AVR-Chat

I would like to measure voltages using the ADC on an AVR (ATmega644P
specifically), but voltages higher than AVcc.  I gather from the data sheet
that AVcc cannot be greater than Vcc by more than 0.3 V.  I intend to use
Vcc = 3.0 V, so that implies AVcc needs to be essentially Vcc. I'm OK with
that, but I need to measure voltages in typical "12 VDC system" range, ie,
11 - 15 V.  

In fact, nowhere in the main text of the ADC section of the data sheet does
it explicitly state, or even imply (as nearly as I can tell), that Vref
cannot be greater than AVcc.  However, the ADC specification section of the
data sheet lists min & max for Vref as 1.0 to AVcc, but the column heading
has a footnote that says these "values are guidelines only".  So this is all
very confusing to me. 

Can I, in fact, input to Vref a voltage reference value equal to the maximum
of what I need to measure, say 15 V, even though that is well above AVcc?

Or does the spec "maximum" truly imply that Vref cannot, under any
circumstance, be > AVcc?  If so, does that mean the only solution to my
problem is an external voltage divider? 

Thanks for any help.  Steve