Schedule the next OS tick inside of the ticker interrupt rather than
in the systick interrupt. Scheduling this while the ticker is
dispatching prevents an unnecissary rescheduling since this
rescheduling is done anyway when dispatching is finished.
This is particularly useful for the low power ticker on devices with
LPTICKER_DELAY_TICKS set to a non-zero value. This is because the low
power ticker cannot be reschduled immediately and needs to fall back
onto the microsecond ticker which temporarily locks deep sleep.
Note - the optimization in this commit is made possible by the commit:
"Don't reschedule ticker while dispatching"
Remove the assert that the tick count of the OS matches the tick
count of the tickless timer as this occurs frequently when
debugging. This is because the function svcRtxKernelResume
only increments the OS's tick count until the next wakeup event
so if the device was halted by a debugger past the next wakeup
event the tick counts will be out of sync.
Some Cortex-M0 devices, such as the nrf51, don't have the SysTick.
Instead, these targets use a software interrupt to simulate SysTick.
Add the hooks in the tickless code to support these devices. Targets
which do not have SysTick should now define NO_SYSTICK in targets.json
and implement mbed_get_m0_tick_irqn to add os suport.
This patch also removes os tick handling from the existing devices
(nrf51) since this is now handled in common code.
Add support for tickless by replacing RTX's SysTick timer code with
with code which uses an mbed timer along with suspending and
resuming the kernel in the idle loop. Tickless is enabled on a
per-target basis by defining the macro MBED_TICKLESS.
Oren Cohen
Russ Butler
Bartek Szatkowski
Cruz Monrreal
Filip Jagodzinski
Martin Kojtal
Jaeden Amero