Re: Z80 its refresh pin

[Alex Freed <alex_news@mirrow.com>]

Z80 is one of the most documented CPUs in the world and there are very 
few mysteries around it. The refresh wire goes active (low)for the 2 
last clocks on an instruction fetch (M1) cycle.

Two things may be relevant here:
1. The clock has to go nearly all the way up to 5V. Not good driving it
with TTL chips. Use a pull-up transistor as in the SoftCard or better a 
CMOS buffer.

2. SoftCard has to use a very special clocking scheme to be able to 
access the Apple's memory transparently. It pulses twice during PH0 and 
stays low during PH1. Well documented.

-Alex.