manager/rtc: Experimental power saving technique

Currently the time is calculated 8 times per second from (relatively) slow
python code. Optimize the power consumed by reducing the number of times
we check for wall time updates to only once-per-second and use native
code generation to reduce VM overhead when executing this critical code.

At the time of writing the difference is battery life has not yet been
measured (but we know the current master branch is worse than v0.4 and,
in theory at least, this should close the gap).

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>

wasp/drivers/nrf_rtc.py

@@ -45,6 +45,7 @@ class RTC(object):
             self._uptime = 0
             self.set_localtime((2020, 3, 1, 3, 0, 0, 0, 0))
 
+    @micropython.native
     def update(self):
         """Check for counter updates.
 
@@ -59,10 +60,12 @@ class RTC(object):
 
         self.lastcount += split
         self.lastcount &= (1 << 24) - 1   
+        uptime = self._uptime
         self._uptime += split
         machine.mem32[0x200039c8] = self._uptime * 125
 
-        return True
+        # Has the seconds count changed
+        return bool((self._uptime ^ uptime) & 0x08)
 
     def set_localtime(self, t):
         """Set the current wall time.
@@ -103,6 +106,7 @@ class RTC(object):
         localtime = self.get_localtime()
         return localtime[3:6]
 
+    @micropython.native
     def time(self):
         """Get time in the same format as time.time"""
         return self.offset + (self._uptime >> 3)

wasp/wasp.py

@@ -430,6 +430,7 @@ class Manager():
 
         watch.touch.reset_touch_data()
 
+    @micropython.native
     def _tick(self):
         """Handle the system tick.