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)