AVR-Chat

Graham, Jim, and John,

Thanks very much for all the information.  It will definitely help me 
in the future to better understand these issues.  And, no, I don't 
want to cut corners.  I will definitely use some type of level 
converter.  John's suggestions looked good.

Thanks again, I learned a lot.  This is a great forum.

Bob


--- In AVR-Chat@yahoogroups.com, "Graham Davies" <Yahoo37849@...> 
wrote:
>
> --- 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.
>