Interfacing 5v Tx to 3.3v Rx?

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1

I have a 5v device sending data out on its Tx and I have to connect it to a MSP430’s 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?

Thanks

2

Someone referenced this a while ago, and it’s a good resource, for 5V-3.3V interfacing.

http://www.microchip.com/stellent/group … 026368.pdf

3

If it is 5V to 3v, I usually use a buffer that has 5V-tolerant inputs. You can use something like a Fairchild NC7SZ125. It is only 1 gate so it doesn’t take up much real estate.

Cheers!

gm

4

gm:
If it is 5V to 3v, I usually use a buffer that has 5V-tolerant inputs. You can use something like a Fairchild NC7SZ125. It is only 1 gate so it doesn’t take up much real estate.

Cheers!

gm

I don’t understand, what is the NC7SZ125 supposed to do? I only see one connection to Vcc. How does it work?

5

Azimon:
I don’t understand, what is the NC7SZ125 supposed to do? I only see one connection to Vcc. How does it work?

The key is that the NC7SZ125 is tolerant of a 5V input when powered at 3.3V, so it only needs a single power rail. The output toggles between gnd and Vcc, while the input can go between gnd and 5V, in your case.