This is an example application demonstrating how to enable interrupts
on a GPIO and register an Interrupt Service Routine (ISR).

On the NS7520 there are only 4 GPIO pins capable of generating interrupts.
These pin are all on PORTC2, specifically PORTC0, PORTC1, PORTC2 and PORTC3.

On the Digi Connect platform's these pins have the following external access
points:

- Connect ME and Wi-ME 
  - PORTC is not accessible

- Connect EM 
  - PORTC2 (pin 5 of CR1, a.k.a. anode of the Yellow LED)
  - PORTC3 (RXD2/GPIO9)
- Connect Wi-EM
  - PORTC2 (pin 1 of CR1, a.k.a. anode of the Red LED)
  - PORTC3 (RXD2/GPIO9)
                            
For the sake of this example we will make use of PORTC3 (RXD2) as our 
interrupt enabled GPIO.

The ISR outputs a '.' (period) to the STDOUT port
and sets the data bit of PORTC3 back to high.

HARDWARE PRE_SETUP
------------------

Make certain that SW4, position 4 (RXD2/IO9) is in the IO9 position. Additionally,
SW2, position 4, should be in the HI position.


EXERCISING THIS APPLICATION
---------------------------

In order to trigger the ISR toggle SW2, position 4, from HI to LO. So long as this 
switch position is in the LO position the ISR (portc3_isr) will be triggered and 
a steady stream of periods ('.') will be outputted to the STDOUT port.
