Posted 06 May 2018

For the last two months I have been working on adding some secondary, but still important capabilities to Wall-E2, my wall-following autonomous robot.   As I noted back in April, Wall-E2 still can’t tell which way he is heading, which (among other problems) means he can’t make accurate turns.   In addition, Wall-E2 can’t tell how long (or even  if) he has been turned off, making it impossible to tell how long it has been since he last was charged.

In the intervening months, I have been able to obtain and test individual modules to address the above issues; the Sparkfun MPU9250 9DOF IMU breakout board to obtain heading information, and the combination of an Adafruit MB85RC256V FRAM breakout board and an Adafruit DS3221 RTC breakout board to capture the date/time of any power interruptions.

Since all three of the above breakout boards are I2C-capable, it should be feasible to run all three from a single I2C bus on Wall-E2’s main controller – an Arduino Mega 2560.   Unfortunately, the Sparkfun MPU9250 module isn’t 5V tolerant, so integrating all three isn’t as simple as just daisy-chaining them all together.   Back to Google for some more research, where I eventually uncovered Philips Application Note AN97055 by Herman Schutte of The Netherlands dealing with bidirectional level-shifters for just this problem. Turns out that Philips introduced the I2C bus back in 1980, so they may know a thing or two about the issues ;-).   In any case, this 1997 paper described a 2-MOSFET bidirectional level-shifter that completely addresses the issue, and seemed to be pretty straightforward to implement.   It took me a couple of tries, but I got it working, with the result that I can now run all three modules (RTC, FRAM and IMU) from an Arduino Mega 2560, with the level-shifting MOSFET’s placed between the IMU and the rest of the circuit, as shown in the photo below.

All three sensors operating a the same time, controlled over a single I2C channel.

In the above photo, the modules are (from left to right): Adafruit MB85RC256K FRAM breakout, Adafruit DS3231 RTC breakout, and Sparkfun MPU9250 9DOF IMU breakout.   The two  2N7000 level-shifter MOSFETs can be seen at the top left-hand corner of the Sparkfun IMU breakout board.

I put together a small test program, combining pieces from the software used to test the power-down date/time capture idea, and the software used to test the ability to use the Sparkfun IMU to accurately manage rotations.   This program doesn’t do much at all, but it does demonstrate that all three modules can be controlled at the same time over a single I2C channel, as shown in the following output.