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MM74HC4538
Timing Diagram
Circuit Operation
The MM74HC4538 operates as follows (refer to logic dia-
gram). In the quiescent state, the external timing capacitor,
C
X
, is charged to V
CC
. When a trigger occurs, the Q output
goes HIGH and C
X
discharges quickly to the lower refer-
ence voltage (V
REF
Lower =
1
/
3
V
CC
). C
X
then charges,
through R
X
, back up to the upper reference voltage (V
REF
Upper =
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 oper-
ation refers to both the logic diagram and the timing dia-
gram.
QUIESCENT STATE
In the quiescent state, before an input trigger appears, the
output latch is HIGH and the reset latch is HIGH (#1 in logic
diagram).
Thus the Q output (pin 6 or 10) of the monostable multivi-
brator is LOW (#2, timing diagram).
The output of the trigger-control circuit is LOW (#3), and
transistors M1, M2, and M3 are turned off. The external
timing capacitor, C
X
, is charged to V
CC
(#4), and the upper
reference circuit has a LOW output (#5). Transistor M4 is
turned ON and transmission gate T1 is turned OFF. Thus
the lower reference circuit has V
CC
at the noninverting
input and a resulting LOW output (#6).
In addition, the output of the trigger-control reset circuit is
LOW.
TRIGGER OPERATION
The MM74HC4538 is triggered by either a rising-edge sig-
nal 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 Truth Table. Either trigger
signal will cause the output of the trigger-control circuit to
go HIGH (#9).
The trigger-control circuit going HIGH simultaneously ini-
tiates three events. First, the output latch goes LOW, thus
taking the Q output of the HC4538 to a HIGH State (#10).
Second, transistor M3 is turned on, which allows the exter-
nal timing capacitor, C
X
, to rapidly discharge toward
ground (#11). (Note that the voltage across C
X
appears at
the input of the upper reference circuit comparator.) Third,
transistor M4 is turned off and transmission gate T1 is
turned ON, thus allowing the voltage across C
X
to also
appear at the input of the lower reference circuit compara-
tor.
When C
X
discharges to the reference voltage of the lower
reference circuit (#12), the outputs of both reference cir-
cuits 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). In addition, transistor M4 is
turned ON and transmission gate T1 is turned OFF. Thus a
high voltage level is applied to the input of the lower refer-
ence circuit comparator, causing its output to go LOW
(#16). The monostable multivibrator may be retriggered at
any time after the trigger-control circuit goes LOW.
When C
X
charges up to the reference voltage of the upper
reference circuit (#17), the output of the upper reference
circuit goes LOW (#18). This causes the output latch to
