AVR-Chat

--- In AVR-Chat@yahoogroups.com, "Graham Davies" <Yahoo37849@...> wrote:
> 
> What isn't in the data sheet, but we all know, is that there are 
> protection diodes between each input and VCC.  At VCC + 0.5V, they 
> will start to turn on.  By 5V, they will be fried.  But, if you limit 
> the current with a resistor, they can be used to prevent the input 
> voltage from going much above the maximum.  Pragmatically, this will 
> be OK and you can get away with it at low switching speeds.  The use 
> of serial ports (I presume you mean the UARTS, "serial" is ambiguous) 
> will qualify as low speed.

There is an Atmel apnote documenting the above unofficial technique.
IIRC the apnote was discussing building an AC utility power meter and
how one could interface high voltage 240 VAC to detect zero crossings.
They said to select the resistor so that about 1 mA or less flowed through
the protection pullup diodes on the AVR inputs and one should be OK.

The classic brute force technique is to use a series resistor and a pulldown
resistor to build a voltage divider.