bitarithm.h is not needed for the interface of shed but may cause conflicts
due to different definitions of SETBIT and CLRBIT
common implementations are: (value, offset) xor (value, mask) bitarithm
implements the later
frac.c and nrf52/usbdev.c use bitarithm.h but where missing the include
sam0/rtt.c defined a bit using mask from bitarithm,
changed that to the soulution used in sam0/rtc.c
From the data sheet:
> The COUNT register requires synchronization when reading.
> Disabling the synchronization will prevent reading valid
> values from the COUNT register.
Without this bit enabled, rtt_get_counter() will always return 0.
The INTFLAGS register is cleared by writing a 1 to the corresponding interrupt
flag bit. From the samr21's manual:
> Writing a zero to this bit has no effect.
> Writing a one to this bit clears the Compare 0 interrupt flag.
This is a common pattern in flag registers.
This RTT driver is using or-equal to clear the flags, which means it can
possibly clear other interrupts. There's a small chance that one event is
missed if it happens very close to another event.
Credits to @benpicco, @dylad for pointing out missing fixes.
The currently supported SAM0 MCUs (samd21, saml21, saml1x) share the
same RTC peripheral, yet each of them carries it's own copy of the RTT
driver.
Unify the drivers and move them to sam0_common.
