Dual Retriggerable Monostable Multivibrator
The SL74HC123 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
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 will be
some possibility of damaging the I.C. with a surge current or
latch-up. If the voltage supply filter capacitor is large enough
and VCC decrease slowly, the surge current is automatically
limited and damage the I.C. is avoided. The maximum forward
current of the parasitic diode is approximately 20 mA.
A B CLR
X L H L* H*
H X H L* H*
X = don’t care
* - except for monostable period