1.\" Copyright (c) 2010 Alexander Motin <mav@FreeBSD.org> 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.\" $FreeBSD$ 26.\" 27.Dd May 26, 2014 28.Dt ATTIMER 4 29.Os 30.Sh NAME 31.Nm attimer 32.Nd i8254 Programmable Interval Timer (AT Timer) driver 33.Sh SYNOPSIS 34This driver is a mandatory part of x86 kernels. 35.Pp 36The following tunables are settable from the 37.Xr loader 8 : 38.Bl -ohang 39.It Va hint.attimer. Ns Ar X Ns Va .clock 40controls support for the event timer functionality. 41Setting this value to 42.Dv 0 43disables it. 44The default value is 45.Dv 1 . 46.It Va hint.attimer. Ns Ar X Ns Va .timecounter 47controls support for the time counter functionality. 48Setting this value to 49.Dv 0 50disables it. 51The default value is 52.Dv 1 . 53.It Va hw.i8254.freq 54allows overriding the default counter frequency. 55The same value is also available at run-time via the 56.Va machdep.i8254_freq 57sysctl. 58.El 59.Sh DESCRIPTION 60This driver uses i8254 Programmable Interval Timer (AT Timer) hardware 61to supply the kernel with one timecounter and one event timer, and to generate 62sound tones for the system speaker. 63This hardware includes three channels. 64Each channel includes a 16 bit counter which decreases with a known, 65platform-dependent frequency. 66Counters can operate in several different modes, including periodic and 67one-shot. 68The output of each channel has platform-defined wiring: one channel is wired 69to the interrupt controller and may be used as event timer, one channel is 70wired to the speaker and used to generate sound tones, and one timer is reserved 71for platform purposes. 72.Pp 73The 74.Nm 75driver uses a single hardware channel to provide both time counter and event 76timer functionality. 77To make this possible, the respective counter must be running in periodic mode. 78As a result, the one-shot event timer mode is supported only when time counter 79functionality is disabled. 80.Pp 81The event timer provided by the driver is irrelevant to CPU power states. 82.Sh SEE ALSO 83.Xr apic 4 , 84.Xr atrtc 4 , 85.Xr eventtimers 4 , 86.Xr hpet 4 , 87.Xr timecounters 4 88