cc2538: lpm: Fix energest context when aborting lpm_enter()

In one of the abort cases in lpm_enter(), the energest context has previously
been set to LPM, so the abort code needs to set it back to CPU.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538: lpm: Let system clock transitions complete before further changes

As a matter of precaution, always make sure that pending system clock
transitions are complete before requesting a new change of the system clock
source.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538: lpm: Speed up the transition to the 32-MHz XOSC after wake-up

As recommended by the CC2538 User's Guide, set SYS_CTRL_CLOCK_CTRL.OSC_PD to 0
before asserting WFI, and set it to 1 after the system clock is sourced from the
32-MHz XOSC following wake-up. This allows to automatically start both
oscillators upon wake-up in order to partially hide the 32-MHz XOSC startup time
by the 16-MHz RCOSC startup time.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538: lpm: Fix RTIMER_NOW() upon wake-up

When returning from PM1/2, the sleep timer value (used by RTIMER_NOW()) is not
up-to-date until a positive edge on the 32-kHz clock has been detected after the
system clock restarted. To ensure an updated value is read, wait for a positive
transition on the 32-kHz clock by polling the SYS_CTRL_CLOCK_STA.SYNC_32K bit,
before reading the sleep timer value.

Because of this RTIMER_NOW() fixup, lpm_exit() has to be called at the very
beginning of ISRs waking up the SoC. This also ensures that all clocks and
timers are enabled at the correct frequency and updated before using them
following wake-up.

Without this fix, etimers could sometimes (randomly, depending on timings)
become ultra slow (observed from 10x to 40x slower than normal) if the system
exited PM1/2 very often. This issue occurred more often with PM1.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538: clock: Request an etimer poll in clock_adjust()

During PM1+, the hardware timer used to implement the Contiki clock is frozen,
so clock_adjust() needs to be called when exiting those modes in order to
compensate for the clock ticks missed while the timer was frozen. Doing so
changes the Contiki clock time, so etimer_request_poll() needs to be called in
order to inform the etimer library that an etimer might have expired.

Note that waiting for the next clock ISR to call etimer_request_poll() is
unreliable because the system might go back to sleep beforehand.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538: clock: Fix secs update in clock_adjust()

Whole elapsed seconds are added to secs first, so only the remaining subsecond
ticks should then be subtracted from second_countdown in order to decide whether
secs should be incremented again.

Otherwise, secs is not correctly updated in some cases, typically if the bit 7
of ticks is 1. E.g., with ticks = 128 (i.e. exactly 1 s elapsed) and
second_countdown = 128, secs was first incremented as expected, then 128 was
subtracted from second_countdown, giving 0 and triggering an unwanted second
increment of secs. Or with ticks = 129 (i.e. 1 s + 1 tick) and
second_countdown = 1, secs was first incremented as expected, then 129 was
subtracted from second_countdown, giving 128 and missing a second increment of
secs that should have occurred because second_countdown wrapped around.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538: clock: Fix time drift occurring with PM1/2

The clock adjustments made when waking up from PM1/2 were very inaccurate. If
relying on ContikiMAC's rtimer to sleep, this led to Contiki's software clock
time, seconds and etimers to be 2.5 s slower after each min, i.e. 1 hour slower
after each day, which is a show stopper issue for most real-life applications.

This was caused by a lack of accuracy in several pieces of code during sleep
entry and wake-up:
 - It was difficult to synchronize the calls to RTIMER_NOW() before and after
   sleep with the deactivation and activation of the SysTick peripheral caused
   by PM1/2. This caused an inaccuracy in the corrective number of ticks passed
   to clock_adjust().
 - The value passed to clock_adjust() was truncated from an rtimer_clock_t
   value, but the accumulated error caused by these truncated bits was ignored.
 - The SysTick peripheral had to be stopped during the call to clock_adjust().

Rather than creating even more complicated clock adjustment mechanisms that
would probably still have mixed results as to accuracy, this change simply uses
the Sleep Timer counter as a base value for Contiki's clock and seconds
counters. The tick from the Systick peripheral is still used as the interrupt
source to update Contiki's clocks and timers. When running, the SysTick
peripheral and the Sleep Timer are synchronized, so combining both is not an
issue, and this allows not to alter the rtimer interrupt mechanism using the
Sleep Timer. The purpose of the Sleep Timer is to be an RTC, so it is the
perfect fit for the clock module, all the more it can not be disturbed by PM1/2.
If the 32-kHz XOSC is used, the Sleep Timer is also very accurate. If the
32-kHZ RCOSC is used, it is calibrated from the 32-MHz XOSC, so it is also
accurate, and the 32753-Hz vs. 32768-Hz systematic error in that case is
negligible, all the more one would use the 32-kHz XOSC for better accuracy.

Besides fixing this time drift issue, this change has several benefits:
 - clock_time(), clock_seconds() and RTIMER_NOW() start synchronized, and they
   change at the same source pace.
 - If clock_set_seconds() is called, then clock_seconds() indicates one more
   second almost exactly one second later, then exactly each second. Before this
   change, clock_seconds() was not synchronized with clock_set_seconds(), so the
   value returned by the former could be incremented immediately after the call
   to the latter in some cases.
 - The code tied to the clock module is simpler and more robust.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>