> > Well, my holy quest for the Z800 has almost drawn to a close. > As you may (or more than likely, may not) be aware, I have for many > years now planned to customise the SAM by the replacement of the > weakest part of the system... the CPU.. :) > > [ nb 'A Z8108 internally clocked at 24Mhz linked to a 6Mhz bus'] > > After a strenous search of our universities' VAST library (how DO you > indicate a sarcastic tone of voice on an email message?... oh, never > mind..) I have finally found.. the 1983/84 Zilog Data book, with full > specs on the Z800 series. wow. I can see you're all impressed > already. > > It would appear that my earlier theory that its just a straight cut > and paste job, so to speak, was ever so slightly misguided. As I > suspected, the Z8108 is indeed fully pin compatible with the Z80 > (weeeelll... after you rearrange and demultiplex them, anyway.. )... > execept for one teeny problem. > > Now, if you would all care to turn to your diagrams of the Z80... > (pause for the rustle of papers), you will see it has a CLK input. > No surprise there. In fact, the surprise would come if... oh, never > mind... I'll get on with it.... To cut a long story short, as we are > all well aware the ASIC dishes out the Z80's clock signal on the SAM, > and thus the ASIC basically drives the bus (not in the literal sense > of the word, but it requires exact timing, hence it gives the CPU > clock). Now, if I were to place a Z800, which has a clock > output, and a direct crystal/oscillator input, into this system, > things get messy... especially since the Z800 requires double the > clock input frequency that the bus is meant to run at. So all I need now > is an accurate clock doubler that feeds from the ASIC into the two > crystal inputs of the Z800, and reference it against its own clock output > somehow so it gives its clock signals at exactly the same time the > ASIC would dish out the clock signals to the Z80...... > heeellllpppp! > >
Could you get the clock using a phase-locked loop (PLL)? Set the PLL to 12MHz, divide the output by two and compare the divide signal with the ASIC's 6MHz output. Actually, I don't even know if you need to worry about tying the Z800 (can you actually still buy them?) clock to the SAM clock - the ASIC is going to insert wait states anyway, and its going to tie those wait to its own 6MHz clock.
