Yet another Mega Drive mod thread, sorry but I think it's better to keep info separated for future reference. Anyway I'd really like to do this mod http://www.clubic.com/forum/jeux-vi...-avoir-une-version-ultime-id279308-page1.html The guy claims that this will over clock the system to 13Mhz without making the games go nuts and a few people have confirmed it. It's something to do with how one chip reads the tick from another in a sort of daisy chain so if you isolate some of them they won't follow the speed of the processor. I wanna try it but the guide is in french and doesn't translate too well. He's also done the 50/60hz and region mods at the same time, and in a different fashion to most people so I'm having trouble discerning what is what in his pictures and description. Does anyone have a better guide or can confirm the method? It seems like a fairly simple mod, just another 3 way on-off-on switch.
Looks like two switches - one to toggle frequency & the other to halt the cpu while you change frequency. The trace from CPU pin 15 (CLK) on the underside of the board is cut. The frequency switch is a toggle switch (SPDT) with the common connected to CPU pin 15, and the other two terminals connected to cartridge port pins B15 and B19. The halt switch is just an on/off (SPST) connected between GND and CPU pin 17 (/HALT). I'll give it a go later this week, once I get a game for my MD.
I've done a bit more reading on this now and it seems fairly straight forward. A lot of people are saying that the 13+MHz from the cart slot can be unstable and the best frequency is 12.5MHz but I'm having a hard time finding a none eBay supplier of oscillators with that freq. I might give this a go at the two "stock" clock rates but is the halt switch 100% necessary? I really have no intention of switching speeds during play...don't really see the point of an extra switch.
Yeah, i'd probably leave the halt switch too, the less switches the better. Theres a guide here: http://kyorune.com/modding/article.php?id=73 The author also mentions that the 13.4MHz can be unstable, and recommends 12MHz. Here's a 5v 12MHz oscillator on Farnell: http://uk.farnell.com/iqd-frequency-products/lf-x354h/crystal-oscillator-12mhz/dp/9712992 And if you want to push it, 16MHz: http://uk.farnell.com/iqd-frequency-products/lf-x355h/crystal-oscillator-16mhz/dp/9713000
I've been thinking of screwing around with this since stumbling across that kyorune guide a few days ago. Don't know if I can bothered to buy in a 12mHz crystal especially but tapping the cart connector for 13mHz sounds like a fun afternoon project.
I did this mod to a Model 1 MD a few years back when it first made the rounds on the internet. Tapping the 13mhz clock from the cart slot does work and you really do need that halt switch. Thing is games are extremely unstable and have some really interesting effects I've only seen when I ran a MD off a 9v battery. Around 7.7-8v there isn't enough power to run everything and for games like Vectorman you get to walk through walls and have your graphics corrupt all over the place. As an added bonus sound is more or less destroyed depending on the game. Sonic and Sonic 2 seem to be mostly unaffected but other games like Vectorman had the majority of their sound channels muted or otherwise corrupted in various fashions. The only game that comes to mind that played well was X-Men 2 with the intro scene with the snow. Silky smooth, but with the horrendous sound coming from the poor thing made the overclock 100% pointless. Now if a lesser overclock can be done that doesn't result in screwed up audio/video that would be totally awesome. FYI, if you have an XBox and I believe NeoGenesis it has an overclocking feature that performs pretty much identically to the actual hardware when overclocked.
Not that I'm for this at all, but if you're going to overclock, overclock it to MCLK / 6 = ~8.8 MHz = 14% faster. The clock divider can be created with a 74HC161 chip.
