This fixes `xtimer` to use `xtimer_now64()` instead of `xtimer_now()`
for updating the `*now` variable during the iteration in
`_update_short_timers()` function. The same function is used to
initialize `*now` in `_timer_callback()` below.
While using `xtimer_now()` in this iteration step does not hinder the
proper execution of all timers in the short term timers (for those the
`xtimer` module only looks at the `start_time` member, not the
`long_start_time` member) at least for the current long term time window
(I did not test higher cases), it sets the `long_start_time` member to 0
for all timers following in the list of timers after this iteration
step. However, external modules that rely on this to be correct,
e.g. evtimer [1], fail their calculations when trying to compare to
the current value to `xtimer_now64()`.
[1] https://github.com/RIOT-OS/RIOT/blob/11f3d68/sys/evtimer/evtimer.c#L118-L121
Co-Authored-By: Cenk Gündoğan <mail+dev@gundogan.net>
This commit adds logic to make xtimer use ztimer_usec as backend
(instead of periph_timer). This allows ztimer_usec and xtimer to
coexist. It also allows xtimer to profit from eventually implemented
power mode blocking in ztimer's periph_timer backend.
1. When the 32 bit target of the xtimer overflowed the timer was not placed in the right list.
2. When the hardware timer overflowed the comparison was wrong for setting next target.
3. Backoff condition
On a fast CPU with a slow timer (e.g. XTIMER_SHIFT > 0) it is possible
that now == _xtimer_now() when spinning for the overflow. In the extreme
case When this happens _next_period() will be called more than once
until the timer overflows for real.
Fault observed in real life when running on a 32.768 kHz timer on a
~96 MHz clocked mulle (Kinetis K60, Cortex-M4). _next_period() was
called 9 times during the same ISR call before the 32 kHz timer
overflowed.
- more robust underflow protection in xtimer_usleep_until()
- use relative target in xtimer_spin()
- honour reference in isr when spinning until timer target
- add XTIMER_BACKOFF to xtimer_spin_until() target when backing off in
_timer_set_absolute()
- doxygen updates
