From f7279c84bbab92f5f2322b996555d6402c028b53 Mon Sep 17 00:00:00 2001
From: Juan Carrano <j.carrano@fu-berlin.de>
Date: Tue, 10 Sep 2019 11:41:28 +0200
Subject: [PATCH] cpu/sam0_common: rtt: correcly clear flags.

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.
---
 cpu/sam0_common/periph/rtt.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/cpu/sam0_common/periph/rtt.c b/cpu/sam0_common/periph/rtt.c
index b40b2c2d2c..c8793f4463 100644
--- a/cpu/sam0_common/periph/rtt.c
+++ b/cpu/sam0_common/periph/rtt.c
@@ -98,7 +98,7 @@ void rtt_init(void)
     _wait_syncbusy();
 
     /* initially clear flag */
-    RTC->MODE0.INTFLAG.reg |= RTC_MODE0_INTFLAG_CMP0
+    RTC->MODE0.INTFLAG.reg = RTC_MODE0_INTFLAG_CMP0
                            |  RTC_MODE0_INTFLAG_OVF;
 
     NVIC_EnableIRQ(RTC_IRQn);
@@ -116,12 +116,12 @@ void rtt_set_overflow_cb(rtt_cb_t cb, void *arg)
     _overflow_arg = arg;
 
     /* enable overflow interrupt */
-    RTC->MODE0.INTENSET.bit.OVF = 1;
+    RTC->MODE0.INTENSET.reg = RTC_MODE0_INTENSET_OVF;
 }
 void rtt_clear_overflow_cb(void)
 {
     /* disable overflow interrupt */
-    RTC->MODE0.INTENCLR.bit.OVF = 1;
+    RTC->MODE0.INTENCLR.reg = RTC_MODE0_INTENCLR_OVF;
 }
 
 uint32_t rtt_get_counter(void)
@@ -158,14 +158,14 @@ void rtt_set_alarm(uint32_t alarm, rtt_cb_t cb, void *arg)
     _wait_syncbusy();
 
     /* enable compare interrupt and clear flag */
-    RTC->MODE0.INTFLAG.reg |= RTC_MODE0_INTFLAG_CMP0;
-    RTC->MODE0.INTENSET.reg |= RTC_MODE0_INTENSET_CMP0;
+    RTC->MODE0.INTFLAG.reg  = RTC_MODE0_INTFLAG_CMP0;
+    RTC->MODE0.INTENSET.reg = RTC_MODE0_INTENSET_CMP0;
 }
 
 void rtt_clear_alarm(void)
 {
     /* clear compare interrupt */
-    RTC->MODE0.INTENCLR.bit.CMP0 = 1;
+    RTC->MODE0.INTENCLR.reg = RTC_MODE0_INTENCLR_CMP0;
 }
 
 void rtt_poweron(void)
@@ -189,16 +189,16 @@ void rtt_poweroff(void)
 void isr_rtc(void)
 {
     if (RTC->MODE0.INTFLAG.bit.OVF) {
-        RTC->MODE0.INTFLAG.reg |= RTC_MODE0_INTFLAG_OVF;
+        RTC->MODE0.INTFLAG.reg = RTC_MODE0_INTFLAG_OVF;
         if (_overflow_cb) {
             _overflow_cb(_overflow_arg);
         }
     }
     if (RTC->MODE0.INTFLAG.bit.CMP0) {
         /* clear flag */
-        RTC->MODE0.INTFLAG.reg |= RTC_MODE0_INTFLAG_CMP0;
+        RTC->MODE0.INTFLAG.reg = RTC_MODE0_INTFLAG_CMP0;
         /* disable interrupt */
-        RTC->MODE0.INTENCLR.bit.CMP0 = 1;
+        RTC->MODE0.INTENCLR.reg = RTC_MODE0_INTENCLR_CMP0;
         if (_cmp0_cb) {
             _cmp0_cb(_cmp0_arg);
         }