In order to work any of USB GPIO Adapters, USB I2C devices or USB SPI Interfaces handy and properly, you’ve better to know how it works on the electric level. So, in this article I will tell you what happen in the USB I2C Device on the electric level.

I want to list you what things participate in this process and its abbreviation:

  • VCC is I2C supply voltage;
  • SDA is Serial Data line;
  • SCL is Serial Clock line;
  • Rp is Pull-up resistance;
  • Rs is Serial resistance.

Now, I tell what happen between two USB I2C devices connected via a USB-Bus.

Two USB-I2C devices are connected to the power supply with the voltage of 3.3 V. The voltage goes through the line and comes to the pull-up resistor, where starts the allocation of the voltage.

If two USB-I2C-SPI-GPIO devices are Open Drain the flow of voltage comes to both devices and they works.

If one of the devices are Open Drain and another is not open drain. The voltage goes only to the USB I2C Adapter with the open drain.

If two USB-I2C-SPI-GPIO Adapters are closed (not open drain) the voltage stores into the Pull-up resistorm, until one of the devices will not be open drain.

And if two I2C Interfaces have different voltages, and the voltage line passes the higher stain, the device with lower voltage will be broken.