cctalk Digest, Vol 56, Issue 26

[Chuck Guzis]

> Date: Wed, 16 Apr 2008 15:49:42 -0700 (PDT)
> From: "Eric Smith" 

> The most common error I've seen in Z80 designs is to drive the clock input
> directly from a TTL part.  The minimum Vih for the Z80 clock input is
> higher than the minimum Voh for a TTL part.  The Mostek Z80 CPU manual
> shows a circuit using a transistor as an active pullup to increase the
> clock swing.

With the clock driven by standard TTL, Zilog specified a 330 ohm 
pullup.  I've seen lots of instances where the Z80/Z80A clock was 
driven from an LSTTL or even a "canned" crystal oscillator with no 
pullups at all.  It might be that later steppings were more tolerant 
of clock voltage.

If one had an 8284A in the hellbox, it would also make a good choice; 
Voh(min) for the CLK output is 4v--but that might not be considered 
to be "vintage".  Some early S100 boards used an 8224 to generate the 
Z80 clock.

Cheers,
Chuck