AVR-Chat

On 08/23/2012 10:21 PM, Steven Hodge wrote:
>  
> 
> I have a question which has been bugging me for some time. The data
> sheet for an AVR, under absolute maximum ratings, usually says something
> like “voltage on any pin…-0.5V to Vcc+0.5V”. 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? Does the chip handle it just fine w/o any supply voltage or
> does it smoke?
> 
> The same question can, of course, be asked of most any IC, and I’d be
> interested in the answer to this generalization as well.
> 
> Thanks, Steve
> 

This is all about the protection diodes.
On the IO pins, there are FETs for sourcing and sinking. Each of these
FETs has, effectively, a diode in parallel. On input-only pins, there
are dedicated diodes.
When you apply a voltage greater than VCC to the pin, current will flow
in the diode from IO pin to VCC, raising VCC. If VCC is raised enough,
the mcu will happily run powered through the diode.
Like any diode, this diode has forward voltage and maximum current. If
you exceed 0,5V, while shorting VCC to 0V, it will draw a lot of
current. If you supply enough current, you kill the diode, the FET it is
part of, and the surrounding area on the die.
For AVR parts, the 'suggested safe current' is 1mA. So if you intend to
short VCC to GND only on the avr, put ~4.5k in the RXD line to limit the
current to 1mA.

The reset pin is different. There is no diode to VCC. If you feed in a
few mA, it does not end well.

Some logic series, such as NC7SZ32 as an example, are different. Instead
of using clamping diodes to VCC they use zeners to GND. Thus, regardless
of VCC to the part, the input may be as high as 6V. They are quite
useful for 5->3.3V translation.

/Kasper Pedersen