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* SMS2-RGB glitch-free overclocking R2 *
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Design by GliGli

This design needs the following parts:
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- ATF16V8B, DIP20 programmable logic device.
- Standard SPTD switch, hole mountable.
- 100nF non-polarized capacitor, "104" marking.
- Wires.
- Soldering hardware.
- Drilling tool.
- A SMS2-RGB Sega Master System, (this version only).

Quick explanation of how it works:
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- Default Z80 clock is half of main quartz Clock (so: 5.3693Mhz).
- On /IORQ low level, the clock becomes the stock ~3.58Mhz clock, but
    sampled using the 5.3693Mhz clock, this way it avoids glitches when
    switching clock (which would cause the Z80 to fail due to transients
    that would clock beyond stable frequency range).
- After 11.92µS (64 cycles of the 5.3693Mhz) without any /IORQ, the clock
    goes back to 5.3693Mhz.

The period of time before the 64 cycles timeout where the Z80 runs at stock
~3.58Mhz is used to prevent accesses to I/O that would cause glitches in the
peripherals, eg: the VDP, which needs 7.26µS (26 cycles at ~3.58Mhz) between
writes to ensure proper VRAM updates.
   
Installation guide:
===================

- Flash the JEDEC fuses file ( SMS_OC_ATF16V8_Fuses.jed ) onto the ATF16V8B,
    eg. using a TL866II programmer.
- From the wiring diagram ( SMS_OC_SMS2-RGB_Wiring.jpg ):
    - Cut the trace using exacto knife or a medium sized drill.
    - Stick the ATF16V8B using 2-sided adhesive tape on top of the Z80.
    - Wire according to the diagram, let longer wires for the switch.
- Drill a hole into the casing to let the switch through and secure the switch
    into the hole.
- Put 2 layers of masking tape on the interior of the RF shielding around the
    Z80 to prevent short circuits with the ATF16V8B and wiring.


Results:
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From all the games I tried, slowdowns are gone, even in 60Hz, even on titles
like Sonic Chaos. Most games run flawlessly.
Sonic the Hedgehog has very minor sprite glitches, but no tile
corruption so far.
Galaxy Force still has tile corruption on the enemy ships.

Limitations:
============

- Do not change the switch position while a game is running.
- This modification only works on SMS2 consoles with a 10.7386Mhz crystal.
- 5.3693Mhz is high for a Z80 but it seems it doesn't make it glitch.
    (maybe because SMS2s are equipped with CMOS Z80s).
   
Have fun :)