[−][src]Module sunrise_kernel::devices::pit

Programmable Interval Timer

There are 3 channels :

channel 0, wired to irq0. We use this one in rate generator mode, to keep track of the time
channel 1, "unusable, and may not even exist" ... whoah
channel 2, wired to pc speaker. We use this one in "one shot" mode to implement a simple wait function. Output is ANDed with a gate controlled by port 0x61 bit #1 before going to the pc speaker, we use this to enable/disable the speaker. However the channel can only track one countdown at a time, so we have to switch to channel 0 timers when we want to wait as soon as we have interruptions working

Each channel can operate in different modes. The ones we use are:

Mode 0 - Countdown (poorly named "Interrupt on Terminal Count"). Used on channel 2 to create countdowns. Set the reset value, countdowns starts. When countdown goes to 0, OUT goes HIGH, and stays high.
Mode 2 - Rate generator. Used on channel 0 to create recurring irqs. Set the reset value. Countdown starts. When countdown goes to 0, OUT goes LOW for 1 clock cycle, and HIGH again, and countdown restarts.

Pit commands are sent on port 0x43. See OSDEV

To write channel reload values we always use the Access mode "lobyte/hibyte"

The PIT makes great use of the IO port 0x61 :

References

PITChannel2	Channel 2
PITPorts	We put the PIT ports in a structure to have them under a single mutex
PIT_PORTS	The mutex wrapping the ports
Port61Flags	The port 0x61 flags we use.

Used internally to select which channel to apply operations to.

CHAN_0_DIVISOR	The channel 0 reset value
CHAN_0_FREQUENCY	The frequency of channel 0 irqs, in hertz. One every 10 millisecond.
OSCILLATOR_FREQ	The oscillator frequency when not divided, in hertz.

disable ^⚠	Prevent the PIT from generating interrupts.
init_channel_0 ^⚠	Initialize the channel 0 to send recurring irqs.
spin_wait_ms	Spin waits for at least `ms` amount of milliseconds