Im working on creating a simple hardware modification to enable over 300mhz sh4 overclock, Now its relatively simple right now but heres my concept. 1) tap into the holly's 2.4V power line (the test point below C202 on back) to the sh4's 1.8V power line (next to IC104 top side test point on C165). Yes the SH4 can handle up to 2.5V VDD internal power. 2) lift 33.3Mhz sh4 clock leg on clockgen chip, route R337 Test point (holly's 54mhz clk line) to test point R331 (sh4 clk line testpoint) NOTE: from this point on you have a 62% speed increase, that % is important to keep in mind when trying to clock the rest of the hardware together in sync. 3) Overclock vmu with faster crystal (62% faster ^^) pretty much the rest of the clocking on the system should match the first thing u overclocked by the same % increase. and then see what happens ^^, ill give this a whirl in a bit. Also to match the clock sync on the holly the input clk should be ~88Mhz oscilator sh4 ~54mhz vmu ~12mhz crystal Some notes: stock 199.8Mhz @ 1.8V oc 324Mhz @ 2.4V http://yam.20to4.net/dreamcast/va1.html PS: obviously the cooling system is going to need to be upgraded.
Assuming everything works and the cooling is working fine, some games will be smoother, some games will be fast-forwarded. It depends on how the timing is done; clock counter or output frame counter I think. IIRC, Shenmue 2 will be smoother, Quake will be 62% faster. If this works and is well documented I might (in the far future) try to sacrifice a DC to test the extreme overclock; I got access to liquid nitrogen and rf sources.
I don't think you should overclock the pvr though, as it'd increase the output framerate, and break compatibility with most monitors I'd guess.
ye i didnt think of that lol, im more concerned with the entire system being able to take this overclock and still function at all, not like breaking or w/e. just stable enough to operate alternative operating systems, with some punch lol. i myself dont play games rly on any of my systems. just like to mess with the "product(s)" of Corporations ^^
http://pdf1.alldatasheet.com/datasheet-pdf/view/586059/SHARP/PQ07VZ5M2Z.html R133 = 1.2Kohm R134 = 2Kohm both those resistors control the sharp mosfet (datasheet link) output power to SH4 Vcore. so with R134 @ 1K and R133 @ ~800ohm should be close enough to 2.4v i couldnt find the datasheet for the actual sharp part used on the va1 :/
I've overclocked a few Dreamcasts. The biggest problem I've always had is games crashing. I've only ever gone to 246 Mhz and most games run lovely (Powerstone 2 when playing 4 player has literally NO slowdown at all and runs a lot smoother). Half Life ran loads smoother too but used to always crash after 10 minutes or so - I don't think it was a heat problem as I beefed up the cooling with 2 big heat sinks and 2 extra fans. Also another problem is saving games - I only ever found 1 memory card (non screen type) that would work. (if you use a normal vmu with normal data on when playing an overclocked DC it scrambles the whole vmu and you'll lose all the data on there)
most of the vmu issues are because the clock was not scaled to match the system. the maple bus speed increases and the stock vmu has no clue, hence swaping the crystal in the vmu for proper functionality but spinksy, u have some pictures of your cooling setup? could give me some idea.s ^^
Yeah I did buy a few 7Mhz crystals to overclock some vmu's but never got round to it for some reason, I just always stuck to my screenless memory card that worked ok. I'll have to try and dig out some old pictures (or open the console and get some new ones) I don't really use that Dreamcast much anymore to be honest - all i did was put some thermal paste on the 2 chips and then hot glued 2 heatsinks on top - both had a small fan attached to the heat sink so was a little over kill but better to be safe than sorry.
iirc, the stock vmu crystal is 7.3XX or somthing like that. for ur overclock maybe 8 mhz or even possibly 10?
324Mhz boots first to blank blk screen, u can hear the gdrom init, then few seconds after the what would be the boot animation, the screen displays the post animation boot logo and hangs. with 2.4v it still seems using the resources on the dreamcast itself cannot provide a stable environment to overclock. ill go ahead and order up some 42 or 46 Mhz oscilators. stock 200Mhz clock seems fine with this voltage jumper
I have a spare pcb from a design I made based on Ozones ATA G1 board (minus the ATA port). It has space for 2 oscillators and a 3v line for them. This might be useful for testing different oscillators. I have intended for one to be an overclock oscillator and the other to allow the GDrom to be removed (providing 33mhz to the HOLLY). Where u based? Let me know if u want it.
you must keep in mind - SH4 is not CPU, but MCU with many other things inside, including Timer Units (TMU), which clock is derived from main clock. so then its speed will significantly differ from game expetations (relative to other events like "end of render", "end of TA list" or Maple DMA IRQs - game code may (and will) go crazy.
I bought two heatsinks with fans to do my overclock. was going to start with 240 and edge up. Also have a pico. I was worried the extra draw might be too much for the factory powersupply plus it would remove some heat. If it works right I'm going to put my usb setup in it and then throw it into my kiosk.
Any chance we could have a binary off/on for the overclock? Having Shenmue II and PowerStone 2 running w/o much slowdown would be nice but scrambling VMUs is not.
Why engineer a proper clock generator PLL ? and make it dynamically clockable in real time ? Toggle a button for boost/normal, and allow in-game saving to not horribly crash due to de-sync'd clocks.
