ATmega128RFA1/ATmega256RFR2 do not have a unique CPU ID.
Use the RC oscillator callibration byte as an impromptu CPU ID and rely
on bootlader constants present on all ATmega families for the remaining
bytes.
This way we can provide a faux CPU ID on all ATmega MCUs and typical hobbyists
with no access to JTAG adapters or high voltage programmer capable of writing
the user signature have a good chance that the CPU IDs of their device do not collide.
Memory management function like `malloc`, `calloc`, `realloc` and `free` must not be preempted when they operate on allocator structures. To avoid such a preemption, wrappers around these functions are used which simply disable all interrupts for the time of their execution.
Functions marked with __atribute__((naked)) may only use basic inline assembly
and must not use any c code. The functions __enter_thread_mode() and
cpu_switch_context_exit() are using C code, so they must not be marked as
naked.
To prevent reordering of accesses to the interrupt control register when link
time optimization (LTO) is enabled, memory barriers are needed. Without LTO
calls to the external functions irq_disable(), irq_restore(), irq_enable() and
irq_is_in() have the same affect as compiler barriers, as the compiler is unable
to prove that reordering of memory accesses is safe (from a single-threaded
point of view). With LTO the compiler can easily prove that reordering is safe
from a single-threaded point of view: Thus, the compiler may move memory
accesses wrapped in irq_disable(), irq_restore() across those calls.
The memory barriers will have no effect on non-LTO builds.
Citing the doc of irq_enable():
@return Previous value of status register. [...]
On atmega however the new value of the status register is returned, not the one
prior to enabling interrupts.
Moving atmega_stdio_init() to cpu_init() just before periph_init() guarantees
that stdio is available to allow DEBUG() in periph_init(). This also helps to
unify the boot up process of ATmega boards and de-duplicates the stdio init from
board_init().
This prevent evaluating `LINKFLAGS` when not needed and when building
in docker so does not produce errors if `avr-ld` is not installed.
```
BUILD_IN_DOCKER=1 BOARD=arduino-mega2560 make --no-print-directory -C examples/hello-world/ clean
/srv/ilab-builds/workspace/git/riot_master/makefiles/toolchain/gnu.inc.mk:18: objcopy not found. Hex file will not be created.
/bin/sh: 1: avr-ld: not found
```
It removes the `/bin/sh: 1: avr-ld: not found`
The brr calculation on the datasheet is different than what is implmented.
This is intentional since it provides better rounding due to truncation.
There was no comment explaining that so this comment should prevent confusion.
- correct number of timers for atmega328p from 2 to 1
- correct number of timer channels for atmega328p from 3 to 2
- adapt atmega periph timer implementation accordingly
cpu.c and startup.c were redundant in most platforms, except for
atmega256rfr2. The common code is now in cpu/atmega_common/cpu.c
and cpu/atmega_common/startup.c. cpu_conf.h is also removed as
it's now in cpu/atmega_common/include thus shared by all atmega
based platforms.
Removes duplicated code for atmega platforms. They were all
basically the same, only with the exception of atmegarfr2,
for which there is an #if statement to use the code in the
same file.
