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.
