Overclocking CPC @6/8 Mhz

[TFM]

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

I have one CPC6128 in which I replaced the Z80A with a Z80H (had no Z80B at hand), and I overclock the whole system witha 24 MHz crystal instead of a 16 MHz crystal. In my hands it works like a charm. But some expansion hardware is to slow to work with it.

[radu14m]

it"s possible to use a 20Mhz crystal, right ? to get 5Mhz

[TotO]

As it was already explain on the first page, changing the main clock as a side effect on the sound and the fdc.
The result is a CPC that no more work properly.

[TFM]

it"s possible to use a 20Mhz crystal, right ? to get 5Mhz

Yes.

As it was already explain on the first page, changing the main clock as a side effect on the sound and the fdc.The result is a CPC that no more work properly.

Well, it works properly for me. The picture on the monitor has a higher resolution (using 32 KB fits on screen w/o a problem). The soundchip is moved to a higher region, more like Atari ST (half the way there). And the FDC can write more on discs (50%). All that side effects are very welcome to me. 

[TotO]

Well, it works properly for me. The picture on the monitor has a higher resolution (using 32 KB fits on screen w/o a problem). The soundchip is moved to a higher region, more like Atari ST (half the way there). And the FDC can write more on discs (50%). All that side effects are very welcome to me. 

Sure, I know that. But, in reality, that made the computer no really running faster. (something like 5% instead of the 50% expected)
The PIO should fail because it is not intended to run 50% faster.
The Audio sound "halftone" and original floppies can't more be read. (and 50% more disc need a custom format)

[TFM]

Well, if your PIO fails then the whole system does run not much more quick.

I did a benchmark and in my case everything worked well, it is 50% faster, tested with some simple BASIC programs and stopped time by hand.

It probably depends which keyboard you are using. Some seem to work at 6 MHz, others don't.

[radu14m]

@TFM:the used cpc6128 was one of the latest model (Cost Down Version) ?
What memory chips & speed ( ns ) are build in ?

[TFM]

It was a 1987 CPC with CRTC1. The RAMs are usually fine (regarding speed). IMHO the GA is the one that makes problems first. But if you try 5 MHz everything is fine.

[MacDeath]

So.
would need heavier mods to get proper display resolutions and this would then need more RAM used as VRAM then... so the 64K or main RAM would be quite fastly used as VRAM.

on the other hand a properly X2 overclocked CPC would basically become an 8bit Atari ST.
Minus the megaRAM management and the 16/32 CPU and the MIDI port and the 512 palette... also what about mode2 ? can it be turned into 640x400x2 ?

Depend if you want a full overclock on every thing, or jsut the CPU running at 8mhz (or 6) but the Video staying at same speed...
if gates array handles both from the original clocking, you can't separate unless you redesign the GA.

Also as some video stuffs are to be synched with the CRTC... (50hz cathodic stuff) there are signal that must not really be overclocked.



BTW... 6mhz and 192x256x16 ? is that a SAM coupé ? 
Anyway wouldn't be the same machine, and wouldn't be really compatible unless you want all display divided by 2 on screen...

Not even sure "fullscreen" could be achieved... if the video if also overclocked (=384x256x16 per exemple)
would need to be able to run 48k of RAM for one screen.

hell a simple normal 320x200x16 creen would be actual "fullscreen" in 32K...


Will not really work on a 464 lol.


in order to do it properly you'll have to replace :
= crystals
=RAMs
=CPU
=GateArray

=Also not sure the AY used can be run at 2mhz (think they could)

Not sure about the CRTC though...those were also used with EGA or VGA cards (may not be the same models though) if I remember well, so may perhaps handle some 320x200x16, 640x200x4 or even 160x200x256...

as a result you would have to design a pin compatible Gate array/Asic that would be somewhat 100% compatible yet with a few tweaks : some clockings would be doubled, others wouldn't... and a few stuff would be redone in order to handle some extra monstruosity.

I mean we all agree a 1280x200x2 mode wouldn't work...


otherwise, such souped up CPC on cocaïne would still have the same CPU to Video ratio... wouldn't be faster, jsut display more colours.
If you keep the existing classic CPC video modes, then you may have a thing quite faster but it would still be slowed down by the video interruption when "VRAM" is read.

As a result such device should be able to handle both overclocked video modes and classic video modes.

[arnoldemu]

The aleste 520ex has two speeds. One where 16Mhz is master clock. This is the one with the CPC's modes.

But you can switch to 13Mhz master clock, in this mode the pixels are wider I think, but this is the mode with the MSX modes.

I don't know how the CPU suffers, it should run slower in 13Mhz mode, but it may be delayed less. I have not looked into the possibilities.

What I am saying here is that the aleste can give a resolution with more colours I think (e.g. 320x200 in 16 colours). So faster speed may not always mean higher resolution.

[MacDeath]

CPC and MSX (or spoeccy) wouldn't produce pixels on the same size nor same display on the same monitor.

I like to say that the CPC can manage to put 320x200 where Speccy can only put 256x192... (need to verify betterly)


So the "video" clock difference and pixel ratio difference on the Aleste may come from the need to stick to original machines display.

When you get a 256x192 on a CPC, you feel like having only a post-stamp displayed despite the exact same resolution.

A but like the 256x192x16 on MSX2 or 192x256x16 on CPC in mode0 ful screen... same resolution but not the same border, not the same pixel ratio, not the same size on the screen, and so on..

[arnoldemu]

I haven't proved it yet but the differenc in pixel clock says the spectrum has almost square pixels. Cpcs mode 1 has thinner pixels, so yes bigger border on CPC. Aleste Msx resolution probably has wider pixels than spectrum.

[radu14m]

I swapp yesterday the 16Mhz with an 24Mhz oscilator on my cpc.
Unfortunately i did not get a stable image  [size=78%] ( even tested on 2 TV's )[/size]
did i miss something ?

[gerald]

did i miss something ?

Yes  .
You now need to update the ROM so the CRTC is programmed according to its new frequency (1.5MHz istead of 1MHz) and generate a 15kHz/50Hz syncs.
As a side note, your floppies can't be read anymore (unless you make them spin at 450rpm  )

[radu14m]

But TFM did not update the ROM....it worked directly.

[gerald]

But TFM did not update the ROM....it worked directly.

He may not have updated the ROM, but you still need to adjust the CRTC timing.
Check there : 6 MHz CPC - CPCWiki . You will have to adjust the value to your clock (20MHz instead of 24MHz)

[radu14m]

is the updated ROM available here somwhere ?

[TFM]

Yes  .
You now need to update the ROM so the CRTC is programmed according to its new frequency (1.5MHz istead of 1MHz) and generate a 15kHz/50Hz syncs.
As a side note, your floppies can't be read anymore (unless you make them spin at 450rpm  )

No, the floppy works fine. Of course you can't read old discs. But where is the problem. Data transfer can be done using a RAM disc. The big gain is that you can use a format which gives you nearly 0.3 MB on a 3" disc. It worked in my hands and the floppy is a regular 3" floppy.


I didn't alter the ROM. Just typed in a couple of OUT instructions under BASIC 

[radu14m]

Can you post here the BASIC OUT instructions that you type in ?



Thanks !

[TFM]

Don't remember the value I used back the day. But it's enough to increase the number of total characters per line by 50%

[radu14m]

understand but how ?

[TFM]

Check out the CRTC page in the Wiki. Then...

CRTC - CPCWiki


Try this in BASIC

OUT &BC00,0        selects CRTC Register 0 (horizontal total)

OUT &BD00,95      sets number of total CRTC characters to 96 (95+1)


That should work IIRC. You can further adjust the horizontal positioning.

[Neil79]

A brand new addon with overclocking cabilities and speedy ram, very nice

[radu14m]

it's working TFM !

I manage to overclock the cpc to 6Mhz, but unfortunately the hxc is not working anymore

the same situation on 5Mhz. ( OUT &BD00,77 )

do i have to change something else too ?

[TFM]

Well, first! Awesome that it works!!!


Second, the FDC is (at 6 MHz) working 50% more quick, so this is out of tolerance for the HxC probably. On the other hand you could try to use a HD format (just an idea, I don't expect it to work).


You should be ok with 3" or 3.5" disc. Now how to get data to them? I would suggest to use a RAM disc. Back the day I used the !COPY command of the X-DDOS ROM and the RDOS which provides a RAM disc C:
The X-DDOS also allows to format discs. And you need to format discs for 6 MHz.


You can try this: Try to format a .HFE file on the HxC, with some luck this could work.