PIT

[I...am your father!]

Hi!
I implementated a PIT to my OS, and I use this function:
void init_timer(unsigned char FREQ)
{
unsigned int counter=1193181/FREQ;
outportb(PIT_CTRL, 0x43);
outportb(PIT_CH_0, counter%0xFF);
outportb(PIT_CH_0, counter/256);
}

what is the exact value that the PIT activates 1000 times a second or how can I calculate this???? Or do I have to calibrate this?
Thanks...!

[Pype.Clicker]

well, just use a short instead of a char and ask for FREQ=1000 ... this will be the best the PIT can offer ...

[Guest]

thank you! Ill try...


May the source be with you

[Guest]

Great it works perfect, but on an emulator like bochs or vmWare the seconds are too long... This made me crazy because i thought it was my fault

Thank you very very much...... :-*

[Dragon88]

I would surmise that that is caused by bochs essentially being a comp inside a comp.

[proxy]

i noticed a bug in your code
you use the modulus operator to get the low byte of the counter variables like this:

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outportb(PIT_CH_0, counter%0xFF);

this will yeild a value between 0 and 0xfe, skiping one value. incorrect.

you have two choices to fix this minor, which depend on your preference of style (compiler will likely emit same code for both).

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outportb(PIT_CH_0, counter%0x100);

or

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outportb(PIT_CH_0, counter&0xFF);

[Neo]

Great it works perfect, but on an emulator like bochs or vmWare the seconds are too long... This made me crazy because i thought it was my fault

Thank you very very much...... :-*

In BOCHS 2.0.2 and above you can use this to get a better simulation of the PIT. add this line to your bochsrc file

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pit: realtime=1