[Avr-libc-corelib] Interrupt Handlers

[Mike Perks]

One of the issues that will need to handled (intentional pun) by the 
library is ISRs. There are several use cases to consider:

1. Library module wants to use an ISR
2. User wants to use an ISR
3. User wants to use an ISR but it is used by a library module
4. Two library APIs (presumably from different modules) both want to use 
the same ISR

Having a first come first served policy will not work. We probably need 
to provide a way to have hooks such as a callback. The problem is that 
this could be too heavyweight for what is supposed to be a fast response.

Do we provide default ISRs as part of a library core and then individual 
library modules hook in using a callback. The advantage of this 
technique is for debugging. Standards for debug support is probably a 
separate thread.

Mike Perks
Oak Micros (oakmicros.com)