8254 timer usage under Sys V/386 R3.2?

[Dick Dunn]

apfiffer at admin.ogi.edu (Andy Pfiffer) writes:
> I'd like to be able to program and poll one of the timers on the 8254
> Programmable Interval Timer under a vanilla System V/386 R3.2.  I have
> no difficulty accessing the ports, but I'm not certain how UNIX uses them.

There really isn't a timer you can use without screwing things up.  For two
of them, it's not so much how UNIX uses them as how the hardware uses
them.

> There are three 1.19 MHz timers on the chip...

Yeah...there's actually one crystal-controlled oscillator driving all three
CLK inputs on the 8254.  The frequency is 1.193182 MHz.  (Some AT tech refs
are imprecise on this; it does matter for how you program it if you want an
accurate clock.)

>...My guesses:...
> 	    0	100Hz clock interrupt
> 	    1	DRAM refresh (about 66 KHz)
> 	    2	beep & buzz
> Are these correct?

Yes.  Note that timer 1 is set up for the 15 us cycle and wired on the
motherboard to drive refresh...so although you probably can reprogram it
(tho I've never done it--at least not that I know of:-), you're asking for
trouble.

In the same vein, timer 2 is wired to the speaker driver, so if you fiddle
with it, you'll make funny noises--and if something decides it needs to
honk at the user, the kernel will reprogram the timer out from under you.

There's no hardware magic to timer 0 other than wiring CLK OUT 0 to
IRQ0.  But you're right that UNIX uses it for the system clock, so if you
go reprogramming it, you're going to time-warp your system.  Even if you
don't mind your clock running funny, there are other dangers in repro-
gramming it:  If you turn it off, kernel timeouts won't happen and you'll
probably hang the system in short order.  If you crank it way up, clock
interrupts will happen too fast and, I'd guess, you'll spend all your time
in the kernel trying to handle them; you might manage to panic it.

It's unfortunate that there isn't a free timer for other uses.  It would be
very useful for both system and application tasks.  It would be nice if
there were a simple I/O card with a programmable timer or two...couldn't
take more than a few chips.  Does anyone know if such an animal exists?
-- 
Dick Dunn     rcd at ico.isc.com -or- ico!rcd       Boulder, CO   (303)449-2870
   ...Mr. Natural says, "Use the right tool for the job."