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 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)