16 Apr
2008
16 Apr
'08
3:49 p.m.
Andrew wrote:
PS, if there are any experienced CCTALK veterans
who'd like to review &
critique my home brew Z80 SBC design, I would much appreciate your
comments.
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.
If you're using CMOS (e.g., 74HC, 74AC, etc.) to drive the clock, it's
not an issue, since the minimum Voh on those is near the positive rail (5V).
I haven't looked at the Vih spec for the clock on CMOS Z80 parts; it's
possible that they might have changed them to 2.0V.
Eric