Dual Retriggerable Monostable
The IN74HC123A is identical in pinout to the LS/ALS123. The
device inputs are compatible with standard CMOS outputs; with pullup
resistors, they are compatible with LS/ALSTTL outputs.
There are two trigger inputs, A INPUT (negative edge) and
B INPUT (positive edge). These inputs are valid for rising/falling signals.
The device may also be triggered by using the CLR input (positive-
edge) because of the Schmitt-trigger input; after triggering the output
maintains the MONOSTABLE state for the time period determined by the
external resistor RX and capacitor CX. Taking CLR low breaks this
MONOSTABLE STATE. If the next trigger pulse occurs during the
MONOSTABLE period it makes the MONOSTABLE period longer.
• Outputs Directly Interface to CMOS, NMOS, and TTL
• Operating Voltage Range: 3.0 to 6.0 V
• Low Input Current: 1.0 µA
• High Noise Immunity Characteristic of CMOS Devices
TA = -55° to 125° C for all packages
PIN 16 =VCC
PIN 8 = GND
(1) CX, RX, DX are external components.
(2) DX is a clamping diode.
The external capacitor is charged to VCC in the stand-by
state, i.e. no trigger. When the supply voltage is turned off
CX is discharged mainly through an internal parasitic diode.
If CX is sufficiently large and VCC decreases rapidy, there
mwill be some possibility of damaging the I.C. with a surge
ocurrent or latch-up. If the voltage supply filter capacitor is
.clarge enough and VCC decrease slowly, the surge current is
uautomatically limited and damage the I.C. is avoided. The
t4maximum forward current of the parasitic diode is
www.datasheeapproximately 20 mA.
A B CLR
X L H L* H*
H X H L* H*
XX L L H
X = don’t care
* - except for monostable period