I2C problem pulling up to 3.3V from existing 1.8V circuit.

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GoExtreme

Joined Mar 4, 2018
52
Hello. I have a existing I2C circuit that communicates with capacitance touch controller. I need to replace the controller with cap touch capable MCU acting as I2C slave.
Lines are originally pulled to 1.8V, but mcu need min 3V. I trying to pull it with 4.7K resistors to 3.3V but the lane does go above 2.4 V - 2.7 V.
Seems like all components on the line are 5V - 5.5V capable. Even with 5V it just goes up to 2.7V. If I disconnect the circuit and measure on resistor end t shows 3.3v. Something is pulling it down and not sure what.
Is there a way to bypass the whole circuit? Meaning, at the end add components to bring the lines to 3V before the mcu ?
 

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DickCappels

Joined Aug 21, 2008
10,153
The problem is probably the electrostatic discharge protection circuits on the pins you are trying to pull higher than the power supply voltage. The give-away is that the pins can only go one diode drop above the power supply
 

Picbuster

Joined Dec 2, 2013
1,047
Is your question can I feed 1.8V into a 3V input?
If so no problem the 3 volt is max input voltage and a logical 1 is seen above the .8V (check datasheet).
The other way around is a small problem feeding 3V into 1.8V circuit.
This easy to solve with a resistor and diode to 1.8V VDD.
If you want to this very well then use a few resistors(voltage dividers) to get rid off Diode Vf.

Picbuster
 
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