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ATTIMER(4)             FreeBSD Kernel Interfaces Manual             ATTIMER(4)

NAME
     attimer - i8254 Programmable Interval Timer (AT Timer) driver

SYNOPSIS
     This driver is a mandatory part of x86 kernels.

     The following tunables are settable from the loader(8):

     hint.attimer.X.clock
     controls support for the event timer functionality.  Setting this value
     to 0 disables it.  The default value is 1.

     hint.attimer.X.timecounter
     controls support for the time counter functionality.  Setting this value
     to 0 disables it.  The default value is 1.

     hw.i8254.freq
     allows overriding the default counter frequency.  The same value is also
     available at run-time via the machdep.i8254_freq sysctl.

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.

     The attimer 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.

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

SEE ALSO
     apic(4), atrtc(4), eventtimers(4), hpet(4), timecounters(4)

FreeBSD 11.1-RELEASE-p4          May 26, 2014          FreeBSD 11.1-RELEASE-p4
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