AVR-Chat

On Feb 23, 2005, at 2:13 PM, kc9dag wrote:

> Will the keyboard work on +3v,

If you are designing a product for mass production then the answer is 
"NO!" But if you are experimenting or playing with a one-off item then, 
"Try it and find out. Some keyboards probably will and some won't."

> or can I power the keyboard from a +5v source and just use a common
> ground between the keyboard and the butterfly (and if I do it that
> way, can the AVR's inputs handle the +5v while it is running at
> +3v)???

There is a pullup diode on the AVR inputs primarily intended for static 
zap protection. One Atmel apnote discusses using a 1M resister between 
an IRQ input pin and the AC power mains for a cheap zero crossing 
detector. Maybe with a series capacitor too. That at 1000 volts peak 
the AVR input pullup diode only has to conduct 1mA. They also go to 
great lengths to point out they have skipped all the safety 
ramifications.

As others have said, use a voltage divider to bring the keyboard 
outputs down to safe range. The AVR at 3 volts shouldn't have any 
problems making valid data levels for 5 volt TTL. 5 volt CMOS could be 
a problem. Would hope the keyboard has HCT logic rather than HC logic.

--
David Kelly N4HHE, dkelly@HiWAAY.net
========================================================================
Whom computers would destroy, they must first drive mad.