[Coco] Instruction cycles per 60Hz frame.

[wrcooke at wrcooke.net]

   I'm no CoCo expert, but I believe, as with most similar systems, the
   clock is derived from the NTSC colorburst frequency.  That frequency is
   3,579,545 Hz.  The CPU clock is that frequency / 4 = 894,886.25 Hz.  (
   divided by 2 it is 1,787,752.5 Hz)  The 60/59.94 Hz are also derived
   from that by a simple divider(3,579,545*2 / (455*525) * 2 = about 59.94
   Hz).

   Crystals aren't perfect, even new, and drift with age.  But, the beauty
   is the ratios will always stay the same.

   The simple math gives 894,886 / 59.94 = 14,929.7 -> 14,929

   The exact math, that will ALWAYS be perfect, is:

   cb = colorburst = 3,579,545

   (cb /4) / (cb*4/ (455 * 525))

   which simplified to 455 * 525 / 16 or 14,929.6875 -> 14,929 for normal
   speed

   For double speed it is 455 * 525/8 or 29,859.375 -> 29,859

   Notice that the actual frequencies aren't in those forumulas.  They
   cancel out, since all the clocks are derived from the same crystal by
   dividers.

   [1]https://en.wikipedia.org/wiki/NTSC

   [2]http://www.colorcomputerarchive.com/coco/Documents/Datasheets/MC6847
   %20MOS%20Video%20Display%20Generator%20(Motorola).pdf

   Sorry, I was bored on a Sunday evening :-)

   Will

   On November 17, 2019 at 3:52 PM Alex Evans < [3]varmfskii at gmail.com>
   wrote:

   Is it worth taking into consideration that the 60Hz should actually be

   59.94Hz assuming NTSC (30/1.001 full frames or 60/1.001 half frames).

   On Sun, Nov 17, 2019 at 2:39 PM Ed Snider < [4]zippster278 at gmail.com>
   wrote:

   >

   Correct. Round down to the nearest whole cycle of course.

   894886 cycles per second at single speed.

   1789772 cycles per second at double speed.

   - Ed

   On Nov 17, 2019, at 12:41 PM, James Ross < [5]jrosslist at outlook.com>
   wrote:

   continued: actually the math can simplified to ...

   cycles_per_60hz_frame = (1d / 60d) * 1000000 * coco_clk; // 0.8948...
   MHz

   double_cycles_per_60hz_frame = (1d / 60d) * 1000000 * double_speed; //
   1.789... MHz

   which gives the same results:

   standard 60hz (vsync) frame = 14914.7727272727 cycles per frame

   double speed 60hz (vsync) frame = 29829.5454545455 cycles per frame

   so, unless I am really confused (which is quite possible!), then the
   #'s above have to be correct ... ok moving on then :) :)

   james

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References

   1. https://en.wikipedia.org/wiki/NTSC
   2. http://www.colorcomputerarchive.com/coco/Documents/Datasheets/MC6847 MOS Video Display Generator (Motorola).pdf
   3. mailto:varmfskii at gmail.com
   4. mailto:zippster278 at gmail.com
   5. mailto:jrosslist at outlook.com
   6. mailto:Coco at maltedmedia.com
   7. https://pairlist5.pair.net/mailman/listinfo/coco
   8. mailto:Coco at maltedmedia.com
   9. https://pairlist5.pair.net/mailman/listinfo/coco
  10. mailto:Coco at maltedmedia.com
  11. https://pairlist5.pair.net/mailman/listinfo/coco