MC54HC4538AJ

Manufacturer Part NumberMC54HC4538AJ
DescriptionDual precision monostable multivibrator
ManufacturerMotorola
MC54HC4538AJ datasheet
 
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CIRCUIT OPERATION

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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, C x , is charged to V CC . When a trigger occurs, the Q out-
put goes high and C x discharges quickly to the lower
[
reference voltage (V ref Lower
1/3 V CC ). C x then charges,
through R x , back up to the upper reference voltage (V ref Up-
[
per
2/3 V CC ), 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, C x , is charged to V CC (#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.
QUIESCENT
TRIGGER CYCLE
STATE
7
TRIGGER INPUT A
(PIN 4 OR 12)
TRIGGER INPUT B
(PIN 5 OR 11)
9
TRIGGER-CONTROL
3
14
CIRCUIT OUTPUT
11
4
15
R X /C X INPUT
12
(PIN 2 OR 14)
13
V ref LOWER
UPPER REFERENCE
5
CIRCUIT
13
LOWER REFERENCE
6
16
CIRCUIT
RESET INPUT
(PIN 3 OR 13)
1
RESET LATCH
10
Q OUTPUT
2
(PIN 6 OR 10)
High–Speed CMOS Logic Data
DL129 — Rev 6

CIRCUIT OPERATION

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, C x , to rapidly discharge toward ground (#11).
(Note that the voltage across C x appears at the input of both
the upper and lower reference circuit comparator).
When C x 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 C x to begin
to charge back up toward V CC , with a time constant t = R x C x
(#15). Once the voltage across C x charges to above the low-
er reference voltage, the lower reference circuit will go low
allowing the monostable multivibrator to be retriggered.
TRIGGER CYCLE
(A INPUT)
(B INPUT)
8
17
V ref UPPER
18
19
Figure 10. Timing Diagram
3–9
MC54/74HC4538A
RESET
RETRIGGER
t rr
24
21
23
25
20
22
+ t rr
MOTOROLA