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

     hint.attimer.X.timecounter
     controls time counter functionality support. Setting to 0, disables it.  Default value is 1.

     hw.i8254.freq
     allows to override default counter frequency.  The same value is also available in run-time
     via machdep.i8254_freq sysctl.

DESCRIPTION

     This driver uses i8254 Programmable Interval Timer (AT Timer) hardware to supply kernel with
     one time counter and one event timer, and generate sound tones for system speaker.  This
     hardware includes three channels.  Each channel includes 16bit counter, counting down with
     known, platform-dependent frequency.  Counters can operate in several different modes,
     including periodic and one-shot.  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 speaker and used to generate sound tones, and one timer is reserved for platform
     purposes.

     Driver uses single hardware channel to provide both time counter and event timer
     functionality.  To make it possible, respective counter must be running in periodic more.
     As result, one-shot event timer mode supported only when time counter functionality is
     disabled.

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

SEE ALSO

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