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.Dd May 26, 2014 26.Dt ATTIMER 4 27.Os 28.Sh NAME 29.Nm attimer 30.Nd i8254 Programmable Interval Timer (AT Timer) driver 31.Sh SYNOPSIS 32This driver is a mandatory part of x86 kernels. 33.Pp 34The following tunables are settable from the 35.Xr loader 8 : 36.Bl -ohang 37.It Va hint.attimer. Ns Ar X Ns Va .clock 38controls support for the event timer functionality. 39Setting this value to 40.Dv 0 41disables it. 42The default value is 43.Dv 1 . 44.It Va hint.attimer. Ns Ar X Ns Va .timecounter 45controls support for the time counter functionality. 46Setting this value to 47.Dv 0 48disables it. 49The default value is 50.Dv 1 . 51.It Va hw.i8254.freq 52allows overriding the default counter frequency. 53The same value is also available at run-time via the 54.Va machdep.i8254_freq 55sysctl. 56.El 57.Sh DESCRIPTION 58This driver uses i8254 Programmable Interval Timer (AT Timer) hardware 59to supply the kernel with one timecounter and one event timer, and to generate 60sound tones for the system speaker. 61This hardware includes three channels. 62Each channel includes a 16 bit counter which decreases with a known, 63platform-dependent frequency. 64Counters can operate in several different modes, including periodic and 65one-shot. 66The output of each channel has platform-defined wiring: one channel is wired 67to the interrupt controller and may be used as event timer, one channel is 68wired to the speaker and used to generate sound tones, and one timer is reserved 69for platform purposes. 70.Pp 71The 72.Nm 73driver uses a single hardware channel to provide both time counter and event 74timer functionality. 75To make this possible, the respective counter must be running in periodic mode. 76As a result, the one-shot event timer mode is supported only when time counter 77functionality is disabled. 78.Pp 79The event timer provided by the driver is irrelevant to CPU power states. 80.Sh SEE ALSO 81.Xr apic 4 , 82.Xr atrtc 4 , 83.Xr eventtimers 4 , 84.Xr hpet 4 , 85.Xr timecounters 4 86