[En-Nut-Discussion] Lost caracter with At91 UART0

[Remi Bilodeau]

To enabling nesting interrupt and get full potential or IRQ priority, just
some little modification of Nut/OS IRQ handler have to be done. (I think?)

Maybe I dig Nut/OS code later this week, here the paragraph in the Atmel doc
about nesting interrupt.

24.7.3.2 Interrupt Nesting
The priority controller utilizes interrupt nesting in order for the high
priority interrupt to be handled
during the service of lower priority interrupts. This requires the interrupt
service routines of the
lower interrupts to re-enable the interrupt at the processor level.
When an interrupt of a higher priority happens during an already occurring
interrupt service routine,
the nIRQ line is re-asserted. If the interrupt is enabled at the core level,
the current
execution is interrupted and the new interrupt service routine should read
the AIC_IVR. At this
time, the current interrupt number and its priority level are pushed into an
embedded hardware
stack, so that they are saved and restored when the higher priority
interrupt servicing is finished
and the AIC_EOICR is written.
The AIC is equipped with an 8-level wide hardware stack in order to support
up to eight interrupt
nestings pursuant to having eight priority levels. 
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