220’s registers, specifically in the .hpp file. The project can be found here.

• I2C guide

This blog provides a good overview of I2C, and a brief tutorial for using an I2C device with a Raspberry Pi. The code given can be used on the jetson nano with only a few small tweaks.

• Nvidia Developer Forums

If you have a unique problem using I2C with the jetson nano, chances are there is someone else who has had the same problem and posted about it on the Nvidia developer forums, found here.

Next week, we will take a look at setting up an experiment to use the accelerometer(s) to get useful data. 

In last week’s blog post, we finished creating our first machine learning model using a public repository dataset. While this is a major step towards our end goal, it is missing a critical aspect: being able to use our own data on the model to train it and test it. The goal for the model is to eventually be able to apply it to many different types of data, and to easily be able to change in between use cases. 

Collecting our own data:

To use our own data on the model, we first need to collect some data. In this case we will be using an accelerometer that outputs x, y, and z data. We are using the SEN0168 accelerometer with a BMA220 chip. This accelerometer uses an I2C or inter-integrated circuit interface to connect to the jetson nano.

• What is I2C?

I2C is actually not eye-two-C, but I-squared-C. This stands for “inter-integrated circuit interface”. I2C is a single-ended serial communication bus that is widely used for connecting integrated circuits (which can come on all kinds of sensors, motors, etc.) to processors and microcontrollers. Regardless of how many pins you have on your microcontroller, you can connect as many I2C devices as you’d like. I2C even supports connecting multiple microcontrollers to allow for more than one controller to communicate with all peripheral devices on the bus. 

• Connecting to the Jetson

The jetson nano comes with a 40-pin header. The pins functions are shown in the image below.

For the I2C connection, we will only need to use 4 pins: VDC (power- in this case 3.3v), GND (ground), SCL (clock), and SDA (data). These pins on the jetson nano (pins 1, 3, 5, and 6) will be plugged into the corresponding pins on the BMA220.

Checking the connection:

If the BMA220 has been properly connected, it should light up. We can now check for the signal on the jetson nano. 

Open the command line and run “ i2cdetect -y -r busnumber”. I am connected to I2C bus 1, and if you followed the same pin setup described above you should be as well. 

We can see that we are detecting our I2C device on 0x0a, or port 10. If you connect to other buses, you can check that as well by changing the number of bus in the i2cdetect command. 

We now have a properly connected I2C device to our Jetson Nano. In the next blog post, we will cover any configuration necessary and write some code to actually access the accelerometer’s data.