PIT Namespace Reference

Abstraction of the historical Programmable Interval Timer (PIT). More...

Enumerations
enum	AccessMode
	Access mode.
enum	OperatingMode { }
	Operating Mode. More...
enum	Format { }
	data format More...

Functions
bool	set (uint16_t us)
	Start timer.
uint16_t	get (void)
	Reads the current timer value.
bool	isActive (void)
	Check if the timer is running.
bool	waitForTimeout (void)
	(Active) waiting for timeout
bool	delay (uint16_t us)
	Set the timer and wait for timeout.
void	pcspeaker (uint32_t freq)
	Play a given frequency on the PC speaker.
void	disable (void)
	Deactivate the timer.

Detailed Description

Abstraction of the historical Programmable Interval Timer (PIT).

Historically, PCs had a Timer component of type 8253 or 8254, modern systems come with a compatible chip. Each of these chips provides three 16-bit wide counters ("channel"), each running at a frequency of 1.19318 MHz. The timer's counting speed is thereby independent from the CPU frequency.

Traditionally, the first counter (channel 0) was used for triggering interrupts, the second one (channel 1) controlled the memory refresh, and the third counter (channel 2) was assigned to the PC speaker.

As the PIT's frequency is fixed to a constant value of 1.19318 MHz, the PIT can be used for calibration. For this purpose, we use channel 2 only.

Note

Interrupts should be disabled while configuring the timer.

Enumeration Type Documentation

Format

enum PIT::Format

data format

Enumerator
BCD	Binary Coded Decimals.

OperatingMode

enum PIT::OperatingMode

Operating Mode.

Warning

Channel 2 is not able to send interrupts, however, the status bit will be set

Enumerator
SQUARE_WAVE_GENERATOR	useful for the PC speaker

Function Documentation

delay()

bool PIT::delay

(

uint16_t

us

)

Set the timer and wait for timeout.

Note

The maximum waiting time is approx. 55 000 us due to the timers being limited to 16 bit.

Parameters

us	Waiting time in us

Returns

true when waiting successfully terminated; false on error (e.g., waiting time exceeds its limits)

get()

uint16_t PIT::get

(

void

)

Reads the current timer value.

Returns

Current timer value

isActive()

bool PIT::isActive

(

void

)

Check if the timer is running.

Returns

true if running, false otherwise

pcspeaker()

void PIT::pcspeaker

(

uint32_t

freq

)

Play a given frequency on the PC speaker.

As the PC speaker is connected to PIT channel 2, the PIT can be used to play an acoustic signal. Playing sounds occupies the PIT, so it cannot be used for other purposes while playback.

Note

Not every PC has an activated PC speaker

Qemu & KVM have to be launched with -soundhw pcspk. If you still cannot hear anything, try to set QEMU_AUDIO_DRV to alsa (by launching StuBS with QEMU_AUDIO_DRV=alsa make kvm)

Parameters

freq	Frequency (in Hz) of the sound to be played, or 0 to deactivate playback.

set()

bool PIT::set

(

uint16_t

us

)

Start timer.

Sets the channel 2 timer to the provided value and starts counting.

Note

The maximum waiting time is approx. 55 000 us due to the timers being limited to 16 bit.

Parameters

us	Waiting time in us

Returns

true if the counter is running; false if the waiting time exceeds the limits.

waitForTimeout()

bool PIT::waitForTimeout

(

void

)

(Active) waiting for timeout

Returns

true when timeout was successfully hit, false if the timer was not active prior to calling.