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

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

     This driver is a mandatory	part of	x86 kernels.

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

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

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

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

     This driver uses i8254 Programmable Interval Timer	(AT Timer) hardware to
     supply the	kernel with one	timecounter and	one event timer, and to	gener-
     ate 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 inter-
     rupt 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 functional-
     ity is disabled.

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

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

FreeBSD	Ports 11.2		 May 26, 2014		    FreeBSD Ports 11.2


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