Better IP initialization sequence:
1. Configure IP pins
2. Select IP clock source and then enable it
3. Reset the IP (SYS_ResetModule)
NOTE1: IP reset takes effect regardless of IP clock. So it doesn't matter if
IP clock enable is before IP reset.
NOTE2: Non-configured pins may disturb IP's state, so IP pinout first and then
IP reset.
NOTE3: IP reset at the end of IP initialization sequence can cover unexpected
situation.
Replace wait_us with nu_busy_wait_us in lp_ticker since wait_us is not allowed in sleep test
which would suspend us ticker layer on which wait_us relies. nu_busy_wait_us is implemented
by calling us ticker HAL API directly rather than relying on us ticker layer.
If us_ticker/lp_ticker is scheduled and then the interrupt is disabled, the originally scheduled
interrupt may still become pending. If this occurs, then an interrupt will fire twice on the next
call to us_ticker_set_interrupt/lp_ticker_set_interrupt - once immediately and then a second time
at the appropriate time.
This patch prevents the first interrupt by clearing interrupts in
us_ticker_set_interrupt/lp_ticker_set_interrupt before calling NVIC_EnableIRQ.
The rework includes the following:
1. Remove ticker overflow handling because upper layer (mbed_ticker_api.c) has done with it.
This makes us_ticker/lp_ticker implementation more succinct and avoids potential error.
2. Refine timer register access with low-power clock source
Chun-Chieh Li
Martin Kojtal
cyliangtw
Deepika