Programmable Interval Timer: Difference between revisions

For the "lobyte/hibyte" access mode you need to send the latch command (described above) to avoid getting wrong results. If any other code could try set the PIT channel's reload value or read its current count after you've sent the latch command but before you've read the highest 8 bits, then you have to prevent it. Disabling interrupts works for single CPU computers. For example, to read the count of PIT channel 0 you could use something like:

 

unsigned read_pit_count(void) {

unsigned count = 0;

return count;

}

 

== Setting The Reload Value ==

For the "lobyte/hibyte" access mode you need to send the low 8 bits followed by the high 8 bits. You must prevent other code from setting the PIT channel's reload value or reading its current count once you've sent the lowest 8 bits. Disabling interrupts works for single CPU computers. For example:

 

void set_pit_count(unsigned count) {

// Disable interrupts

return;

}

 

It should be noted that a reload value of zero can be used to specify a divisor of 65536. This is how the BIOS gets an IRQ 0 frequency as low as 18.2065 Hz.

To begin with, this following code contains all of the data used by this example. It is assumed that the ".bss" section is filled with zeros.

 

section .bss

system_timer_fractions: resd 1 ; Fractions of 1 ms since timer initialized

PIT_reload_value: resw 1 ; Current PIT reload value

section .text

 

Next is the handler for IRQ 0. It's fairly simple (all it does it add 2 64 bit fixed point values and send an EOI to the PIC chip).

 

IRQ0_handler:

push eax

pop eax

iretd

 

Now for the tricky bit – the initialization routine. The PIT can't generate some frequencies. For example if you want 8000 Hz then you've got a choice of 8007.93 Hz or 7954.544 Hz. In this case the following code will find the nearest possible frequency. Once it has calculated the nearest possible frequency it will reverse the calculation to find the actual frequency selected (rounded to the nearest integer, intended for display purposes only).

For some extra accuracy, I also use "3579545 / 3" instead of 1193182 Hz. This is mostly pointless due to inaccurate hardware (I just like being correct).

 

;Input

; ebx Desired PIT frequency in Hz

popad

ret

 

Note: you also need to install an IDT entry for IRQ 0, and unmask it in the PIC chip (or I/O APIC).

==Uses for the Timer IRQ==

===Using the IRQ to Implement <tt>sleep</tt>===

Next, every time the timer interrupt is called, decrement this variable until 0 is stored.

section .text

global TimerIRQ

push eax

mov eax, [CountDown]

jz TimerDone

dec eax

mov [CountDown], eax

pop eax

iretd

Finally, create a function <tt>sleep</tt> that waits the interval, in milliseconds.

 

In a multitasking system, consider using a linked list or array of these CountDown variables. If your multitasking system supports interprocess communication, you can also store the semaphore/exchange where two processes can talk to, have the interrupt send a message to the waiting process when the timer is done, and have the waiting process block all execution until that message comes:

 

#define COUNTDOWN_DONE_MSG 1

struct TimerBlock {

WaitForMessageFrom(t->e = getCrntExch());

}

 

In your documentation, note the interval of the timer. For example, if the timer interval is 10 milliseconds per tick, tell the programmer to issue

 

Sleep(100);

 

to sleep for a single second.

 

[[Category:Common Devices]]

[[de:Programmable_Interval_Timer]]