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32 .Nd i8254 Programmable Interval Timer (AT Timer) driver
34 This driver is a mandatory part of x86 kernels.
36 The following tunables are settable from the
39 .It Va hint.attimer. Ns Ar X Ns Va .clock
40 controls support for the event timer functionality.
46 .It Va hint.attimer. Ns Ar X Ns Va .timecounter
47 controls support for the time counter functionality.
54 allows overriding the default counter frequency.
55 The same value is also available at run-time via the
56 .Va machdep.i8254_freq
60 This driver uses i8254 Programmable Interval Timer (AT Timer) hardware
61 to supply the kernel with one timecounter and one event timer, and to generate
62 sound tones for the system speaker.
63 This hardware includes three channels.
64 Each channel includes a 16 bit counter which decreases with a known,
65 platform-dependent frequency.
66 Counters can operate in several different modes, including periodic and
68 The output of each channel has platform-defined wiring: one channel is wired
69 to the interrupt controller and may be used as event timer, one channel is
70 wired to the speaker and used to generate sound tones, and one timer is reserved
71 for platform purposes.
75 driver uses a single hardware channel to provide both time counter and event
77 To make this possible, the respective counter must be running in periodic mode.
78 As a result, the one-shot event timer mode is supported only when time counter
79 functionality is disabled.
81 The event timer provided by the driver is irrelevant to CPU power states.