I2C – NVIDIA Jetson AGX Xavier Developer Kit

It is straightforward to connect an I2C device to a Jetson AGX Xavier. Looky here:

Background

I2C is a straightforward serial protocol. There are usually two wires, one is for transferring data (SDA), the other is a clock which is used to mark the begin and end of data packed (SCL). Most devices will also require power (VCC) and ground (GND). There are several I2C busses on the NVIDIA Jetson AGX Xavier Kit. You can access I2C bus 8 and I2C bus 1 on the GPIO Expansion Header.

The Xavier GPIO Expansion Header is basically the same layout as the previous generation Jetson TX series. However, there is a slight difference in the software to interface with I2C devices. The Jetson TX series use a derivative of Ubuntu 16.04 (L4T 28.x), the Xavier is an Ubuntu 18.04 variant (L4T 31.x). Ubuntu 18 shifts the libi2c-dev smbus API to a separate library, and a different header file in comparison to the earlier version.

Hardware

Note:A Jetson Xavier using L4T 31.0.2 (JetPack 4.1) is shown in the demo.

First, before powering up the Jetson, let’s wire up the LED Segment Display. Avoid wiring the Xavier when there is power connected. Here’s the pinout of the GPIO Expansion Header. In our example, we power the display from the Jetson GPIO header at 5V.

For this example project, a Adafruit 0.56″ 4-digit 7-segment Display W/i2c Backpack – Green is wired to a Jetson. The Display is assembled per the Adafruit instructions.

On the Jetson AGX Xavier, Pin 1 of the GPIO Expansion Header is the pin closest to the power indicator light:

Jetson AGX Xavier Pin 1
Jetson AGX Xavier Pin 1

The odd number pins (right to left, pins 1,3,5 and so on) are on the top row. The bottom row is the even number pins.

On a Jetson Xavier, here’s the wiring combination for I2C Bus 8:

GND Pin 6 -> LED Backpack (GND)
VCC Pin 2 -> LED Backpack (VCC – 5V)
SDA Pin 3 -> LED Backpack (SDA)
SCL Pin 5 -> LED Backpack (SCL)

Note that the Xavier also has a I2C Bus 1 interface. See the Xavier GPIO Pinout Diagram.

If you wish to interface with I2C Bus 1:

GND Pin 6 -> LED Backpack (GND)
VCC Pin 2 -> LED Backpack (VCC – 5V)
SDA Pin 27 -> LED Backpack (SDA)
SCL Pin 28 -> LED Backpack (SCL)

Note: To use Bus 1 with the example, you will need to modify the example source code.

Software Installation

Once the board is wired up, turn the Jetson on.
Install the libi2c-dev library. In order to be able inspect the LED Display, you may find it useful to also install the i2c tools:

$ sudo apt-get install libi2c-dev i2c-tools

After installation, in a Terminal execute (8 is the I2C bus in this case):

$ sudo i2cdetect -y -r 8

ubuntu@tegra-ubuntu:~$ sudo i2cdetect -y -r 8
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: — — — — — — — — — — — — —
10: — — — — — — — — — — — — — — — —
20: — — — — — — — — — — — — — — — —
30: — — — — — — — — — — — — — — — —
40: — — — — — — — — — — — — — — — —
50: — — — — — — — — — — — — — — — —
60: — — — — — — — — — — — — — — — —
70: 70 — — — 74 — — —

You should see an entry of 0x70, which is the default address of the LED Segment Display. Note that if you have soldered the address pins on the Display to change the address, you should see the appropriate address.

Next, install the library and example code which is available in the JHLEDBackpack repository on the JetsonHacks Github account. As of this writing, the Xavier version is in the L4T31 branch of the repository. There is a tagged release version for the Xavier in the v2.0 tag of the repository. To install:

$ git clone https://github.com/jetsonhacks/JHLEDBackpack.git
$ cd JHLEDBackpack
$ git checkout v2.0
$ cd example

You are then ready to compile the example and run it.

$ make
$ sudo ./displayExample

The display will go through a couple of examples, a blinking set of dashes, a hexadecimal display, a floating point number display, a count down timer and a clock example. Hit the ‘Esc’ key during the clock example to end the demo.

The source library defaults to I2C Bus 1. When we use bus 8 as in our example, displayExample.cpp reads:

HT16K33 *displayMatrix = new HT16K33() ;
// Default I2C Bus 1
// Bus 8 for Xavier Expander Header pins 3,5
displayMatrix->kI2CBus = 8;
int err = displayMatrix->openHT16K33();

Note: If you wish to use Pin 27 and 28, change the kI2CBus to 1. Make sure you save the file, and run make on it.

More Notes

Equipment and Supplies

The segmented LED display is a kit. You will need some elementary soldering skills for assembly. We tend to use:

New to electronics? This is a pretty easy project, looky here: Electronics Tutorials for some introductory material on how to start becoming a master.

Conclusion

Accessing the I2C bus on the Jetson AGX Xavier GPIO Expansion Header is straightforward, and makes for easy prototyping!

Be the first to comment

Leave a Reply

Your email address will not be published.


*