3.3 Volt AVR + 5V LCDisplay

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Hi,

I am trying to interface a megaAVR (it'll be a 103L, even though at the moment I only got a 163L sample), operating at 3.3V, to a 5V LCDisplay. Unfortunately I will have to read data from the display occasionally (and the Busy line), so that I need some kind of 5V --> 3.3V conversion. The AVR is aparantly not 5V tolerant (datasheet says Vcc +0.5V max).
Are the port outputs of the AVR clamped to Vcc with internal diodes ?
I mean, in order to protect the AVR port outputs, would it be sufficient to put e.g. 1kOhm resistors in series ? (AVR port - resistor - LCD port)
Of course it would increase current consumption, but most of the time the lines are tri-stated anyway. I just don't want to damage the AVR.

Another question:
The 4MHz megaAVR103L will work from 2.7V, according to the datasheet.
The 6MHz megaAVR103 starts at 4V ...
can I naivly just draw a line between 2.7V/4MHz and 4V/6MHz, and operate a megaAVR103L at 3.4Volts and 4.9152MHz ?

Any help is appreciated,
Thanks,
Volker

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all i know is that you can't draw a line 2.7v/4MHZ and the 4v/^MHZ

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the 163L can run @ 5v
the range is 2.7 to 5.5V

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Probably not a good idea for production, but the standard 5V ATMEGA103 will work at 3.3V (4Mhz).

To interface, I had a similar problem. Use Schottky diodes (cathode connected to MEGA I/O pin) with a pull-up resistor to 5V on the anode, and the display line at the junction . The 103L pulls the line low, the resistor pulls it high. The resistor needs to be about 1K-4K7.

Other option is to get a 3.3V display???

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andy:

hmm, i think i've missed something. imho your suggestion doesn't work.

i propose to put the kathode of the schottky-diode to 3.3V and the anode to the i/o-pin. then connect the i/o-pin to the 5V-display with a resistor of 1K.

greetings
tom

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