Exploring the i2c Bus

 

In this project I am using three separate sensor chips that are built into the AltiMU-10 sensor board:  Gyro, Accelerometer and Magnetometer.  By careful review of the the documentation on the Pololu web page for the MiniMu-9 V5

MiniMu9

The MiniMu9 circuit board contains three devices, a gyro, a magnetometer and a compass.  Each of the three devices has three sub sensors, X, Y and Z each representing a different dimension / orientation.

We communicate with the MiniMu9 using the Raspberry Pi's i2c serial bus.  The i2c bus is a serial bus that allows a computer to communicate with a variety of devices connected to the bus.  Broadly, this is how it works:

• Each device has an address on the bus
• Each device has a set of registers which can either be readable, writable or both
• Some of the registers are control registers, where you can read or write settings
• Some of the registers are data registers, where you can read or write data

A reading from one of the 9 sensors is read by sending a value to a register to tell the circuit which value we wish to read, then reading the value from a specific address.  We will need to read all nine addresses in order to take one complete set of readings.

Physical Connectivity

The i2c bus is accessed through the Raspberry Pi's serial input/output port.  4 wire connections are needed as follows:

Raspberry Pi Port       Altimu-10 Port        Purpose      My Cable Color

1   3V                             VDD                         3V Power   Red

3  GPIO-0                      SDA                          SDA           Yellow

5  GPIO-1                      SCL                           SCL            Orange

9  Ground                      GND                          GND           Black                                 

The picture above shows the orientation of the pin-out on the Raspberry Pi (note, this is picture of the Raspberry Pi V2 - I will retake the pic with a V3).

Raspberry Pi i2c Bus

i2c bus is a software convention for multiple devices to communicate with a computer over the same serial bus.  The Raspberry Pi's General Purpose Input Output (GPIO) connector (bottom right in the picture above) communicates with the Pi's processor using i2c.

The i2c bus documentation is available online, though is generally not needed here as the developer is abstracted from the complexities the electronic circuits by using a software library.  More to follow on the software later.

First, check that the i2c capability is enabled by accessing the Raspberry Pi's configuration menu:
sudo raspi-config

From the menu select Advanced Options, then select I2C.   On the next screen you are asked if you wish to enable the ARM I2C interface - select YES.

Reboot the Raspberry Pi to make the I2C interface active.

Next install the package that allows Python to communicates with the bus:

sudo apt-get install python-smbus

Next I install i2c tools, a package that allows you to see if devices are active on the i2c bus using the i2cdetect application.  From the Linux command line:
sudo apt-get install i2c-tools libi2c-dev

After saving and exiting from Nano, the Raspberry Pi must be rebooted from the command line with:
 sudo reboot 

Testing The i2c Bus

After rebooting I can use i2c tools to identify if the connections are working correctly.  From the Linux command line I run:
sudo i2cdetect -y 1
The resulting grid is displayed:

I  see the device addresses 6bH and 1eH are visible on the ic2 bus, and that no others are visible.  I check that against the port addresses used by the Gyro, Accelerometer and Magnetometer from my device documentation.  This looks good as the Gyro and Accelerometer both use 6b and the compass uses 1e.

Port Addresses

Gyro - LSM6DS33

Address                     0x6b
Control Register 2_G  0x11
Message                     0x40  01000000b  104Hz, 245dps, Gyro full scale at 125dps
Control Register 7_G  0x16
Message                     0x38  00111000b - X, Y, Z Gyro on
X Data Low               0x22
X Data High              0x23
Y Data Low               0x24
Y Data High              0x25
Z Data Low               0x26
Z Data High              0x27

Accelerometer - LSM6DS33

Address                       0x6b    1101011b
Control Register 1XL 0x10
Message                      0x40     01000000b - 104Hz, +/- 2g, 400Hz BW
Control Register 9XL 0x18      
Message                      0x38     00111000b - X, Y, Z Accel on
X Data Low                0x28
X Data High               0x29
Y Data Low                0x2a
Y Data High               0x2b
Z Data Low                0x2c
Z Data High               0x2d

Magnetometer - LIS3MDL

Address                   0x1e
Control Register 1   0x20
Message                  0x10
Control Register 4   0x23
Message                   0x00
X Data Low             0x28
X Data High            0x29
Y Data Low             0x2a
Y Data High            0x2b
Z Data Low             0x2c
Z Data High            0x2d
Temp Low               0x2e
Temp High              0x2f