Re: Z80 its refresh pin

[srkh28@gmail.com]

Hi Alex,

>
> 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.

Ah, I hope this could well be the problem. Although it looks great on
the CRO, it only peaks at about 4.2V (due to the translators, as im
executing off a real chip for the moment). I might try putting a
120ohm pull up on the clock pin to see if that helps, otherwise an
inline sot-23 buff should do the trick. I didnt realise the clock was
so sensitive on the Z80's.

>
> 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.


I think i might have this bit ok. An image of the captured FPGA wave
form shows it seems to look the part;

http://users.tpg.com.au/srkh//PH0%2BPH1%2BPIN6.png


dang REFRESH is still low!..

thanks,
Steve

.