Overclocking CPC @6/8 Mhz

[TotO]

You are speaking about heavy hacking just to enjoy a faster CPU. Nobody will sacrify his CPC for that and nobody will program for that.
If you need to change the RAM, better to use dual-ported chip... So, you don't get more conflict betwin the GA and the Z80 and can unconnect the WAIT pin.

That said, you could build a CPC-like system from scratch but it'd be a complete redesign.

Sure. And not a all-in-one FPGA...

[Sykobee (Briggsy)]

Yeah, a 16MHz Z80 core in an FPGA accessing SRAM that can handle 16MHz accesses sounds like the best plan.


Even better, if the clock speed is controllable via a port, so you can have RSXs like |16 and |4, with the default being 4MHz for compatibility.


But the limited number of these CPC++'s would limit the software created for the configuration.

[TFM]

On A500, this method (doubling the clock) allow a non sinificative gain. (around 15%)
Here, it's exactly the same... +50% speed allow 7% gain... Because, in both case, the RAM is not enough faster to take advantage on this hack.
You may have problem with the PPI too... Because, it will not work faster to drive properly the peripherals.

I'm not goint to change that in the Wiki page, because I don't want to have another war here. However my 6 MHz CPC runs definitivelz 50% more quick. But I did not use a BASIC program for it (which uses the software clock), but I did check out a couple of games, f.e. Starfox, Nebulus etc... it's a 50% gain!

[ZbyniuR]

Okay, you convinced me that overclocking creates so many problems that it is too difficult and not worth the effort. But what about the processor eZ80?

http://en.wikipedia.org/wiki/Zilog_eZ80  If it's compatible enough, it could accelerate the programs even without overclocking. Perhaps this is only hope for a significant acceleration of the classics. Maybe someone smart and brave to consider? Or try out in redundant 464 or Spectrum. If this works, you can gain fame and start promoting it.   RSX |16 and |4 sounds great.

I've heard about Turbo CPC FPGA such as C-One and T-Rex, or would they fit in a standard case 6128 or CPC+?

[TFM]

With the right 6128 keyboard (quick RAMs!) overclocking is as easy as adding a second crystal and some wires 

The eZ80 brings on problem, it's its IO, better use a Z280 (Actually there are cards replacing a Z80 with a Z280 and RAM).

But if you want a solution like the Super-CPU for the C64 then just make a new computer ;-)

You may also like to check out the CPCnG project.

[TotO]

With the right 6128 keyboard (quick RAMs!) overclocking is as easy as adding a second crystal and some wires 

Yes, some CPC get faster RAM chips.

[TFM]

Yes, I guess they were made around 1988, but I'm not sure. However it's written on them.

Also some hardware expansions run at 6 MHz and some don't.
RRB does not work, Eprom-card does ;-)

[ZbyniuR]

I guess you talk about LC version with small ASIC, palette 4096 without sprites (1988-89). I just looking for this version.

[TFM]

No, actually not the cost down model, but the cost down model has a good chance for quick RAM - on the other hand the GA may be not quick enough.

[Bryce]

The MegaFlash would also run quite happily on a 6 or 8MHz CPC.

Bryce.

[arnoldemu]

I guess you talk about LC version with small ASIC, palette 4096 without sprites (1988-89). I just looking for this version.

I don't think the cost down has a 4096 colour palette??

if it does, how is this accessed?

[TFM]

The MegaFlash would also run quite happily on a 6 or 8MHz CPC.

Yeah! And now I have something I REALLY can look forward to! Will enjoy it during my holidays in May 

I don't think the cost down has a 4096 colour palette??

That's correct. Of course you're right.

[Sykobee (Briggsy)]

It appears that if you can get hold of a C-One, then you can run a CPC core on its FPGA at up to 80MHz - from 2009: C-ONE Reconfigurable Computer


Looks like there is source code on the download page too. Might be useful for relferoo's project?

[ralferoo]

Looks like there is source code on the download page too. Might be useful for relferoo's project?

Well, I've already implemented everything myself from scratch apart from the Z80 core, so I don't really need the C-one core. I'm sure it could be fairly easily ported to my board though. I have some recollection of looking at it a couple of years ago, but I can't remember why I didn't like it now...

I'm currently using the opencores T80 softcore which claims to be able to be clocked at 100MHz, although I've never tried it that far. Certainly, when I synthesised it on the Atmel chip I was prototyping with before I built my own board it wouldn't synthesize above 16MHz but the Xilinx tools are happy letting me run it at 48MHz.

That said, there's a couple of bugs in the T80 implementation that I have mostly fixed in my source, but I still don't really like the way they've implemented the Z80 soft core, so I'm planning to rewrite my own Z80 core at some point too. There are certain things they've done that don't accurately reflect how the Z80 actually works, e.g. sampling WAIT on the clock rising edge not the falling edge that means I have to implement the gate array logic to work to a broken Z80 implementation. But mostly, I want the satisfaction of having independently recreated every aspect of the CPC...

[TFM]

The C-One has a Z80 core in which a lot of bugs are killed, which you usually find in the normal Z80 core.

[MacDeath]

What about overclocking a PLUS and its Asic?


(enjoy it explode...)

[TFM]

Haven't tried it, because the crystal it tightly linked to the PCB and I don't want to screw it up. However if may be worth trying to increase the frequency of the whole system by 25% (5 instead of 4 MHz Z80). That should stay inside the reality tolerance levels. But nobody knows how the ASIC will react...

[radu14m]

Did anybody try to overclock the cpc @5Mhz ? ( if 6Mhz creates problems )

[arnoldemu]

It would be good if the wait states could be removed for extra ram and ROM, these are not in contention with video hardware. If Tue z80 was clocked at 8mhz we could have a bit more throughput? 2 z80 clocks instead of 1. No change to system clock but separate clock for z80. Has anyone tried giving the z80 a different clock and keeping system one at normal?

[gerald]

It would be good if the wait states could be removed for extra ram and ROM, these are not in contention with video hardware. If Tue z80 was clocked at 8mhz we could have a bit more throughput? 2 z80 clocks instead of 1.

With a dedicated memory bus (for ram/rom), that should give us twice the throughput, but we end up with amiga like slow/fast mem issue when accessing the on board HW and video RAM.

No change to system clock but separate clock for z80. Has anyone tried giving the z80 a different clock and keeping system one at normal?

This cannot work because of the GA that deals with the DRAM address muxing. Only one RAM access is possible per µs. 8MHz Z80 could try 2.
The only way is to either overclock the GA, with all the associated problems (video), or a dedicated bus with an interface to access the slow HW.

[SyX]

@Gerald: And what do you think about using Dual Ported RAM for disabling completely the GA contention? Because GA and Z80 could access simultaneously to the RAM (GA = Read and Z80 = Read/Write).

[gerald]

@Gerald: And what do you think about using Dual Ported RAM for disabling completely the GA contention? Because GA and Z80 could access simultaneously to the RAM (GA = Read and Z80 = Read/Write).

That's a possibility. However, you still need to heavily modify the GA controlled datapath. Also dual ported RAM are not that common (at least for CPC hacking  ).
Then you hope that other HW can cop with the 8 MHz Z80 (PPI/FDC, extension)

But we are no more talking CPC, are we ?

[ZbyniuR]

Once my friend had the Amiga 500, with system clock frequency doubled but only for the CPU, motherboard and the rest work normally. With glued cooler on processor, and it worked well. This idea was described in Amiga Magazine.
As I know in CPC - 16MHz goes to GA and this generates a 4MHz for Z80 and 1MHz for AY, maybe something else I don't know. It may be possible to speed up the CPU only in the same way as in Amiga, just doubling frequency somewhere between GA and Z80? And if necessary, change the RAM and ROM into faster one. And I hope disc drive and picture on screen still will be fine.

[SyX]

But we are no more talking CPC, are we ?

Well, those are too many hacks for an old  CPC motherboard, but not for a new one, jejeje.

[TotO]

Once my friend had the Amiga 500, with system clock frequency doubled but only for the CPU, motherboard and the rest work normally. With glued cooler on processor, and it worked well. This idea was described in Amiga Magazine.

He can update the CPU for a 16MHz version to not have to cool it... But, in both cases the fact to increase the CPU clock and not provide it the adequate fast RAM, is just useless... Probably, he had increased it by 5% or 10% ... Not 50%... Wrong?