CGA & clock speeds

From: Tony Duell <>
Date: Mon Nov 24 04:14:55 1997

On Sun, 23 Nov 1997, ACC student wrote:

> could tell), but the display was all messed up. I then did some
> research
> 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
> is
> find a way to send a proper clock signal to the CGA card while clocking
> the

The 14.318 MHz signal is sent to the CGA card via pin B30 on the expansion
slot (1 pin from the front on the left hand side). This signal comes from
the master clock crystal on a PC or XT, but on an AT there's a separate
14.318MHz Xtal and 8284 clock chip just for that clock (OK, and the
real-time-clock heartbeat to the 8253 / 8254).

IBM state that there is no fixed phase relationship between this signal
(Osc) and any other clock signal on the expansion connector. In fact, if
you have a 5161 expansion box, the Osc signal is not fed down the cable -
there's a second 14.318MHz Xtal in the expansion box just to drive the B30
pins of the slots in there. (Yes, I know the CGA card doesn't work in the
expansion box...)

So, if you want to modify your CGA card, all you have to do is cut the
trace going to pin B30 and feed a 14.318MHz TTL level clock signal in
there. The real time clock will run fast, but do you care about that?

The other thing to do would be to pull the 8284 clock chip from the
motherboard, and feed the appropriate signals to the PCLK (Processor
Clock), CLK (bus clock, was 4.77MHz, should now be the same as PCLK?, or
maybe left at 4.77MHz), and Osc (14.318Mhz). You'll need a pair of 8284s
to ensure that things like 'ready' are synced to the appropriate clock
edge (look at the Intel datasheet on the 8284 for details).

> main board at 8MHz. Or will changing to a different monitor (like VGA)
> make the clock speed irrelevant? Is CGA the only clock-dependant
> display

AFAIK the MDA and EGA board have their own clock generators, and don't
depend on Osc. Can't comment on VGA as I've never seen a schematic (I
don't think I even own a VGA card), but if there's no trace to pin B30,
then it should work fine.

> type?
> Thanks in advance,
> Dave

Received on Mon Nov 24 1997 - 04:14:55 GMT

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