CGA & clock speeds

From: jpero_at_cgo.wave.ca <(jpero_at_cgo.wave.ca)>
Date: Sun Nov 23 20:33:50 1997

> <booted but did not give a display. I could tell it was working by
> <entering
> <DOS commands. I tried then at 5.33 MHz. It booted & ran OK (as far as
> <and found that the CGA card depends on the 14.31818 oscilator to run
> <properly.
> <Since the processor seemed to be running OK at 8 MHz, what I want to do
>
> Simple make sure you supply 14.31818 to the ISA bus from a seperate
> oscillator (or put it on the CGA card and bypass the buss connection).
> The problem is that much of the machine is timed off that clock. it is
> generated either using then 8284A clock generator or or a TTL oscillator
> can. The 8088 is driven off the 8284A (it supplies the correct 33% duty
> cycle closk) so it could be done as two seperate sources. There were
> "upgrades" that did exactly that yo yeild turbo systems.
>
> Allison
Before I go in about my saying after this, also for same reason, lot
of CGA/EGA/MDA/HGC clones (quality and craps likewise) has this
14.31818mhz cystral or oscillator among other different frequencies.
Now we're back to using one 14.31818mhz source to generate different
freqs and use that 14.31818mhz clocking to drive a PLL chip producing
these clocks selectable by jumpers: 20, 25, 33, 40, 50mhz
frequencies for the x86's and P5's, PPro and P5II's. :) One who is
very knowledgeable knocked me out saying this is not correct...I did
not believe him and let this matter drop peacefully. Video cards
currently does that same too some use one 14.31818, 20mhz or 40mhz on
board driving the PLL chip to get different clocking by user
programmable. That last one, you can do that too easily with right
s/w like linux, BSD, etc which makes this very useful to drive these
fixed frequency monitors.

Now to my comment and a Q:

Great!

I was not quite sure how that was done by what you have wrote, "...so
it could be done as two seperate sources. These were "upgrades that
did exactly that yo yeild turbo systems..." What!? I get the
feeling something have left out how they make that work without
screwing up that important 14.31818mhz but I would prefer to try to
speed up the sustem bus as well if I could.

I have few 8mhz and 10mhz 8088 CPU's and rather like to experiment
with that OLD XT motherboard.

That would be amusing to show the IBM XT to someone and hear them
exlaim it's not XT, it's not a XT, and confidently telling me this
was fitted with a trash clone mobo! And open that box and look...
Gasp...great secret XT motherboard!

:)

A big hint, have anyone tried that on XT or PC to disable parity by
removing that 74LS280 parity/comparer generator chip? That is what
exactly that is done on generic XT mobos. On same issue, I was
puzzled on a old 1piece Zenith LP 286 mobo, it has more than 2 LS280
chips on it. What gives!? Maybe that is needed for 16bits but this
is hard to believe as that works best if using 1 16bit parity
generator/compare chip.

Troll
Received on Sun Nov 23 1997 - 20:33:50 GMT