sam0_common/periph/rtt: Interrupt flags are not correctly cleared

[jcarrano]

Description

The INTFLAGS register is cleared by writing a 1 to the corresponding interrupt flag bit. Flags that must not be cleared should be written as zero. This is a pretty common behaviour for interrupt 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.

see:

RIOT/cpu/samd21/periph/rtt.c

Line 201 in 12049a5

rtcMode0->INTFLAG.reg |= RTC_MODE0_INTFLAG_CMP0;

and

RIOT/cpu/samd21/periph/rtt.c

Line 206 in 12049a5

rtcMode0->INTFLAG.reg |= RTC_MODE0_INTFLAG_OVF;

The correct thing to write is: rtcMode0->INTFLAG.reg = RTC_MODE0_INTFLAG_CMP0;

References

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.

[stale]

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[jcarrano]

This is still a bug, and it is now in sam0_common:

RIOT/cpu/sam0_common/periph/rtt.c

Line 192 in 741d543

RTC->MODE0.INTFLAG.reg |= RTC_MODE0_INTFLAG_OVF;

RIOT/cpu/sam0_common/periph/rtt.c

Line 199 in 741d543

RTC->MODE0.INTFLAG.reg |= RTC_MODE0_INTFLAG_CMP0;