---
title: "Time and delays"
canonical_url: https://developer.repebble.com/pebbleos/docs/kernel/time/
last_updated: 2026-10-21
sdk_version: 4.33.1
---
# Time and delays

Source: https://developer.repebble.com/pebbleos/docs/kernel/time/

The kernel keeps one monotonic clock, the tick count, advanced by the tick interrupt and by `pbl_idle_slept()` after tickless sleep. `pbl/kernel/time.h` reads it and the CPU cycle counter; `pbl/kernel/delay.h` waits on them.

## Uptime

`pbl_uptime_ticks()` and `pbl_uptime_ms()` count from the scheduler start in 64 bits, so they do not wrap during the life of a device. The 32-bit `pbl_tick_t` remains for relative timeouts only.

`pbl_cycle_get_32()` reads the CPU cycle counter (DWT `CYCCNT` on Cortex-M) at `pbl_cycles_per_sec()`, the current CPU clock. Use it to time short sections: it wraps within seconds and stops while the CPU sleeps.

## Waiting

| 

Need

 | 

Use

 | 

Resolution

 | 

Context

 |
| --- | --- | --- | --- |
| 

A few microseconds, or no thread

 | 

`pbl_busy_wait_us()`

 | 

cycles

 | 

anywhere

 |
| 

Microseconds, being late is fine

 | 

`pbl_sleep_us()`

 | 

one tick

 | 

thread

 |
| 

Milliseconds and longer

 | 

`pbl_sleep_ms()`

 | 

one tick

 | 

thread

 |
| 

A timeout or a deadline in ticks

 | 

`pbl_thread_sleep()`

 | 

one tick

 | 

thread

 |
| 

A condition, spinning

 | 

`PBL_POLL_BUSY_US()`

 | 

cycles

 | 

anywhere

 |
| 

A condition, sleeping

 | 

`PBL_POLL_SLEEP_MS()`

 | 

one tick

 | 

thread

 |

The `pbl_sleep_*()` and `pbl_busy_wait_us()` calls wait at least the requested time, which is what hardware timings from a datasheet need: sleeps round up to whole ticks and add one for the tick already in progress. `pbl_thread_sleep()` counts tick boundaries instead, so its first tick may be partial.

The polling macros return 0 once the condition holds and `-ETIMEDOUT` otherwise, evaluating it once more after the timeout so that preemption cannot turn a ready condition into a false timeout, as Linux `read_poll_timeout()` does.

Busy waiting keeps the CPU awake and starves lower priority threads; past a tick or so, sleep instead.

Generated from PebbleOS [3a7b71785](https://github.com/coredevices/pebbleos/commit/3a7b71785fd439666c4a8bc116f561140e8dec01). This page is maintained in the pebbleos repository: [docs/kernel/time.md](https://github.com/coredevices/pebbleos/blob/3a7b71785fd439666c4a8bc116f561140e8dec01/docs/kernel/time.md).