diff --git a/ports/rp2/modmachine.c b/ports/rp2/modmachine.c
index cd73552dd9874..82be2bbbbcfd3 100644
--- a/ports/rp2/modmachine.c
+++ b/ports/rp2/modmachine.c
@@ -197,6 +197,7 @@ static void mp_machine_lightsleep(size_t n_args, const mp_obj_t *args) {
xosc_dormant();
} else {
bool timer3_enabled = irq_is_enabled(3);
+ uint64_t endtime_us_64 = timer_time_us_64(timer_hw) + delay_ms * 1000;
const uint32_t alarm_num = 3;
const uint32_t irq_num = TIMER_ALARM_IRQ_NUM(timer_hw, alarm_num);
@@ -216,7 +217,7 @@ static void mp_machine_lightsleep(size_t n_args, const mp_obj_t *args) {
#error Unknown processor
#endif
timer_hw->intr = 1u << alarm_num; // clear any IRQ
- timer_hw->alarm[alarm_num] = timer_hw->timerawl + delay_ms * 1000;
+ timer_hw->alarm[alarm_num] = ((uint32_t)((endtime_us_64) & 0xffffffff));
} else {
// TODO: Use RTC alarm to wake.
clocks_hw->sleep_en0 = 0x0;
@@ -245,8 +246,24 @@ static void mp_machine_lightsleep(size_t n_args, const mp_obj_t *args) {
#endif
#endif
- // Go into low-power mode.
- __wfi();
+
+ // A timer other than #3 could fire.
+ // Repeatedly go into low-power mode until timer 3 is no longer armed or
+ // there is less that 30 microseconds left to go.
+ //
+ // This means lightsleep can return 50 microseconds early.
+ // That is still close, even for a 1 millisecond sleep.
+ // But this masks a race condition where and interrupt occurs after the armed bit
+ // is checked but before __wfi() is called.
+ // Note: 50 microseconds is a magic number.
+ // Given that all interrupts are masked except the interrupt for timer_hw
+ // This should be more than enough to mask the race condition.
+
+ uint64_t currenttime_us_64 = timer_time_us_64(timer_hw);
+ while (timer_hw->armed & (1u << alarm_num) && (endtime_us_64 - currenttime_us_64) > 50) {
+ __wfi();
+ currenttime_us_64 = timer_time_us_64(timer_hw);
+ }
if (!timer3_enabled) {
irq_set_enabled(irq_num, false);
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