Add a weak attribute to serial IRQ allowing real time communication lib.

[nicolas-rabault]

There is no way to have a complexe software communication protocol without IRQ.
Arduino should comply with it allowing real time constraints protocols.

The best way could be to be able to avoid Serial object creation and call a custom object into IRQ, but I can't find any way to do it without breaking everything.

Another good way could be to make something like an attach_interrupt function on Serial object. But it's not really standard into Arduino ecosystem and impact everyone with a little bit more CPU time on each byte reception for the benefits of some specific guys like me.

The last way is to declare IRQ as weak allowing someone to take the lead on IRQ. In this case, there is a dead Serial object but this modification doesn't do anything in any other case and the custom serial lib developper is the guilty one.

[matthijskooijman]

This seems like a reasonable approach, but I think we can do better. For AVR, we already set things up so that if Serial is not references, the entire object and ISR is omitted. See #489 for brief discussion and links about this approach.

[nicolas-rabault]

Thank you for pointing me into this direction.
Indeed it seems to be a better solution, but a more time consuming one to make.
Like you I would love to make it happen but I don't have a lot of time.
Anyway I will make a post-it on my blackboard about it...

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[Adminius]

@nicolas-rabault hi, any news? I would also like to use my own serial IRQs