Interfacing 5v Tx to 3.3v Rx?

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blapcb

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I have a 5v device sending data out on its Tx and I have to connect it to a micro's (MSP430) Rx which is 3.3v level. I tried using a simple resistive divider (5K and 10K) but behavior is erratic (sometimes works, sometimes does not, sometimes starts when you touch it...). What is a better way of doing this? Using a MOSFET?
 

What is the speed of your communication?
Maybe, lower values of R, did you try 1K and 2K2?
If resistors doesn't work, you need to use a transistor. Bipolar transistors works well.
If transistors still don't work, you must use a chip voltage level translator.
 

No I did not try another resistor value, I guess I should.

A transistor? I never saw a design using a regular transistor. Do you mean a MOSFET? That is the only type of design I have seen for 5v -> 3.3v
 

A bipolar transistor will work ok. Let me see if can I found a circuit called "front dog" or something similar.
 

it should be 6.8K paired with 10K
this should you a voltage level well within the minimum-maximum specification
 

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