Russell McMahon <apptech@paradise.net.nz> wrote:
> I wrote:
> > > 55ms (or 55.55555ms to be more exact)  is about 18 pulses per 
> > > second.
> 
> > ... 4.772727 MHz.
> 
> which was chosen because that's the NTSc color burst frequency and the 
> crystals are cheap.

Nope. The NTSC color-burst frequency is 3.579545 MHz, which is
14.31818 MHz divided by four, not three.

The PC used the 14.3 MHz crystal partly because it was cheaply available
as a result of its wide use in the TV industry, and partly because the
early video cards wanted to be compatible with TV monitors. The early
8088s could run at up to 5 MHz, and they figured 4.77 MHz was "close
enough" without requiring a separate oscillator.

> > If you want 55.55555 ms, that's *exactly* 18 pulses/second.
> 
> In engineering at least, 'about' includes 'exactly' :-)

Yes, but I was emphasizing the point that this rate is not the PC's rate
of 18.2 Hz. I wasn't sure if that's what the OP was looking for or if he
cared about the difference.

-- Dave Tweed
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