In a project of mine I have two ICs that work at different voltages.
IC_A at 3V and IC_B at 3.3V and they talk to each other through UART.
In my prototype I use a mosfet with two resistors for each signal in UART( TX, RX ) as a level converter.
My question is:
Could I omit the mosfet and connect them directly or with only a resistor in series? And if yes how to find the resistor's value? e.g.Ohms law?
edit: 3V and 3.3V are the theoretical voltages. When I measure them they are 2.85V and 3.3V…
Best Answer
You actually don't care what the VCC or VDD voltage level is, but are more interested in the V(input lo) and V(input high) requirements for both devices.
For your 3 V and 3.3 V system elements the real questions are:
For many interface elements the input is advertised as 5 V safe, so for those elements the 3.3 V output can interface directly to the 3 V system.
I'd suggest there are no conditions where this would not work or cause damage.
Some logic has clamp structures in the input gates, so there is more to consider. For these elements it may look a bit like this:
Under these conditions (high signal) where the input diode clamps are planar diodes with a Vf of typically 650 mV you unlikely to get current flowing from the 3.3 V output into the 3 V elements VDD structure/supply.
For safety your could add a small value resistor to limit current.
Since the V(output high) of the 3 V element will approach 3 V, this will easily meet the requirements which are likely in the 1-1.8 V level. You can directly connect the 3 V element output to the 3.3 V element input.