cpu/atmega2560: reworked timer implementation

[haukepetersen]

fixes also #5252

The timers were kind of broken (at least did not function correctly). This implementation should now work fine.

NOTE: the periph timer is not able to run at 1MHz (missing prescaler for that), so the tests/periph_timer test needs to be adjusted to run the timer either with 2MHz or better with 250khz.

Also the xtimer tests will most probably fail, as the xtimer requires the low-level timer to run at 1MHZ.

[kYc0o]

Small murdock complain...

xtimer_shift_on_compare:arduino-mega2560:
Building application "xtimer_shift_on_compare" for "arduino-mega2560" with MCU "atmega2560".

/data/riotbuild/tests/xtimer_shift_on_compare/main.c: In function 'main':
/data/riotbuild/tests/xtimer_shift_on_compare/main.c:58:38: error: '_high_cnt' undeclared (first use in this function)
                 a = _lltimer_now() | _high_cnt;

[kaspar030]

xtimer requires the low-level timer to run at 1MHZ.

Not true, it currently requires 1MHz or a frequency that can be reached by dividing with a power of two.

[haukepetersen]

yapp, you are right, the XTIMER_USEC_TO_TICKS(1000000ul) macro confused me for a second...

[kYc0o]

So we need to provide _high_cnt by calculating it using xtimer_shift_on_compare is it?

[kaspar030]

No, XTIMER_SHIFT needs to be set, probably to "2" (divide by 4), when setting the periph timer to 250khz.

XTIMER_SHIFT_ON_COMPARE probably also has to be set. It masks bits when reading two timer values in a row. Some MCU's (the atmega is a candidate) are not fast enough to return two times the same value, so some bits have to be ignored.

[haukepetersen]

fixed doxygen issue and added missint XTIMER_MASK definition to the arduino-mega2560.

[kYc0o]

Nice! ACK and go!

[kYc0o]

Oh, squash first?

[haukepetersen]

IMHO no need for squashing, as each commit contains a logical group of changes -> go

[cgundogan]

backport needed