High Speed Z80 test Board

Use this forum to describe how you are using, or planning to use, the parts kits generously donated to a number of members by Lez Anderson
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Martin A
Posts: 417
Joined: 09 Nov 2013 21:03

High Speed Z80 test Board

Post by Martin A » 09 Nov 2014 18:47

As part of the MTX+ project Dave and I are building CPU boards that mimic to a certain extent the original MTX, that includes the fairly complex memory paging arrangement.

As a side project, I've built a much simpler CPU board with the intention of seeing just how fast I can get the CPU to go.

In order to use the minimum possible logic, I am using 32k rom and 32k ram. Unlike it's 32k and 512k cousins, a 128k static ram chip has 2 chip selects. The usual active low CE1 and a second active high CE2 located to allow a 32k chip to occupy the same socket.

So, I have A15 from the Z80 connected to the Chip enable on the Flash and to CE2 on the Ram, CE1 on the ram is tied low, as are A15 and A16 on both chips.

With A0-A14 and the data lines connected to both chips, this pages in the rom in the lower 32k and the ram in the upper 32k without needing any decode logic.

The only logic on the board apart from the clock is a single 74F32 which is a quad 2 input OR gate. I use 2 of the 4 gates to combine the Z80's memory request line with the Read and Write, to create Memory read and memory write signals to feed the read and write inputs on the ram and the read on the rom.

The F series chip is used for speed. It's propagation time is in the region of 4ns, somewhat better than the usual HC or LS logic.

I started using a 20Mhz Z80 CPU supplied by Lez and a 4 speed clock from a 40mhz oscillator driving a 74HC193, which gave me choices of 2.5,5,10 or 20mhz.

After testing Lez's chip, I put in a 10mhz CMOS Z80 (from Mouser) just to see how that would respond. Surprisingly, it ran faultlessly at 20Mhz.

After an hour running the test code without any apparent problems, and the chip remaining at room temperature I decided to push it further.

It wasn't able to push the HC193 any faster, so I built a crystal clock based on the normal resonant circuit, with a spare 21.4Mhz Crystal from the V9958, the 10Mhz Z80 coped with that too.

Finally I replaced the crystal with a 27mhz version, and replaced the flash chip with fastest I have available, and tested again.

Net result I have a 10mhz Z80, running at 27Mhz, and no sign of any over heating or other problems....

In order to remain within spec as much as possible, the ram is rated at 55ns, the Flash at 45ns. I don't think I can push it any further without needing new ram, I'd need faster crystals too, 27Mhz is currently the quickest I have.

It's not MTX compatible, as there is no provision for rom or ram paging, nor will it run CPM.

However it is capable of driving the V9958 board from the MTX+ project and hopefully the other I/O boards once we get that far.

And of course it's FAST !
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1024MAK
Posts: 569
Joined: 24 Dec 2012 03:01
Location: Looking forward to summer, in Somerset, UK

Re: High Speed Z80 test Board

Post by 1024MAK » 09 Nov 2014 20:56

Excellent :D

You are not the only person to over clock Z80 CPU's. There are a number of other reports about this...

Mark

Tony Brewer
Posts: 62
Joined: 08 Jan 2014 20:50

Re: High Speed Z80 test Board

Post by Tony Brewer » 09 Nov 2014 22:22

Possible to connect Z80 signals direct to RAM:
/CE1 = /MREQ, CE2 = A15, /OE= /RD, /WE = /WR

Could do similar with 'ROM':
/CE = A15, /OE = /RD
if not bothered about I/O reads at all, or any input ports => 32768

74F32 would then not be needed.

Martin A
Posts: 417
Joined: 09 Nov 2013 21:03

Re: High Speed Z80 test Board

Post by Martin A » 10 Nov 2014 22:28

Hmm, the RAM mod might help squeeze a little more speed out of the current board.

The flash chip holding the ROM is the faster of the two memory devices, so keeping the F32 on the ROM wouldn't be problem.

The VDP is, and eventually a keyboard and maybe some storage will be, connected via the I/O ports so I'll need I/O read as well as write.

Depending on the relative timing, maybe an OR of A15 and MREQ connected the Flash Chip select, and connecting read direct to OE might allow a little more leeway than the current setup?

I wonder just how fast the CMOS Z80 can be pushed, if it does 27, maybe it can do 30 or 33??

I've done some searching of the net, ignoring the spoof articles claiming 1ghz etc, the fastest overclock I'd been able to read up on was 24.

The speed of the Flash might be the limiting factor, 45ns was the best I could find that was 5v friendly. 5v Cache ram is still available so I could maybe get the ram access time down to 20ns instead of the 55ns parts currently in use.

Martin A
Posts: 417
Joined: 09 Nov 2013 21:03

Re: High Speed Z80 test Board

Post by Martin A » 15 Nov 2014 12:49

Update

I've now got the 10Mhz Z80 clocked at 32Mhz, which is pretty much the limit for the flash chip holding the ROM code.

I tried 40Mhz, but got no output from the display board. Given that that's too fast for both the ROM and the RAM, I've not investigated further.

Still a Z80 running at 8 times the speed of the one in the MTX is still pretty quick....

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1024MAK
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Location: Looking forward to summer, in Somerset, UK

Re: High Speed Z80 test Board

Post by 1024MAK » 15 Nov 2014 13:17

ImageImage

Mark

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