CPU Upgrade/Overclock

[RobIvy64]

Hey guys,

I just attempted a CPU upgrade on my X68000 Super. I installed a 12 MHz mini-DIP Toshiba 68000 CPU in place of the Hitachi, and wired up a 4-pin metal oscillator also rated at 12 MHz. While technically an upgrade and not an overclock, the system did not boot. The CPU was tested good before install, and the oscillator was providing the proper frequency as tested by a counter.

I simply lifted pin #15 (CLK) and directly soldered the frequency from the oscillator. Does the DMA have to match the speed of the CPU?

Obviously I had to return mine back to stock which was simple since only 2 wires were soldered to the X68000 itself (+5, GND).

Has anyone else tried to upgrade their CPUs?

[eidis]

  Hi !

I compiled some info on X68000 overclocking. Check it out and pardon the excessive use of Engrish:

http://www.megaupload.com/?d=7UKEQK4V

A word of warning. I strongly believe that overclocking kills hardware.

Eidis

P.S. Please let us know if you succeeded with the mod.

[RobIvy64]

i'll check that out, thanks!

As mentioned, this is not overclocking, but a CPU upgrade. The new CPU I installed was rated at 12 MHz, and the oscillator was also 12.

[RobIvy64]

Hmmm, looks like you need to replace the oscillator on the motherboard. Since I don't want to modify the motherboard (I am a purist in that sense!), I think i'll keep chugging along at 10 MHz 

[eidis]

 I had the same idea and wanted to upgrade the CPU. I bought a 16mhz 68000 but there was a small drawback. Check out this massive piece:

http://foto2.inbox.lv/eidis/X68000/cpu.jpg

How do you think, if I made a DIP64 to Mini DIP64 adapter and plugged the CPU in my X68000, would it work ? If it did, would it work in 16Mhz or 10Mhz ? Thank you in advance.

Eidis

[RobIvy64]

Well, you would be better finding a mini-DIP 68000 CPU, although they can be hard to find.

If you plugged a 16 MHz 68000 without any modifications, the CPU would still operate at 10 MHz. What I attempted to do was lift the clock input pin (pin 15) on the CPU and feed it my own clock signal, in this case 12 MHz.

[lydux]

Hi  there !

This is an OHM related mod. This chip is responsible of the whole system synchronization (memory decoding, clock generation and synchronization, dram refresh, plus some other features).
OHM is connected to a 40Mhz oscillator (XTAL3), and divide it by 4 for obtaining the 10Mhz for cpu. The 40Mhz clock is also used as a counter for dram refresh.

RAM chips are MB81C4256 -10psz and according to the datasheet, will respond to a CAS access in a maximum of 25ns (1/0.000000025=40Mhz).

So you need to also overclock OHM, while maintaining a correct sync with dram.

I'm not sure (I haven't tried), but you could try to replace this oscillator with a 48Mhz one as 48/4 = 12Mhz. And OHM will be in sync with cpu.
But you could get a dram CAS access issue : 1/48Mhz = 21ns. Dram will be too slow then...

Have fun !

[eidis]

 Hi Lydux,

Is there a method which lets us use faster processors without overclocking other components ? What would you do if you wanted to replace 10mhz CPU with 16mhz ?

Eidis

[lydux]

Hi eidis,

No, this is an hardware design restriction that is even relevent today. Especially nowadays ! With higher and higher frequency hardware that need more and more precision in synchronization.

Basically, the cpu will request to read for a device, then will wait for an answer in a certain amount of unit time, and AT a certain amount of unit time (called a "cycle", the time between a logic 0 then a 1 provided by these oscillators). If the requested device doesn't respond in the given time OR respond but not when the cpu have to read for this acknowledge (in a middle of a cpu cycle), then the cpu can just forgive the request (switching to a timeout routine, considering the device is not available), or hang-up falling into an error state, or also wait forever ! (probably what occured in RobIvy64's case).
In an x68k, the M68K, OHM and DRAM really need a mutual respect about timing.

For your 16Mhz version of 68000, I'll personnally not spend my time on this...
A method could be based on the 48Mhz osc I talk earlier (if it works of course !), it will need to take the direct 48Mhz frequency output, and divide it by 3 keeping a 50% duty cycle. Could be a pain ! (will request 4 or more chip...)
Or else, a better aproach is keeping the 40Mhz oscillator, building a pcb for stacking on the 68000 socket (you'll need this anyway, as a 16mhz version doesn't exist in PDIP package to my knowledge). The board owns a 16mhz oscillator, the cpu and something for synchronizing with some signals coming from OHM (Let's say a cpld).
The trick is similar to building an Xellent30 clone ! So largely better interest in working directly on this one !

Hope this is clear enough...

Lydux

[RobIvy64]

Very interesting, thank you for the thorough reply.

So basically, I won't be doing this.

I don't want to overclock other components, and I don't want to overstep the limitations of the DRAM access.

I'm not too bothered by 10 MHz

[eidis]

Thank you Lydux for the thorough explanation. You convinced me that my X68000 is beautiful as it is.

Eidis

[amigo-mexicano]

I compiled some info on X68000 overclocking. Check it out and pardon the excessive use of Engrish:

http://www.megaupload.com/?d=7UKEQK4V

Sorry for the necro-posting. Eidis, could you please re-upload this? Since Megaupload is long gone. I'm looking for X68000 overclock info, and I wanna check these from you!

Thanks in advance.

[neko68k]

I compiled some info on X68000 overclocking. Check it out and pardon the excessive use of Engrish:

http://www.megaupload.com/?d=7UKEQK4V

Sorry for the necro-posting. Eidis, could you please re-upload this? Since Megaupload is long gone. I'm looking for X68000 overclock info, and I wanna check these from you!

Thanks in advance.

So the way it is before XVI you can replace the 40MHz MPU clock. The MPU and System clock derive from that OSC. The output is OSC_FREQ/4=MPU_FREQ. So 40/4=10, 50/4=12.5, 60/4=15, etc. There are two main issues that arise when you do this. One is that since the system clock is now up also the SCC is overclocked. At 15MHz it's 7.5MHz. At 10MHz it's 5MHz. This interferes with the mouse and MuTERM(without a patch). You can fix it by cutting the CLK pin on the SCC and attaching a 5MHz OSC to it directly. The other side effect is that the DMAC may or may not be stable >10MHz. I was told that 12.5 is typically stable. Cooling the DMAC may help as might overvolting the DMAC. There is also a 12MHz DMAC part but I haven't been able to find any to purchase. I run my PRO at 15MHz and it's not super stable.

I don't remember the divider for the XVI but it's possible to clock the system and the MPU separately. This is great because you can get a 24MHz MPU clock and a 12.5MHz DMAC clock. I'm not sure if the system clock impacts the SCC like on the older machines. I suspect it does and the remedy is likely the same.

Check these scans for details where to attach things. FYI you don't need that complex switching setup. Mine is a 74157, a resistor, an OSC, a bypass cap, and a switch.

Scans