Z80 / Z84C Swap (Doh!)

Eric Smith spacewar at gmail.com
Sun Aug 16 23:33:06 CDT 2015

On Sun, Aug 16, 2015 at 8:48 PM, drlegendre . <drlegendre at gmail.com> wrote:
> The Zilog P/N of the chip is "Z84C0008PEC", and it's further marked "Z80
> CPU" with a datecode of "8904" - April of 1989?

Assuming that your Z84C00 is not damaged, I think there's one
difference that might make it not work in a circuit that works with
the NMOS Z80.  The clock input signal of the Z80 CPU, either NMOS or
CMOS, is *not* TTL-compatible, but rather has a Vihc min spec of
Vcc-0.6V, which for a 5V supply is 4.4V.  Back in the day, many Z80
system designers ignored that problem, and drove the Z80 clock input
from a normal TTL gate, which was marginal at best since the TTL Voh
min spec is only 2.4V. In practice, you could usually get away with
that in a 2.5 MHz design (original Z80), but at 4 MHz or higher it
tended to be noticeably unreliable.   The Mostek Z80 databook gives
suggested drive circuits; I imagine that the Zilog documentation must
have also.

Anyhow, even though the Vihc spec is the same for NMOS and CMOS, the
NMOS part might better tolerate a clock input that didn't reach as
high as the spec.

The other thing that could do it is if the circuit might have been
designed such that a Z80 with very short clock-to-signal delays
wouldn't work. That would be bad design since the Z80 has never had a
minimum spec for those delays, so in such a case even a 6 or 8 MHz
NMOS part might not work.

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