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.\" $FreeBSD: releng/12.2/share/man/man4/attimer.4 266709 2014-05-26 19:02:34Z brueffer $
.\"
.Dd May 26, 2014
.Dt ATTIMER 4
.Os
.Sh NAME
.Nm attimer
.Nd i8254 Programmable Interval Timer (AT Timer) driver
.Sh SYNOPSIS
This driver is a mandatory part of x86 kernels.
.Pp
The following tunables are settable from the
.Xr loader 8 :
.Bl -ohang
.It Va hint.attimer. Ns Ar X Ns Va .clock
controls support for the event timer functionality.
Setting this value to
.Dv 0
disables it.
The default value is
.Dv 1 .
.It Va hint.attimer. Ns Ar X Ns Va .timecounter
controls support for the time counter functionality.
Setting this value to
.Dv 0
disables it.
The default value is
.Dv 1 .
.It Va hw.i8254.freq
allows overriding the default counter frequency.
The same value is also available at run-time via the
.Va machdep.i8254_freq
sysctl.
.El
.Sh DESCRIPTION
This driver uses i8254 Programmable Interval Timer (AT Timer) hardware
to supply the kernel with one timecounter and one event timer, and to generate
sound tones for the system speaker.
This hardware includes three channels.
Each channel includes a 16 bit counter which decreases with a known,
platform-dependent frequency.
Counters can operate in several different modes, including periodic and
one-shot.
The output of each channel has platform-defined wiring: one channel is wired
to the interrupt controller and may be used as event timer, one channel is
wired to the speaker and used to generate sound tones, and one timer is reserved
for platform purposes.
.Pp
The
.Nm
driver uses a single hardware channel to provide both time counter and event
timer functionality.
To make this possible, the respective counter must be running in periodic mode.
As a result, the one-shot event timer mode is supported only when time counter
functionality is disabled.
.Pp
The event timer provided by the driver is irrelevant to CPU power states.
.Sh SEE ALSO
.Xr apic 4 ,
.Xr atrtc 4 ,
.Xr eventtimers 4 ,
.Xr hpet 4 ,
.Xr timecounters 4