---
name: "atrtc"
section: "4"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/atrtc"
---

# atrtc(4)

AT Real-Time Clock (RTC) driver

## NAME

**atrtc** – AT Real-Time Clock (RTC) driver

## SYNOPSIS

This driver is a mandatory part of i386/amd64 kernels.

The following tunables are settable from the [loader(8)](/loader/8.md):

- *hint.atrtc.**X**.clock*

  controls event timers functionality support. Setting to 0, disables it. Default value is 1.

- *hw.atrtc.enabled*

  Forces enabling or disabling of the device(s). Setting to 0 disables it, setting to 1 enables it, bypassing any platform configuration hints. Default value is -1 (autodetect).

## DESCRIPTION

This driver uses RTC hardware to supply kernel with time-of-day clock with 1 second resolution and one event timer. This hardware uses base frequency of 32768Hz for advancing time-of-day clock and generating periodic interrupts. Interrupts could be generated with fixed number of frequencies, from 2Hz to 8192Hz, obtained by dividing base frequency by one of supported power-of-2 divisors.

Event timer provided by the driver is irrelevant to CPU power states.

## SEE ALSO

[apic(4)](/apic/4.md), [attimer(4)](/attimer/4.md), [eventtimers(4)](/eventtimers/4.md), [hpet(4)](/hpet/4.md)