Logic Levels - Mixing 5v and 3v3

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#1
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Hey all,

I have a need to run a custom board at 5v. The issue is the board needs an xBee as well. I'm pretty sure the xBees only run on 3v3 so I'm wondering about talking between the two. (Just the serial lines are needed)

The AVR (ATmega128) datasheet says that the Input High Voltage needs to be above 0.6 VCC to read. 5v * .6 = 3V. Can I thus just use the xBee output directly into the input pins? Or is it really worth throwing a small transistor with a couple pull-ups

In the opposite direction, I was just gonna use a voltage divider but it would be nice to just drive the 3v3 inputs with 5v. Anyone know if you can just drive the xBee with 5v? Will it hurt it? I can't find any electrical characteristics for the xBee.

Pushing AVRs to their limits

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If it doesn't say that the inputs are 5V-compatible, you can't assume that they are!

Leon

Leon Heller G1HSM

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why not run the avr with 3.3v!

I love Digital
and you who involved in it!

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If you want a complete solution $4.90 buys you this:

http://www.futurlec.com/Mini_Log...

Otherwise just borrow their idea of using a 74LC245

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5V -> 3.3V
Use a voltage divider (2 resistors)
http://www.makingthings.com/docu...

3.3V -> 5V
Use a voltage buffer (a transistor)
http://125.7.56.8/ocau/logic.jpg

uC's: Atmega16, 32, 64, 128 and Attiny13
Lang.: C
Interests: Small scale robots AND sensor monitoring system

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I've just started experimenting with XBee and have been using the Adafruit Industries XBee Adapter kit with a +5V ATmega setup.

See this link for information on the Adafruit Industries XBee Adapter kit. You can download EAGLE schematic and board files from this link.

Don