AVR-Chat

I am trying to enable external interrupt 2 to trigger on a falling edge on
an ATMega1284p but I am unable to get the proper bits in the EICRA register.
I'm using the following:

(1) EIMSK &= ~_BV( INT2 );				// Disable external
interrupt 2
(2) EICRA &= ~(_BV( ISC21 ) | _BV( ISC20 ));		// Clear the
external interrupt 2 control bits
(3) EICRA |= (_BV( ISC21 ) /*| _BV( ISC20 )*/);		// Setup interrupt 2
for interrupt on falling edge
(4) EIFR |= _BV( INTF2 );				//  Clear any
pending interrupt
(5) EIMSK |= _BV( INT2 );				// Enable external
interrupt 2

(Yes, I know that lines 2 & 3 can be combined but I split them in an attempt
to diagnose the problem)

A JTAG breakpoint at line 4 shows the EICRA register unchanged from its
initial value of 0x00 ...

ISC21 has a value of "5" ...

Even if I replace line 3 with "EICRA = 32" the register remains unchanged
...

When I look at the disassembled code it appears to be storing to location
0x69 (the EICRA reg) ...

It is as though there is some control bit that I have to turn on to enable
changes to the EICRA register ...

Note: at this time Global Interrupt Enable happens to be disabled (it is
enabled later in the code).

This is code that is being converted from an ATMega32 where it is working
fine just using:

GICR |= _BV( INT2 ); // Enable interrupt on (default) falling edge of
external interrupt 2

I know that this has to be something simple and I've read the datasheet
section on external interrupts over and over.  Can someone point out where
I'm going wrong?
 
Cheers,

Chuck Hackett
"Good judgment comes from experience, experience comes from bad judgment"
7.5" gauge Union Pacific Northern (4-8-4) 844
http://www.whitetrout.net/Chuck