/* Copyright (c) 1999, 2000, 2001, 2002, 2005 Rich Neswold
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/* $Id: interrupts.dox,v 1.11 2007/10/29 14:01:18 joerg_wunsch Exp $ */

/** \defgroup avr_interrupts <avr/interrupt.h>: Interrupts

\note This discussion of interrupts was originally taken from Rich Neswold's
document. See \ref acks.

<h3>Introduction to avr-libc's interrupt handling</h3>

It's nearly impossible to find compilers that agree on how to handle interrupt
code. Since the C language tries to stay away from machine dependent details,
each compiler writer is forced to design their method of support.

In the AVR-GCC environment, the vector table is predefined to point to
interrupt routines with predetermined names. By using the appropriate name,
your routine will be called when the corresponding interrupt occurs. The
device library provides a set of default interrupt routines, which will get
used if you don't define your own.

Patching into the vector table is only one part of the problem. The compiler
uses, by convention, a set of registers when it's normally executing
compiler-generated code. It's important that these registers, as well as the
status register, get saved and restored. The extra code needed to do this is
enabled by tagging the interrupt function with
<tt>__attribute__((signal))</tt>.

These details seem to make interrupt routines a little messy, but all these
details are handled by the Interrupt API. An interrupt routine is defined with
ISR(). This macro register and mark
the routine as an interrupt handler for the specified peripheral. The
following is an example definition of a handler for the ADC interrupt.

\code
#include <avr/interrupt.h>

ISR(ADC_vect)
{
    // user code here
}
\endcode

Refer to the chapter explaining \ref ass_isr "assembler programming" for an
explanation about interrupt routines written solely in assembler language.

<h3>Catch-all interrupt vector</h3>

If an unexpected interrupt occurs (interrupt is enabled and no handler is
installed, which usually indicates a bug), then the default action is to reset
the device by jumping to the reset vector. You can override this by supplying
a function named \c BADISR_vect which should be defined with
ISR() as such.  (The name BADISR_vect is actually an alias for __vector_default.
The latter must be used inside assembly code in case <avr/interrupt.h> is
not included.)

\code
#include <avr/interrupt.h>

ISR(BADISR_vect)
{
    // user code here
}
\endcode

<h3>Nested interrupts</h3>

The AVR hardware clears the global interrupt flag in SREG before
entering an interrupt vector.  Thus, normally interrupts will remain
disabled inside the handler until the handler exits, where the RETI
instruction (that is emitted by the compiler as part of the normal
function epilogue for an interrupt handler) will eventually re-enable
further interrupts.  For that reason, interrupt handlers normally do
not nest.  For most interrupt handlers, this is the desired behaviour,
for some it is even required in order to prevent infinitely recursive
interrupts (like UART interrupts, or level-triggered external
interrupts).  In rare circumstances though it might be desired to
re-enable the global interrupt flag as early as possible in the
interrupt handler, in order to not defer any other interrupt more than
absolutely needed.  This could be done using an sei() instruction
right at the beginning of the interrupt handler, but this still leaves
few instructions inside the compiler-generated function prologue to
run with global interrupts disabled.  The compiler can be instructed
to insert an SEI instruction right at the beginning of an interrupt
handler by declaring the handler the following way:

\anchor attr_interrupt
\code
ISR(XXX_vect, ISR_NOBLOCK)
{
  ...
}
\endcode

where \c XXX_vect is the name of a valid interrupt vector for the MCU
type in question, as explained below.

<h3>Two vectors sharing the same code</h3>

In some circumstances, the actions to be taken upon two different
interrupts might be completely identical so a single implementation
for the ISR would suffice.  For example, pin-change interrupts
arriving from two different ports could logically signal an event that
is independent from the actual port (and thus interrupt vector) where
it happened.  Sharing interrupt vector code can be accomplished using
the ISR_ALIASOF() attribute to the ISR macro:

\code
ISR(PCINT0_vect)
{
  ...
  // Code to handle the event.
}

ISR(PCINT1_vect, ISR_ALIASOF(PCINT0_vect));
\endcode

\note There is no body to the aliased ISR.

Note that the ISR_ALIASOF() feature requires GCC 4.2 or above (or a
patched version of GCC 4.1.x).  See the documentation of the
ISR_ALIAS() macro for an implementation which is less elegant but
could be applied to all compiler versions.

<h3>Empty interrupt service routines</h3>

In rare circumstances, in interrupt vector does not need any code to
be implemented at all.  The vector must be declared anyway, so when
the interrupt triggers it won't execute the BADISR_vect code (which by
default restarts the application).

This could for example be the case for interrupts that are solely
enabled for the purpose of getting the controller out of sleep_mode().

A handler for such an interrupt vector can be declared using the
EMPTY_INTERRUPT() macro:

\code
EMPTY_INTERRUPT(ADC_vect);
\endcode

\note There is no body to this macro.

<h3>Manually defined ISRs</h3>

In some circumstances, the compiler-generated prologue and epilogue of
the ISR might not be optimal for the job, and a manually defined ISR
could be considered particularly to speedup the interrupt handling.

One solution to this could be to implement the entire ISR as manual
assembly code in a separate (assembly) file.  See \ref asmdemo for
an example of how to implement it that way.

Another solution is to still implement the ISR in C language but take
over the compiler's job of generating the prologue and epilogue.  This
can be done using the ISR_NAKED attribute to the ISR() macro.  Note
that the compiler does not generate \e anything as prologue or
epilogue, so the final reti() must be provided by the actual
implementation.  SREG must be manually saved if the ISR code modifies
it, and the compiler-implied assumption of \c __zero_reg__ always
being 0 could be wrong (e. g. when interrupting right after of a MUL
instruction).

\code
ISR(TIMER1_OVF_vect, ISR_NAKED)
{
  PORTB |= _BV(0);  // results in SBI which does not affect SREG
  reti();
}
\endcode

<h3>Choosing the vector: Interrupt vector names</h3>

The interrupt is chosen by supplying one of the symbols in following table.

There are currently two different styles present for naming the
vectors.  One form uses names starting with \c SIG_, followed by
a relatively verbose but arbitrarily chosen name describing the
interrupt vector.  This has been the only available style in
avr-libc up to version 1.2.x.

Starting with avr-libc version 1.4.0, a second style of interrupt
vector names has been added, where a short phrase for the vector
description is followed by \c _vect.  The short phrase matches the
vector name as described in the datasheet of the respective device
(and in Atmel's XML files), with spaces replaced by an underscore
and other non-alphanumeric characters dropped.  Using the suffix
\c _vect is intented to improve portability to other C compilers
available for the AVR that use a similar naming convention.

The historical naming style might become deprecated in a future
release, so it is not recommended for new projects.

\note The ISR() macro cannot really spell-check
the argument passed to them.  Thus, by misspelling one of the names
below in a call to ISR(), a function will be created
that, while possibly being usable as an interrupt function, is not
actually wired into the interrupt vector table.  The compiler will
generate a warning if it detects a suspiciously looking name of a
ISR() function (i.e. one that after macro replacement does not
start with "__vector_").

*/
