diff --git a/boards/airfy-beacon/include/board.h b/boards/airfy-beacon/include/board.h
index a94b061e8eb335352456cc8c686067ae48813952..49b17c03e7ce7f16a76df69e40e56e18d5bcfc78 100644
--- a/boards/airfy-beacon/include/board.h
+++ b/boards/airfy-beacon/include/board.h
@@ -34,7 +34,6 @@
#define XTIMER (0)
#define XTIMER_CHAN (0)
#define XTIMER_MASK (0xff000000)
-#define XTIMER_SHIFT_ON_COMPARE (2)
#define XTIMER_BACKOFF (40)
/** @} */
diff --git a/boards/arduino-mega2560/include/board.h b/boards/arduino-mega2560/include/board.h
index fb5fc9b57a1e82a7c1799a9075c1c539d2e898c9..8bd917f531fb81230b4e1f26b6b37b7557cc2116 100644
--- a/boards/arduino-mega2560/include/board.h
+++ b/boards/arduino-mega2560/include/board.h
@@ -53,7 +53,6 @@ extern "C" {
*/
#define XTIMER_MASK (0xffff0000)
#define XTIMER_SHIFT (2)
-#define XTIMER_SHIFT_ON_COMPARE (8)
#define XTIMER_BACKOFF (40)
/** @} */
diff --git a/boards/chronos/include/board.h b/boards/chronos/include/board.h
index 319eb4f81773a973fd99748be5e6ff2d0532e069..00dab1df8a694eb8b69265fb22de61ec918fb147 100644
--- a/boards/chronos/include/board.h
+++ b/boards/chronos/include/board.h
@@ -41,7 +41,6 @@ extern "C" {
#define XTIMER (0)
#define XTIMER_CHAN (0)
#define XTIMER_MASK (0xffff0000)
-#define XTIMER_SHIFT_ON_COMPARE (4)
/** @} */
/**
diff --git a/boards/msb-430-common/include/board_common.h b/boards/msb-430-common/include/board_common.h
index 24953643ae9c990681fabd7c7a54d22f528cfa14..72d856dcb78b21cfd0773bf7a4b209a2332e62ab 100644
--- a/boards/msb-430-common/include/board_common.h
+++ b/boards/msb-430-common/include/board_common.h
@@ -40,7 +40,6 @@ extern "C" {
#define XTIMER (0)
#define XTIMER_CHAN (0)
#define XTIMER_MASK (0xffff0000)
-#define XTIMER_SHIFT_ON_COMPARE (4)
#define XTIMER_BACKOFF (40)
/** @} */
diff --git a/boards/nrf51dongle/include/board.h b/boards/nrf51dongle/include/board.h
index 39223c8db024f6a4a87aa821bd07645b3e4b1368..3e3ee91c6671b64a1aa9cf94d9070aab2a97708d 100644
--- a/boards/nrf51dongle/include/board.h
+++ b/boards/nrf51dongle/include/board.h
@@ -35,7 +35,6 @@ extern "C" {
#define XTIMER (0)
#define XTIMER_CHAN (0)
#define XTIMER_MASK (0xff000000)
-#define XTIMER_SHIFT_ON_COMPARE (2)
#define XTIMER_BACKOFF (40)
/** @} */
diff --git a/boards/pca10000/include/board.h b/boards/pca10000/include/board.h
index 67e31bbbd34b9a5066c708e343dbc3006dd2d369..11ee1dcbb3f88609e3a28a2eb975eef1c81c5926 100644
--- a/boards/pca10000/include/board.h
+++ b/boards/pca10000/include/board.h
@@ -35,7 +35,6 @@ extern "C" {
#define XTIMER (0)
#define XTIMER_CHAN (0)
#define XTIMER_MASK (0xff000000)
-#define XTIMER_SHIFT_ON_COMPARE (2)
#define XTIMER_BACKOFF (40)
/** @} */
diff --git a/boards/pca10005/include/board.h b/boards/pca10005/include/board.h
index a5ce739b6db5b2a8fc3816ee3f529d2c228934ae..ab8a291f774aad9f99e7ef096f3abfb0e2465f9f 100644
--- a/boards/pca10005/include/board.h
+++ b/boards/pca10005/include/board.h
@@ -35,7 +35,6 @@ extern "C" {
#define XTIMER (0)
#define XTIMER_CHAN (0)
#define XTIMER_MASK (0xff000000)
-#define XTIMER_SHIFT_ON_COMPARE (2)
#define XTIMER_BACKOFF (40)
/** @} */
diff --git a/boards/telosb/include/board.h b/boards/telosb/include/board.h
index 78c67772f30bcf41fd3368b1b3e803a5c8454164..a3e36ab4738be26100b94309a3771fb79b3907ff 100644
--- a/boards/telosb/include/board.h
+++ b/boards/telosb/include/board.h
@@ -57,7 +57,6 @@ extern "C" {
#define XTIMER (0)
#define XTIMER_CHAN (0)
#define XTIMER_MASK (0xffff0000)
-#define XTIMER_SHIFT_ON_COMPARE (4)
#define XTIMER_BACKOFF (40)
/** @} */
diff --git a/boards/wsn430-common/include/board_common.h b/boards/wsn430-common/include/board_common.h
index 56a0b6b5c6a6024004490c210a6e1ea4cf962a8b..1942986328c6b11cc912aeaafadd27e47c33f358 100644
--- a/boards/wsn430-common/include/board_common.h
+++ b/boards/wsn430-common/include/board_common.h
@@ -40,7 +40,6 @@ extern "C" {
#define XTIMER (0)
#define XTIMER_CHAN (0)
#define XTIMER_MASK (0xffff0000)
-#define XTIMER_SHIFT_ON_COMPARE (4)
#define XTIMER_BACKOFF (40)
/** @} */
diff --git a/boards/yunjia-nrf51822/include/board.h b/boards/yunjia-nrf51822/include/board.h
index 4b5509d560d9d700bea9bb548c8f90cac15fa095..22f21e99054bb3a8b28043323085a09fe01397e0 100644
--- a/boards/yunjia-nrf51822/include/board.h
+++ b/boards/yunjia-nrf51822/include/board.h
@@ -34,7 +34,6 @@ extern "C" {
#define XTIMER (0)
#define XTIMER_CHAN (0)
#define XTIMER_MASK (0xff000000)
-#define XTIMER_SHIFT_ON_COMPARE (2)
#define XTIMER_BACKOFF (40)
/** @} */
diff --git a/boards/z1/include/board.h b/boards/z1/include/board.h
index 431931cd916544594a4368ab1d7f155e484c8c4f..4698181e049a315f6aeae8579ba7f1b4205e5eaa 100644
--- a/boards/z1/include/board.h
+++ b/boards/z1/include/board.h
@@ -51,7 +51,6 @@ extern "C" {
#define XTIMER (0)
#define XTIMER_CHAN (0)
#define XTIMER_MASK (0xffff0000)
-#define XTIMER_SHIFT_ON_COMPARE (4)
#define XTIMER_BACKOFF (40)
/** @} */
diff --git a/sys/include/xtimer.h b/sys/include/xtimer.h
index 05c68270b9cee29359a17204998d308ddfa478b3..a600777c802df859d2f2b337d949cda3db951fdf 100644
--- a/sys/include/xtimer.h
+++ b/sys/include/xtimer.h
@@ -432,25 +432,6 @@ void _xtimer_sleep(uint32_t offset, uint32_t long_offset);
static inline void xtimer_spin_until(uint32_t value);
/** @} */
-#if XTIMER_MASK
-#ifndef XTIMER_SHIFT_ON_COMPARE
-/**
- * @brief ignore some bits when comparing timer values
- *
- * (only relevant when XTIMER_MASK != 0, e.g., timers < 32bit.)
- *
- * When combining _lltimer_now() and _high_cnt, we have to get the same value in
- * order to work around a race between overflowing _lltimer_now() and OR'ing the
- * resulting values.
- * But some platforms are too slow to get the same timer
- * value twice, so we use this define to ignore some of the bits.
- *
- * Use tests/xtimer_shift_on_compare to find the correct value for your MCU.
- */
-#define XTIMER_SHIFT_ON_COMPARE (0)
-#endif
-#endif
-
#ifndef XTIMER_MIN_SPIN
/**
* @brief Minimal value xtimer_spin() can spin
@@ -461,12 +442,18 @@ static inline void xtimer_spin_until(uint32_t value);
static inline uint32_t xtimer_now(void)
{
#if XTIMER_MASK
- uint32_t a, b;
+ uint32_t latched_high_cnt, now;
+
+ /* _high_cnt can change at any time, so check the value before
+ * and after reading the low-level timer. If it hasn't changed,
+ * then it can be safely applied to the timer count. */
+
do {
- a = _lltimer_now() | _high_cnt;
- b = _lltimer_now() | _high_cnt;
- } while ((a >> XTIMER_SHIFT_ON_COMPARE) != (b >> XTIMER_SHIFT_ON_COMPARE));
- return b;
+ latched_high_cnt = _high_cnt;
+ now = _lltimer_now();
+ } while (_high_cnt != latched_high_cnt);
+
+ return latched_high_cnt | now;
#else
return _lltimer_now();
#endif