Hi Simon,

The macro _sys_clock_tick_announce() call _nano_sys_clock_tick_announce(_sys_idle_elapsed_ticks), so _sys_idle_elapsed_ticks might off, can you check the value of _sys_idle_elapsed_ticks before every _sys_clock_tick_announce on the timer driver?, I think this line might be suspect on

drivers/timer/arcv2_timer0.c on the function _timer_idle_exit, look for the lines

uint32_t remaining_cycles = programmed_limit - current_count;

_sys_idle_elapsed_ticks = current_count / cycles_per_tick;
if (_sys_idle_elapsed_ticks > 0) {
update_accumulated_count();
_sys_clock_tick_announce();
}

The problem here is maybe _sys_idle_elapsed_ticks is going off,

also take a look at _timer_idle_exit when is coming off a timer interrupt,

if (control & _ARC_V2_TMR_CTRL_IP) {
/*
* The timer has expired. The handler _timer_int_handler() is
* guaranteed to execute. Track the number of elapsed ticks. The
* handler _timer_int_handler() will account for the final tick.
*/

_sys_idle_elapsed_ticks = programmed_ticks - 1;
update_accumulated_count();
_sys_clock_tick_announce();

return;
}

Is announcing the ticks minus one to all the timers (if any), so if no other interrupt happens (just the timer) the count will be fullfilled at the next tick, but if other interrupt happens and add more that one tick to _sys_idle_elapsed_ticks, that might set the timers with wrong count, so we have to revisit the non timer interrupt tick anounce