Interrupt On Analog Input4

Hi All !

I am new to arduino.

I have arrange M8870 (tone decoder) on analog 0 to analog3. ( I make these pin as digital input) & i want to use analog 4 pin for interrupt purpose.

When valid dtmf tone received then program will jump on interrupt.

My question is when interrupt occur then how can i read complete portc in variable & mask upper 4 bits.

Thanks

Roshani

Upper 4 bits of PORTC inputs:

unsigned char value = (PINC>>4)&0x0F;