Hardware Reference
In-Depth Information
Figure 2.10
Circuit capable of detecting pulses of any width, producing a pulse with i xed length (one clock
period) at the output (
outp
).
propagation delays left intentionally between the clock transitions and the corre-
sponding responses in
outp
in order to portray a more realistic situation.
In some cases, the pulse (of any duration) must be detected and converted into a
pulse whose duration is one clock period. This is illustrated in i gure 2.9b, where both
pulses are detected, and each produces an output pulse with duration
T
clk
.
A circuit that shortens the output pulse down to a predei ned length (
T
clk
in the
present case) is called a
one-shot
circuit, whereas one that detects short pulses is called
an
edge detector
. Both are present in the example of i gure 2.10, so pulses of any length
can be detected and converted into pulses with one-clock-period duration.
The i rst circuit in i gure 2.10 is the edge detector, which consists simply of a DFF
plus a reset mechanism. Note that to be able to detect short pulses,
inp
is connected
to the clock port instead of the data port. Because the data input is connected to
V
DD
('1'), the output goes immediately to '1' when a positive edge occurs in
inp
. Some time
later (see exercise 2.4), this '1' reaches
inp
sync
, resetting the input DFF, which will remain
so until a new positive transition occurs in
inp
.
The second circuit in i gure 2.10 is the synchronizer, already seen in the previous
section.
The third and i nal circuit before the application is the one-shot circuit. Because
of the AND gate, as soon as
inp
sync
goes to '1',
outp
goes to '1'. However, at the next
(positive) clock edge, this value of
inp
sync
crosses the DFF, bringing
outp
back to '0',
which it will remain until another pulse occurs at the input and the whole procedure
is repeated. This one-shot circuit, however, works well only when the input is syn-
chronous (see exercise 2.3), which is the case here.
Another circuit with the same purposes as that in i gure 2.10 is discussed in exercise
2.5. In the chapters ahead, examples employing this kind of circuit are seen.
2.5 Glitches
Glitches are short voltage (or current) pulses produced involuntarily by combinational
circuits. It is said that a hazard exists when the possibility of glitches in the circuit exists.
