AVR-Chat

First, you can forget the diode if you choose the resistors for the 
voltage divider to deal with the minimum and maximum values that the 
digital inputs recognize as logic 1 (VCC).
Second, choose the resistor values high enough (hundreds of K) so that 
you have negligible current through the clamping diode even if you have 
more than 5V in the digital input pin (it will not damage your input). 
In addition, if you use high value resistors you will avoid the battery 
to run down when in Off mode.
That's all you need for automotive applications.

Adeilton Oliveira
GMB


Jim Wagner wrote:
>  
>
> One way is to add a series resistor to each input (say 4.7K to 10K),
> then add a diode (preferably Schottkey,j but silicon junction will
> work) from each input to the processor Vcc, cathode to Vcc. That will
> limit each input at the micro to the current Vcc )(plus one diode drop).
>
> Jim Wagner
> Oregon Research Electronics
> On Jul 28, 2009, at 7:40 PM, Brian wrote:
>
> > I made a module for a car to monitor inputs. Since inputs can only
> > be VCC +/-0.5V for a high signal, how do you keep the inputs from
> > getting to the micro before VCC is applied? It keeps blowing my
> > inputs so they dont work.
> >
> > One thing I thought about is using a mosfet switch controlled by an
> > output of the micro to allow the signal only once it is started up.
> >
> > Brian
> >
> >
> >
>
> [Non-text portions of this message have been removed]
>
>