19 Dec
2008
19 Dec
'08
10:28 a.m.
From: "Philipp Hachtmann"
Sent: Friday, December 19, 2008 6:58 AM
Not sure what the epithet "sequential coding" means in this context.
There is combinatoric logic elsewhere computing the new values for
these 6 latches, which take their new values at the beginning of TP4.
Your code above does not result in latches. It results in D type
flipflops!
They're set on the rising edge of tp4.
You would get latches if you omit the tp4'event part of the conditional
expression.
Agreed. I stand corrected on the terminology. The part that is being
modeled was originally a 7474 edge triggered device, though, so the
model seems correct.
The real question about correctness would seem to be whether it is safe
to refactor the "gated clock" into the combinatorial inputs. Which
basically depends, on whether the gating elements are stable by the end
of TSn and throughout TPn.
Me too, though it's generally getting easier. Basically there are a
mess of delay lines which establish the spacing of the major timing
signals, with a few delay chains that are conditionally inserted for
special (longer) cycles.
Vince
if
(tp4'event and tp4 = '1') then
fetch <= f_set;
defer <= d_set;
execute <= e_set;
word_count <= wc_set;
current_address <= word_count;
break <= b_set;
end if;
That's sequential coding!! every time I
look at the 8/i drawings, they seem more synchronous, so
maybe it's just that I haven't fully understood the older machines yet.
I
have my problems understanding the control and sequencing in the old
machines.