Re: Information ROM List

[dempson@actrix.gen.nz (David Empson)]

Quadrajet1 <quadrajet1@aol.com> wrote:

> David Empson wrote:
> >Bryan Parkoff wrote:
> >> 197-0001 Crystal, 14.318630 MHZ $1.38 N/A
> >> 
> >> (What is this?)
> >
> >It is a 14.318630 MHz crystal, as the description says.
> 
>    I have also seen Apple use a 14.38383 or something like that.  The 14.31863
> is a color timing crystal, or maybe I have the two mixed up.

The correct frequency is supposed to be 14.31818 MHz, which is four
times the colour reference frequency of NTSC (3.579545 MHz).

I'm not sure what this slightly faster crystal is used for.  It is
probably close enough to the ideal value not to matter (fast by 31 parts
per million, assuming ideal crystals).

14.31818 MHz is the standard frequency of the the primary crystal (14M)
in a US (NTSC) ][ or ][+.  It is divided by two to get 7M (7.15909 MHz),
which is the reference clock for all the CPU and slot bus timing
signals, including Phi0 and Phi1 (1.022727143 MHz, not allowing for the
"65th cycle stretch").

Each horizontal line of video is 65 normal cycles of Phi0/Phi1 plus one
cycle of 7M, or 456 cycles of 7M, for a period of 63.695 microseconds
(about 15.7 kHz). Every 65th cycle of Phi0/Phi1 is stretched by one
period of 7M to stay in sync with the video output.

The same frequency is used in the NTSC versions of the IIe.  The IIgs
uses 28 MHz as its primary clock source, (actually 28.63636 MHz, twice
14M).

PAL models of the IIe use a 14.25 MHz master crystal, which means that a
PAL IIe is a little slower than an NTSC IIe.

-- 
David Empson
dempson@actrix.gen.nz