Merge pull request #15003 from kaspar030/ztimer_now64_tick

ztimer/core: ensure half-period tick when ztimer_now64 is used

sys/ztimer/core.c

@@ -295,11 +295,17 @@ static void _ztimer_update(ztimer_clock_t *clock)
         if (clock->list.next) {
             clock->ops->set(clock, clock->list.next->offset);
         }
+        else {
+            if (IS_USED(MODULE_ZTIMER_NOW64)) {
+                /* ensure there's at least one ISR per half period */
+                clock->ops->set(clock, clock->max_value >> 1);
+            }
             else {
                 clock->ops->cancel(clock);
             }
         }
     }
+}
 
 void ztimer_handler(ztimer_clock_t *clock)
 {