AVR-Chat

On Apr 28, 2008, at 9:34 PM, Bob Gardner wrote:

> This is probably in the data sheet but I can't find it or maybe don't
> understand it. I'm working on a project with two Mega324p processors.
> One is running at 5 volts and the other is running at 3.3 volts. I am
> planning to communicate between them using the built in serial ports.
> Do I need to do something between the two ports or I can I connect the
> serial ports diretly together? I'll probably use a 1K resistor to
> protect the ports but am not sure if I have to reduce the output from
> the 5 volt processor to 3.3 volts.
>
> Thanks for any suggestions.
>
> Bob
>
>
> 
I;ve done this several ways. The best , over and over, has been to use  
a 74LCX125. It has an open-drain with 5V compliance (IN and OUT) even  
when powered from 3.3V. When the enable is tied LOW, it is a simple  
non-inverting buffer that needs an output pullup.

On the 5V output, just connect the pullup to +5V and, voila, you have  
a 3.3V in, 5V out, buffer. On the 3.3V output, connect that output  
pullup to 3.3V power and bring the 5V logic into its input.

Works great. Little pain. Almost guaranteed.

Jim Wagner
Oregon Research Electronics




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