Electronic – Is this communication I²C


I need to decode communication between two devices, but I have no information about these devices. All I know is that four wires are needed (GND, VCC and two wires of communication). I suspect that it is I²C communication.

I'm trying to decode it with the oscilloscope decoding tool, but I'm not quite sure about it. I can not identify elements of I²C communication appropriately when I visually check the waveforms.

Looking at the waveforms I made the following assumptions, and maybe someone can help. These were my assumptions:

  1. Everything leads to believe that the clock is the blue signal and the data is the red signal.
  2. The clock seems to be inverted because its idle state is not at high level.
  3. I'm not sure if the data signal is also inverted, but it seems to be.

Are my assumptions correct?

In the last figure, the figure with the number 5 indicated in a circle, and there is a part of the signal. I can not identify the start, ack and stop bits. Can anyone identify these elements just looking at the figure?

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[Edited] Some people asked me about the devices that are in the communication. The communication is between a car key and a tool that I'm not allowed to say, but I'm trying to do a reverse engineering on it.

Best Answer

My guess is that it's some company's homegrown "I2C-like" protocol. There were some of those back in the day when using I2C meant having to give money to Philips.

It appears to have an ACK (the short pulse on the data line prior to the clock stretch looks a lot like the data line getting passed from master to slave).

Oddly, it appears to transmit 7 bits at a time.