Replace the 16mhz clock with a low-freqncy external crystal based clock source at statup.
This results in a big win in power consumption.
add a missing call to us_ticker_disable_interrupt from us_ticker_remove_event.
reimplement the micro-second ticker on nRF51822 using the app_timer module (RTC)
add a minor comment to explain some complex branching in us_ticker_irq_handler()
add a cachedInterruptTimestamp to us_ticker_set_interrupt() to short-circuit duplicate calls.
introduce a timestamp_t to track timeouts for TimerEvents.
The previous type used for this purpose was unsigned int, which was both
un-portable and liable to wrap-around.
updated call to APP_TIMER_INIT() to use contants from projectconfig.h
simplify the algorithm which keeps only one app_timer timer running.
white space diffs.
update definitions for us_ticker_set_interrupt() across all platforms to use timestamp_t
use constants from projectconfig.h as arguments for APP_TIMER_INIT()
minor simplification of a reused expression
Merge work for creating timestamp_t and for reimplementing us_timer on nRF51 using RTC.
use timestamp_t for Ticker::_delay
add a destructor for Ticker.