AVR-Chat

On Aug 23, 2012, at 7:45 PM, Steven Hodge wrote:

> So the chip can actually be powered on, eh? That explains a few  
> mysterious
> things. Thanks, Steve

Generally you can protect the chip and related circuitry by putting a  
resistor between the pin and power supplying circuitry.  On inputs  
this is usually pretty easy.  It gets more complicated with outputs

The parasitic diodes can be used to deal with external inputs that  
exceed 0-Vcc.  You put a series resistor (Rh) between the input pin  
and the higher voltage and another resistor (Rl) from the input pin to  
ground.  You size them so that with maximum input voltage the voltage  
at the pin will be about Vcc.  Rh should be high enough so that any  
current that does flow into the pin and the parasitic diode is well  
below the limit for the diode.

I use this method along with a 74LVC1G14 (a SMD single inverter  
version of the 74HC14 Hex Schmidt trigger) as an RS-232 receiver.   
That and another 74LVC1G14 as output inverter and it takes care of the  
serial interface.

>
> From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On  
> Behalf
> Of John Samperi
> Sent: Thursday, August 23, 2012 2:30 PM
> To: AVR-Chat@yahoogroups.com
> Subject: Re: [AVR-Chat] voltage on an AVR pin when powered off
>
> At 06:21 AM 24/08/2012, you wrote:
> >So what happens if there is no Vcc, i.e., there is no supply power
> >applied to the AVR,
> >but at the same time there happens to be Vcc on, say, a digital
> >input pin or a UART RX line?
>
> The chip will try and draw power from that line. It's VCC (and the
> boards' VCC) will be the
> voltage present at the pin less 0.5V.
>
> So it is never a good idea to have power on pins of a chip which is  
> off.