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):
-
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.
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.
apic(4),
atrtc(4),
eventtimers(4),
hpet(4),
timecounters(4)