How to Connect an I²C Device to the Robotics TXT?

fischertechnik Robotics TXT controller has one extension port, EXT. This port operates at 3.3 V level.

The extension port EXT supports I²C connection, with 3 active wires: 
  • ​Ground, 
  • I²C Clock, and
  • I²C Data.
Contrary to what has been posted in the ft:c forum, video and even issue 2015-4 of ft-pedia, pin 9 of the extension port does not provide +3.3V power supply. 

​Do not use pin 9 as 3.3 V source.
The EXT connector for I²C on the Robotics TXT controller

I²C only defines a bus protocol, nothing more. 
  • Each I²C device is different: it may require 5 V or 3,3 V or even 1,8 V, runs at 100 or 400 kHz, and brings its own specific command set.
  • Before using any I²C device, read the specification sheet and the application notes very carefully! 
Otherwise, the I²C device and even the Robotics TXT controller may be damaged irremediably.
​
How to power the sensor with 3.3 V?

There's no special difficulty in assembling a separate 3.3 V power line with a LM2937-3.3 or LM1117-3.3, or similar based on the estimated power consumption of the devices.

The unregulated input is connected to the +9V output of the Robotics TXT, and so does Ground to Ground.

Connecting 5 V I²C Devices to Robotics TXT Controller

Some of the I²C devices I've developed libraries for, like the AMIS-30624 stepper controller or the 4 lines x 20 characters LCD display, run at 5 V.

​The circuit requires dual regulators for 3.3 V and 5 V power lines, and MOSFETs or PCA9306 to step-up I²C signals from 3.3 V to 5 V. Please refer to the different I²C Logic Level Converter options and the official Level shifting techniques in I2C-bus design application note from NXP. 

​Some voltage regulators require a minimum power usage to ensure good stability. In such a case, just place a LED with a limiting resistor on the 3.3 V power rail. 

The two circuits on the right combine the solutions mentioned above.
​
• Caution
The schematics supplied are for illustrative purpose only.
​Please read the specification sheets carefully and check the voltage and current requirements. Otherwise, the I²C device and even the Robotics TXT controller may be damaged irremediably.

Compatibility Issues

The I²C implementation in RoboPRO 4.2.4 for the Robotics TXT controller suffers from three limitations:
  • Although the Robotics TXT controller supports I²C clock stretching, one slave I²C device can no longer drive the clock line low in a MultiMaster configuration, as it was possible with the previous generation RoboTX controller.
  • Similarly, RoboPro 4.2.4 doesn't configure the I²C mode properly on the Robotics TXT controller. Even if standard mode (100 kHz) is selected, the controller runs in fast mode (400 kHz). Some I²C devices may not work if they don't support fast mode (400 kHz).
  • Despite the two cores (MPU and MCU), the Robotics TXT controller doesn't support fast mode plus (1 MHz).

Luckily, the RoboTX controller fully supports I²C fast mode (400 kHz).
​
I do not recommend the logic level converter for I²C described in issue 2016-4 of ft-pedia, as it doesn't match with the Level shifting techniques in I2C-bus design application note from NXP.
The circuit proposed on issue 2016-4 of ft-pedia doesn't match with the Level shifting techniques in I2C-bus design application note from NXP.

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