Electronics > Projects, Designs, and Technical Stuff
Z80 Single (Perf)Board Computer
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grumpydoc:

--- Quote from: obiwanjacobi on April 12, 2017, 07:34:06 am ---Looks good!

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Thanks :)


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--- Quote from: grumpydoc on April 12, 2017, 07:17:58 am ---I'm still in two minds whether to get a right angle header with longer pins and filp the CF car 180o so that it sits over the PIA & SIO which would be neater.

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I would first get it working. The way it is now gives you access to the chips for debugging/probing.
Just try to target each pin with only one wire (on the bottom side I gathered) and it'll be easy enough to replace later on.

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I've more or less decided to flip it around, it looks much uglier than I expected with the CF card sticking out from the board, and it fits very nicely the other way around. If needed I can always put the CF card on a short length of 40-way ribbon cable - the signals are buffered so that shouldn't fry the CPU data & address lines with too much capacitance.

I have to put this aside for a couple of weeks over Easter but I can use the time to order some more headers.
nockieboy:
I know it's been more than a year grumpy, but are there any updates on your project at all?

I've been pretty quiet on EEVblog recently, but that's mostly because my hardware is all working nicely now and I've been learning Z80 assembler and writing BIOS's and such for CP/M 3 on my system, alongside listening to old classics playing on the AY-3-8910 PSG, which pumps out chiptunes quite nicely now!  ;D
grumpydoc:

--- Quote from: nockieboy on May 12, 2018, 08:49:50 pm ---I know it's been more than a year grumpy, but are there any updates on your project at all?

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It's stalled, as has just about everything electronics-wise for lack of time.

It's still on the bench though, reminding me I need to get on with making some progress every time I look at it :(
kde:
Hi! I see there were no new posts within more than a year here. grumpydoc, how is your project going? Were you able to achieve 20 MHz with Z80?

I was a big fan of Z80 in the nineties, so I had constructed a few different devices with it. I've checked your original design and find out the following:

- Why do you check the MREQ line twice to access memory? You check it on the memory CS line and on the MRD/MWR lines. You just need to use it for RAM/ROM CS line only and directly connect CPU RD to the memory OE line and CPU WR to the memory WE line.

- If you look the AS7C4096A datasheet (e.g. the 10-ns version), you'll see that 10 ns delay is the address-to-output delay, i.e. it is the time that passes from the moment when you change the address line until the moment when data are available on chip's outputs. And the OE access time is just one half of that, i.e. 5 ns! And this is true for all other static RAM/ROM chips I know. E.g. the old good 200 ns 2764 has 200 ns address-to-output or CS-to-output delay and only 75 ns OE-to-output delay. And if you look the Z80 datasheet, you'll see that address lines are set a little bit before MREQ and RD. So thus you get a few extra ns for your memory to be read. This is also true for your SST39SF010A-45 - the timings are 45 and 30 ns correspondingly. But you may consider adjusting your schematics so you have ROM CS always low (active) and just add one more logic gate to its OE pin. Or you may connect your ROMEN pin directly to ROM CS and do not use it anywhere else.

- You do not need to provide the data to the bus 12 ns before the T3 rising clock edge - Z80 has internal delays, so it's OK to provide the data right at the rising edge. Thus the 45-ns ROM should be working fine with 20 MHz Z80. A long time ago I tried to overclock Z80 to understand how fast can I make it go. I had 6 MHz original Zylog Z80B with an ordinary 2764 ROM chip (I do not remember now which version I had, probably it was the 200 ns one) on my test board and I was able to clock it from 11 or 12 MHz! And there was no any special action taken (like the CS trick I described above) to speed it up. So I'm pretty sure that 45-ns ROM will work OK with 20 MHz Z80 with no additional wait states.

Also if you think your ROM is still not fast enough, you can try using the M1 line which is asserted a little bit earlier.
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