MegaDrive/Genesis Overclocking Topic

hehehe i saw you on techtv hehehehe kevin knows this forum :S

 

Megapost!

Wow, that is a lot of replies. I'll do this all in one big post then!

1. LEGITIMACY / ORIGINS:
No, I did not lift any work from any publications or documented works, save for well known pinouts. All tasks completed and all research done on this project was original and independent. I only learned of its existance about a month ago, in fact.

2. GAME SPEED:
The games will not run faster than they are intended to in 99.9% of the time. Most games are coded to time the work the system does based on the VBLANK, an interrupt sent to the CPU 60 times, or 50 times a second (60/50hz depending on the region you are operating the machine in.) Since changing CPU clock does not alter the Vertical Syncrate, the system stays at its proper speed. Slowdown is eliminated, as what causes it is the rest of the system waiting on the 68000 to finish its work, and consequently drawing the same frame over and over again.

The only notable exceptions to this rule are rushed coding jobs wherein the gameplay IS dependent on strict VDP/68000 timing. The best example I can find is the Special Stages in Sonic the Hedgehog 2. As CPU clock increases, game speed increases. This is welcome to me, though; I find the levels too slow ..

3. STABILITY AND HEAT CONCERNS:
The 68000, at 7.6 MHz, operates at an approximate surface temperature of 70 Degrees Fahrenheit. Overclocked to 13.4 MHz, the temperature at heavy load is approximately 75 Degrees Fahrenheit. At 19.6 MHz, the temperature averages closer to 79-80 Degrees Fahrenheit.

As for stability, most games run fine at any speed up to 13.4 MHz. At 13.4 MHz, (presumable) system load on the clock generator causes sound sync issues (occasional failure for sound effects to play or music to change between levels.) Above 13.4 MHz, random palette switching, VRAM corruption and crashes often occur, presumably due to RAM/VRAM refresh rating being exceeded.

4. PROJECT COMPLEXITY:
While the basic concept is just 'attach a crystal', it's a bit more complex in actuality. In addition to the crystal, the driving hardware must also be present, so a self-contained crystal oscillator is required to use any speed besides the 13.4 MHz signal found on the cartridge slot. This clock needs to be coherent, carried by a short, thin, preferably copper wire without many bends, far from sources of EMI like chips, other wires, et cetera.

In addition to clock feed, a method to change CPU clock while the machine was running was needed, as large problems would occur when the system was 'cold booted' at a clock speed above the 7.6 MHz spec, often caused by the BIOS itself crashing. To do so, I tested a theory that halting the CPU would make it indifferent to a rapid clock switch. This theory was correct (yay).

5. CPU REPLACEMENT :
The 68000 is only pinout and instruction set compatible with one other chip- the 68010 from Motorola (and clones). This chip is often swapped in Amiga/Macintosh computers, as it is capable of getting more work done PER clock tick, and has an instruction cache to store needed data prior to its use. Installation of the 68010 would likely cause a performance increase equal to or nearly equal to the raising of CPU clock to 12.0 MHz. However, this process entails desoldering and resoldering 64 pins and is not guaranteed to work, as it is untested on this system. Its probability of success, however, is very high. I lack the mechanical skills to attempt this myself.

Replacement of the 68000 with the 68020, 68030, 68040, et cetera is not possible due to incompatible pinouts and instruction sets, as well as differing form factor.

6. MODIFICATION OF THE MEGADRIVE/GENESIS MODEL 2 AND 3
The MegaDrive/Genesis Model 2 can be overclocked, with much greater difficulty and lesser compatibility. The Model 2's CPU requires that Pin 19 be used for halting and not 17. Determining the proper Clock pin from the back of the board is a challenge, and lifting it is hazardous to the CPU (it is very easy to break the clock pin when attempting to lift it.) The Model 2 board is coated with a solder-resistant layer that makes work on the PCB very difficult.

I have tested the Model 2 with this procedure and have found it does function. Others have reported the machine is not very stable and has a tendency to crash very often.

The Genesis 3 from Majesco does not appear to be overclockable. 95% of the ICs in the system are integrated into one large chip. Since this chip contains the clock divisors, and CPU hardware, the CPU's clock feed cannot be isolated without clean-room microprocessor-internals level tools. The clock oscillator IS external, but since it provides the 53 MHz clock which is divided to ALL components, raising it would almost certainly break stability for the other parts, and would CERTAINLY raise the Vertical Syncrate, making the system useless on a video display designed for 50/60 Hz.

7. FUTURE PROJECTS:
I am building a guide on NES overclocking and it should be ready very soon, once I can afford one particularly expensive testing part (which fortunately will not be needed by the end user.)


ACTUAL PROCEDURE
--------------------------

This is a quick rundown as the entire guide would be excessive to post here. It is a guide for the MODEL 1 MEGADRIVE/GENESIS. The model 2 is the same with different pin locations (and pin 19 is halt, not 17.) Stability may vary. All modification is done at your own risk, I will not be held responsible for any damages incurred by use of this information:

#1. Open the system by removing the 6 screws on the bottom of the unit.

#2. Lift off the lid, take care not to break the 2 wires connecting the lid's LED to the board.

#3. Check the CPU type. It is the large black rectangular chip between the cartridge slot and the back ports (video, power.) It should read MC68000, SCN68000 or HD68HC000. If it reads SCN68000 and/or has a large S logo, it CAN NOT BE OVERCLOCKED, according to 2 machines of this type I have tested. Most of these cannot even halt properly.

#4. If your system had an MC68000 (Motorola) or an HD68HC000 (Hitachi) type CPU, you're all set. Take off the RF shielding by removing all its screws.

#5. Take out the board, it has about 5 screws holding it in. Don't unscrew the Regulator Heatsink (huge piece of metal) from the board. It's an important ground point and may ruin stability if it comes loose from the ground trace on the board.

#6. Find the trace coming out of Pin 15 on the 68000. Cut it CAREFULLY with a sharp tool like an X-Acto knife. Do not cut any other traces and do not go through the back of the board under any circumstances! Once the system cannot boot, continue to step #7..

#7. Find pins B15 and B19 on the cartridge slot. Pin B32 is on the end of a row, nearest the controller ports and the Mega CD / Sega CD expansion connector. B1, B32, A1, A32 are usually labelled on the front of the board. Solder a wire to pin B15 and B19. Do not connect these wires to anything yet.

#8. Solder a wire to Pin 15 on the CPU and Pin 17 on the CPU.. do not connect these to anything yet.

#9. Make Pin 15 connect to the common ground on an ON-ON switch with 3 terminals. Pins B19 and B15 connect to the 2 switched terminals on either side of the ON-ON switch.

#10. Connect Pin 17 on the CPU to an ON/OFF switch, either terminal. The other terminal connects to ground, i.e. the metal edge of the board. Which terminal connects to ground and which to the Halt pin does not matter.

#11. Switch on the system at 7.6 MHz. Halt the CPU by flipping ON the Halt Switch (pin 17/ground). Change to 13.4 MHz with the other switch. Unhalt, and you should be running.

OTHER SPEEDS:

Buy 4-pin Full Size or Half-Size metal can oscillators.. I get them from Digi-Key. Get them in the speed you wish the CPU to run at, i.e. 12.000 MHz (best frequency to use in my experience.) These have 4 pins. The * is Pin 1. Orient it like below:

Pin 4 Pin 3

* PIN 1 Pin 2

Pin 1 connects to NOTHING. Pin 2 connects to Ground. Pin 3 connects to the CPU's 15th pin, or where 13.4 Mhz (pin B15) goes on your switch. Pin 4 goes to +5VDC (I get this from the fourth pin on the MegaDrive's existing oscillator.) This should generate a usable clock. Keep wires short, use THIN copper wire. To disable the oscillator, if for some reason you need to, any oscillator with the specification 'output enable' can be turned off by grounding Pin 1.

There you have it! Please post any other questions .. I'm more than happy to answer them.

 

Think I brought this up a few times but havent got a definitive answer yet:



Thats a picture of one of my model 1 PAL Megadrives (modded by yours truly), which came with a resistor on one of the CPUs legs :smt017
My other MD1's are tucked away so I cant check them, but if I recall correctly this resistor (well I think its a resistor, its got color rings on it) isnt in all of them. You seem like the right person to ask :-D

Moreover, 680x0's are supposed to be backwards compatible with the 68000 on the instruction level at least I'm pretty sure...

 

So does overclocking the genesis make a noticeable difference in slowdown without blowing it up?

 

Yes, it does.

This is only on some revisions of the system. Whether you have this resistor or not is inconsequential in the sense of modifying the machine,

 

Ah, thanks for the info guys :smt023
I knew it hadnt anything to do with OCing the machine but I was just wondering for the sheer geekiness of it all :smt043

But what puzzles me is why you would place a resistor on something like the address strobe... isnt that a TTL or whats-it-called signal that only has 0v and +5v? Wouldnt make sense to me to stick a rezzie on unless there's something wrong electrically somewhere down the line... (I know assembler already said something along those lines but its the abovementioned geekiness again making me want to get to the bottom of this)

 

I've seen that resistor on MANY MegaDrive model 1's, but it's usually attached from the back of the board.

 

I agree... resistor is actually on the last revision PAL units, so its not an early "temp fix" per se (well maybe, and then they just moved it to the back)

From what I remember, the overclock has to be locked to one frequency, right? How about having a multi-switch, for a gradual step up? I guess more to demonstrate the differences easily than anything else.

 

The system can tolerate multiple frequencies on a switch, as long as it's not deprived of a clock for too long, it will not crash. I am, in fact, working on such a setup right now. Once I can afford a part I will be setting up my MegaDrive to pick a clock speed anywhere from 4 to 30 MHz using a pretty versatile control board. I will record video footage of it in operation when set up.

 

Another noob question. Where/How can I do the multi-region switch mod for a Model 2 Genesis??

Also, for that OCing procedure. Can I solder the wires to underside of the Cartridge slot pinouts instead of soldering them on the top side?

 

You can solder from the back, in fact, it's pretty much the only way to go unless you want to use wire hooks to grab the pins under the slot. Soldering to the tops of the pins would make cartridges unable to fit. So yeah, solder from the back.

As for a region switch, just search google for MegaDrive/Genesis Import Modification .. there are tons of pages documenting this.

 

Epic Gaming is back online!
http://www.epicgaming.uk.ro
in the meantime.

Epicgaming.net will point to that worldwide in 24 hours, once all the DNS servers have properly refreshed to this new URL.

Thanks for the patience, people!

 

back

Everything is finally back to normal. The new server is located in Europe, but speeds seem really good even here in the 'States.

http://www.epicgaming.net

.. now points to the right location. I just hope my new host doesn't go nazi like the old one did. I guess they didn't figure I'd actually USE the 'unlimited' bandwidth they offered me ..

 

sorry for reviving such an old thread, but i cant seem to find the answer to this anywhere.

i'm trying to overclock a genesis model 2 with a mc68h000fn8 cpu.do i go by how the pin numbers on the board because pin 1 is in the middle and theres no trace connected to pin 15 following the labels on the board.

 

This the problem with pruning inactive posters. Yes it gets rid of inactive members but then in instances like this where links in the original post are dead and the member has been pruned you cant get their email (if they had it set to show) and you cant send them a message either. You could say google his name but 'Epicenter' Is a real word and would be impossible to find online probably. Something like Cyantist is less used (Im on the first page of results twice)

 

yea its frustrating, not even a cached version of the site works

 

These work;

http://web.archive.org/web/20050209141413/http://www.epicgaming.uk.ro/
http://web.archive.org/web/20050205174434/http://www.epicgaming.uk.ro/md_oc/
http://web.archive.org/web/20050205192419/http://www.epicgaming.uk.ro/staff.php