Hardware Reference
In-Depth Information
Figure 2.20
a) It is said in section 2.4 that this circuit is i ne only if the input is synchronous.
In i gure 2.19b, two illustrative pulses are shown for
x
, the i rst assumed to be
synchronous (produced by the same clock that commands this DFF; the delays
between the clock edges and the pulse edges are just
t
pCQ
) but the second not.
Draw the waveforms for
q
and
y
, and coni rm what was said about the output
pulse duration.
b) It is also said that the input must last at least one clock period. Why?
c) Why is this circuit called a “one-shot” circuit? Does
x
need to return to '0' for it to
produce the intended pulse?
Exercise 2.4: Fast Synchronized One-Shot Circuit #1
Figure 2.20a shows the same arrangement of i gure 2.10, implementing a fast one-
shot circuit with asynchronous, and possibly short, input. This circuit is capable of
detecting input pulses shorter (and also longer, of course) than the clock period,
producing at the output a pulse whose duration is always one clock period. Figure
2.20b shows the main signals involved in this circuit, with the plots for the clock
and for the input (
x
) already completed (some helping arrows are also included in
the i gure).
a) Draw the waveforms for the internal (
i
1
to
i
4
) and output (
y
) signals. Do not forget
to leave a little delay between a signal transition and the corresponding response.
b) What are the minimum and maximum durations of
i
1
(in clock periods)? Why is
the initial (edge detector) DFF also called a “stretcher”?
c) What are the durations of
i
2
,
i
3
,
i
4
, and
y
(in clock periods)?
d) At which clock edge (i rst, second, etc.) after
x
goes up does
y
go up?
