Re: i386: Selectable Frequency of the Timer Interrupt

Re: i386: Selectable Frequency of the Timer Interrupt

[Jan Engelhardt]

Hi,


Vojtech Pavlik @ Tue, 12 Jul 2005 14:10:08 +0200 wrote:
(http://lkml.org/lkml/2005/7/12/122)

>On Mon, Jul 11, 2005 at 05:38:05PM -0700, George Anzinger wrote:
>> HZ  	TICK RATE	jiffie(ns)	second(ns)	 error (ppbillion)
>>  100	 1193180	10000000	1000000000	       0

>The PIT crystal runs at 14.3181818 MHz (CGA dotclock, found on ISA, ...)
>and is divided by 12 to get PIT tick rate
>	14.3181818 MHz / 12 = 1193182 Hz

What exactly is the frequency of the PIT? Many internet resources say 1193180 
(including the original post), some say 1193181, and you say 1193182 Hz.
Which one is correct? (Ignoring temperature for now..)

IMO 1193181 could be, because it matches the almost-magical number 0x1234DD.


Jan Engelhardt
-- 

Re: i386: Selectable Frequency of the Timer Interrupt

[Richard B. Johnson]

On Tue, 12 Jul 2005, Jan Engelhardt wrote:

> Hi,
>
>
> Vojtech Pavlik @ Tue, 12 Jul 2005 14:10:08 +0200 wrote:
> (http://lkml.org/lkml/2005/7/12/122)
>
>> On Mon, Jul 11, 2005 at 05:38:05PM -0700, George Anzinger wrote:
>>> HZ  	TICK RATE	jiffie(ns)	second(ns)	 error (ppbillion)
>>>  100	 1193180	10000000	1000000000	       0
>
>> The PIT crystal runs at 14.3181818 MHz (CGA dotclock, found on ISA, ...)
>> and is divided by 12 to get PIT tick rate
>> 	14.3181818 MHz / 12 = 1193182 Hz
>
> What exactly is the frequency of the PIT? Many internet resources say 1193180
> (including the original post), some say 1193181, and you say 1193182 Hz.
> Which one is correct? (Ignoring temperature for now..)
>

The original specification was based upon the NTSC color subcarrier
frequency of 3.579545. The dotclock is 4 times this:

 	3.579745 * 4 = 14.31818000

You can type 3.579545 into google and see it's used practically
everywhere as the magic number that started it all ;^) The
crooked-earth society uses this for PI (just kidding).


> IMO 1193181 could be, because it matches the almost-magical number 0x1234DD.
>
>
> Jan Engelhardt
> -- 
>
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Cheers,
Dick Johnson
Penguin : Linux version 2.6.12 on an i686 machine (5537.79 BogoMips).
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Re: i386: Selectable Frequency of the Timer Interrupt

[Vojtech Pavlik]

On Tue, Jul 12, 2005 at 04:05:16PM +0200, Jan Engelhardt wrote:

> >> >     HZ   ticks/jiffie  1 second      error (ppm)
> >> > ---------------------------------------------------
> >> >    100      11932      1.000015238      15.2
> 
> I was not quite able to reproduce these values, probably because I got the
> math wrong. I used:
>   $oneSecond = $ticksJiffie * $HZ / 1193182
> which yields 11932*100/1193182 = 1.00001508571198693912, !=1.000015238
> Math corrections welcome.

I used 1.19318[18] MHz periodic as the true clock speed - 1/3rd of the
NTSC color subcarrier frequency.

1193182 Hz is already a rounded value, and as such introduces some error
by the rounding.

It is possible the standard value is 1.1931816[6] MHz periodic, as
Richard B. Johnson corrected me, being 1/12th of 14.31818000 MHz, the
CGA dotclock. 

Anyway, both 14.31818 MHz and 14.3181818 MHz crystals are being
manufactured, and thus we'll see both these numbers in the wild.

> Anyway, I've done some graphs. Intersting that the smaller the HZ, the less
> error (seen on a whole, esp. view_1k and view_8k.png) we get. 20Hz seems to
> be the 0.0 case, and 18Hz is not bad either. IIRC, DOS used 18HZ ;)
> http://jengelh.hopto.org/tick/

DOS used 65535 as the divisor (ticks/jiffie), which doesn't give an
integer HZ.

-- 
Vojtech Pavlik
SuSE Labs, SuSE CR

Re: i386: Selectable Frequency of the Timer Interrupt

[Jan Engelhardt]

Hi,

>> >     HZ   ticks/jiffie  1 second      error (ppm)
>> > ---------------------------------------------------
>> >    100      11932      1.000015238      15.2

I was not quite able to reproduce these values, probably because I got the
math wrong. I used:
  $oneSecond = $ticksJiffie * $HZ / 1193182
which yields 11932*100/1193182 = 1.00001508571198693912, !=1.000015238
Math corrections welcome.

Anyway, I've done some graphs. Intersting that the smaller the HZ, the less
error (seen on a whole, esp. view_1k and view_8k.png) we get. 20Hz seems to
be the 0.0 case, and 18Hz is not bad either. IIRC, DOS used 18HZ ;)
http://jengelh.hopto.org/tick/




Jan Engelhardt
-- 
| Alphagate Systems, http://alphagate.hopto.org/