Re: Source for 20Mhz. DIP-package Z80?

[Alex Freed <alex_news@mirrow.com>]

Steven Hirsch wrote:
> I'm wondering how Alex was lucky enough to get an item that DigiKey 
> shows as non-stock.  And, yes, I'd also like to know if it works!

Just luck. Because I didn't really care all that much :)

> That would be a definite "no".  By running 80nsec. DRAM (VERY expensive 
> at the time and hard to find in a 7-bit refresh flavor),

If you use 256K chips and ground the unused address pin, 70 ns parts
are easy to find. However that is not the total access time. For
marketing reasons the DRAM was rated in *different* ns.
If you only use 64K out of 256, 7 bit refresh makes no difference.
So the 256K 70 ns DRAMs are likely good up to 11-12 MHz on a good day.

> replacing a few 
> 74LS parts with HCT and trying a handful of Z80H (8Mhz. rated) chips I 
> was able to get one of my Applicard running reliably at 10Mhz.  

Actually HCT is not much faster than LS. 74F is the answer these days.

> Alex's card uses a far simpler design with modern parts and static RAM.  
> It's not out of the question that it could run at those speeds, but it's 
> purely in the realm of experimentation.

Actually it was *designed* to run at 20 MHz. I just didn't test it above 
10 because I only got a 20 MHz z80 last week.


-Alex.