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)

Debian                                         September 14, 2010                                     ATTIMER(4)