MC54/74HC4538A Просмотр технического описания (PDF) - Motorola => Freescale
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MC54/74HC4538A
Motorola => Freescale
MC54/74HC4538A Datasheet PDF : 13 Pages
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MC54/74HC4538A
CIRCUIT OPERATION
Figure 12 shows the HC4538A configured in the retrigger-
able mode. Briefly, the device operates as follows (refer to
Figure 10): In the quiescent state, the external timing capac-
itor, Cx, is charged to VCC. When a trigger occurs, the Q out-
[ put goes high and Cx discharges quickly to the lower
reference voltage (Vref Lower 1/3 VCC). Cx then charges,
[ through Rx, back up to the upper reference voltage (Vref Up-
per 2/3 VCC), at which point the one–shot has timed out
and the Q output goes low.
The following, more detailed description of the circuit op-
eration refers to both the logic detail (Figure 9) and the timing
diagram (Figure 10).
QUIESCENT STATE
In the quiescent state, before an input trigger appears, the
output latch is high and the reset latch is high (#1 in Fig-
ure 10). Thus the Q output (pin 6 or 10) of the monostable
multivibrator is low (#2, Figure 10).
The output of the trigger–control circuit is low (#3), and
transistors M1, M2, and M3 are turned off. The external tim-
ing capacitor, Cx, is charged to VCC (#4), and both the upper
and lower reference circuit has a low output (#5).
In addition, the output of the trigger–control reset circuit is
low.
TRIGGER OPERATION
The HC4538A is triggered by either a rising–edge signal at
input A (#7) or a falling–edge signal at input B (#8), with the
unused trigger input and the Reset input held at the voltage
levels shown in the Function Table. Either trigger signal will
cause the output of the trigger–control circuit to go high (#9).
The trigger–control circuit going high simultaneously initi-
ates two events. First, the output latch goes low, thus taking
the Q output of the HC4538A to a high state (#10). Second,
transistor M3 is turned on, which allows the external timing
capacitor, Cx, to rapidly discharge toward ground (#11).
(Note that the voltage across Cx appears at the input of both
the upper and lower reference circuit comparator).
When Cx discharges to the reference voltage of the lower
reference circuit (#12), the outputs of both reference circuits
will be high (#13). The trigger–control reset circuit goes high,
resetting the trigger–control circuit flip–flop to a low state
(#14). This turns transistor M3 off again, allowing Cx to begin
to charge back up toward VCC, with a time constant t = RxCx
(#15). Once the voltage across Cx charges to above the low-
er reference voltage, the lower reference circuit will go low
allowing the monostable multivibrator to be retriggered.
QUIESCENT
STATE
TRIGGER CYCLE
(A INPUT)
TRIGGER CYCLE
(B INPUT)
RESET
RETRIGGER
7
TRIGGER INPUT A
(PIN 4 OR 12)
TRIGGER INPUT B
(PIN 5 OR 11)
TRIGGER-CONTROL 3
CIRCUIT OUTPUT
4
RX/CX INPUT
12
(PIN 2 OR 14)
Vref LOWER
UPPER REFERENCE 5
CIRCUIT
9
14
11
15
13
13
LOWER REFERENCE 6
16
CIRCUIT
8
17
Vref UPPER
18
RESET INPUT
(PIN 3 OR 13)
1
RESET LATCH
10
Q OUTPUT 2
19
(PIN 6 OR 10)
τ
τ
Figure 10. Timing Diagram
trr
24
21 23
25
20
22
τ + trr
High–Speed CMOS Logic Data
3–9
DL129 — Rev 6
MOTOROLA
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