MC74HC4538A Motorola, MC74HC4538A Datasheet - Page 9

• Manufacturer Part Number: MC74HC4538A
• Description: Dual Precision Monostable Multivibrator
• Manufacturer: Motorola

High–Speed CMOS Logic Data
DL129 — Rev 6
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
through R x , back up to the upper reference voltage (V ref Up-
per
and the Q output goes low.
eration refers to both the logic detail (Figure 9) and the timing
diagram (Figure 10).
QUIESCENT STATE
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).
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).
low.
Figure 12 shows the HC4538A configured in the retrigger-
The following, more detailed description of the circuit op-
In the quiescent state, before an input trigger appears, the
The output of the trigger–control circuit is low (#3), and
In addition, the output of the trigger–control reset circuit is
[
LOWER REFERENCE
UPPER REFERENCE
TRIGGER-CONTROL
2/3 V CC ), at which point the one–shot has timed out
TRIGGER INPUT A
TRIGGER INPUT B
CIRCUIT OUTPUT
RESET LATCH
RESET INPUT
(PIN 4 OR 12)
R X /C X INPUT
(PIN 2 OR 14)
(PIN 3 OR 13)
(PIN 6 OR 10)
(PIN 5 OR 11)
Q OUTPUT
CIRCUIT
CIRCUIT
V ref LOWER
QUIESCENT
STATE
3
5
6
1
4
2
12
[
7
1/3 V CC ). C x then charges,
9
10
13
13
11
14
15
16
TRIGGER CYCLE
(A INPUT)
18
17
19
V ref UPPER
Figure 10. Timing Diagram
CIRCUIT OPERATION
8
3–9
TRIGGER CYCLE
TRIGGER OPERATION
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).
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).
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.
(B INPUT)
The HC4538A is triggered by either a rising–edge signal at
The trigger–control circuit going high simultaneously initi-
When C x discharges to the reference voltage of the lower
21
20
22
RESET
23
25
t rr
MC54/74HC4538A
24
RETRIGGER
+ t rr
MOTOROLA