AVR-Chat

--- In AVR-Chat@yahoogroups.com, "Bob Gardner" 
<bob.linda.gardner@...> wrote:

> So, I'm assuming from the replies
> that, even though the 324P can 
> operate at 2.7V to 5.5V, if I power
> it from 3.3V it can't tolerate 5V 
> at an input?  Is that correct?

Well, why don't we go to the data sheet, where you said you "can't 
find" or "maybe don't understand" the information you're looking for.

You want the DC Characteristics table on page 326 (8011H–AVR–04/08).  
The maximum "Input High Voltage" is given for all pins as VCC + 0.5.  
If VCC is 3.3V, this evaluates to 3.8V.  So, yes, an ATmega324P 
powered from 3.3V cannot tolerate 5V on an input.

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.

We also need to ask whether the 3.3V device will be able to produce a 
logic one that the 5V device is happy with.  The minimum Output High 
Voltage is given as 2.3V for a 3V supply, so it is reasonable to 
expect 2.6V at 3.3V.  The minimum Input High Voltage is given as 
0.6VCC, which at 5V is 3V.  So this doesn't look as if it will work.  
Again, pragmatically, the minimum Output High Voltage is given with a 
10 mA current load.  Driving just another device's input, we know 
that you can expect almost the supply voltage.  Here it's close, but 
it will probably work.

So, the advice you're getting is that to do the job properly you need 
level converters.  But, if you're willing to cut corners you can 
probably get away with the resistors you originally proposed.

Graham.