- Joined: 14 Sep 2016
- Posts: 82
- Location: Vienne, France
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RGB SMS2 glitch-free overclocking
Posted: Fri Aug 14, 2020 3:43 pm
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Hello!
Last few weeks I tried to find a simple / elegant solution to overclock my French RGB SMS2 and I finally found a single-chip contraption that works and lets the Z80 at a quite high speed of 5.37Mhz when no IO access is needed.
It really makes a difference in terms of slowdown in games when the TV standard is 60Hz and is quite cheap and not very hard to build.
It only needs an ATF16V8B programmable logic device and off-the-shelf parts.
I wonder if there are other SMS consoles that would be compatible besides the French RGB SMS2...
In the mean time I can now enjoy my childhood games in 60Hz and with perfect timing :)
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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:
==================================
- 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:
========
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 :)
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- Joined: 14 Sep 2016
- Posts: 82
- Location: Vienne, France
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Posted: Sat Aug 15, 2020 3:04 am
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Picture of my own RGB SMS2 wiring.
I used a socketted ATF16V8B in case I had to upgrade the fuses but it's the same wiring (I hope :) as the diagram.
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- Joined: 27 Apr 2024
- Posts: 2
- Location: France
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Posted: Sat Apr 27, 2024 8:20 am
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Hi ! It's a shame no one replied to this, so I've created an account just to thank you !
I've done this mod yesterday and it's incredible, all the slowdowns are gone, Sonic 1 now plays like it always should have !
The only downside seems to be that in 60Hz, music are too fast with games that have their music optimized for PAL (Sonic Chaos and Land of Illusion for example), but for Sonic Chaos the tradeoff is well worth it !
A BIG THANK YOU !
PS : There is a small error on the wiring diagram : the 5V should be taken on Z80 pin 11, not 12
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- Joined: 14 Sep 2016
- Posts: 82
- Location: Vienne, France
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Posted: Sat Apr 27, 2024 2:23 pm
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Hey, happy to see someone using this mod :)
PS: I'll check, it's been a while since I worked on it!
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