Fixes#5745
For AVR based boards, three defines must be defined AVR_CONTEXT_SWAP_INIT,
AVR_CONTEXT_SWAP_INTERRUPT_VECT, and AVR_CONTEXT_SWAP_TRIGGER.
These defines are used to trigger a software interrupt used for context
switching.
When AVR_CONTEXT_SWAP_INTERRUPT_VECT is handled, the scheduler is run
and a context swap will happen if necessary, with the resulting thread
starting following the reti instruction. This results in threads running
at normal priority instead of at interrupt priority.
Atmega devices do provide a pure software interrupt. The method used
here for waspmote-pro and arduino-mega2560 is to use pin change
interrupts, set the pin to act as an output, and toggle the value to
simulate a software interrupt. The main limitation here is that a
physical pin is now occupied and must be defined for each board
supported by RIOT. On the plus side, it provides an easy method for
detecting context swaps with an oscilloscope.
* Add support for U2X switch to double transmission speed
* Provides lower error rates when needed
* Add support for static baud rate calculation, reduces computation by 10x
* adds support for statically enabling double speed baud
use generic avr/libc definition for conditionnal compilation.
eg: use #ifdef(PORTG) or #ifdef(TIMER_0_ISRC) instead of #ifdef(Atmega328p)
This is more generic and simplify future boards support.
Makes AVR register definitions dependent on what avr-libc defines
for a given MCU, rather then duplicating that effort here.
Definitions done in this way are based on functionality provided,
rather than a specific MCU device.
- Move generic implementation of atomic_set_return to core/atomic.c
- Generic implementation of atomic compare and swap in core/atomic.c
- atomic_cas is used to implement atomic counters in core/include/atomic.h
- atomic_int_t is an atomic integer type
- ATOMIC_INIT can be used as an initializer for atomic_int_t
- ATOMIC_VALUE gets a reference to the value of an atomic integer
