1. General description
The 74AHC123A; 74AHCT123A are high-speed Si-gate CMOS devices and are pin
compatible with Low power Schottky TTL (LSTTL). They are specified in compliance with
JEDEC standard no. 7A.
The 74AHC123A; 74AHCT123A are dual retriggerable monostable multivibrators with
output pulse width control by three methods. The basic pulse time is programmed by
selection of an external resistor (R
EXT
) and capacitor (C
EXT
). The external resistor and
capacitor are normally connected as shown in Figure 11
.
Once triggered, the basic output pulse width may be extended by retriggering the gated
active LOW-going edge input (nA
) or the active HIGH-going edge input (nB). By repeating
this process, the output pulse period (nQ = HIGH, nQ
= LOW) can be made as long as
desired. Alternatively an output delay can be terminated at any time by a LOW-going edge
on input nR
D, which also inhibits the triggering.
An internal connection from nR
D to the input gate makes it possible to trigger the circuit by
a positive-going signal at input nR
D as shown in Table 3. Figure 8 and Figure 9 illustrate
pulse control by retriggering and early reset. The basic output pulse width is essentially
determined by the value of the external timing components R
EXT
and C
EXT
. When
C
EXT
10 nF, the typical output pulse width is defined as: t
W
=R
EXT
C
EXT
where
t
W
= pulse width in ns; R
EXT
= external resistor in k; C
EXT
= external capacitor in pF.
Schmitt-trigger action at all inputs makes the circuit highly tolerant to slower input rise and
fall times.
2. Features and benefits
All inputs have a Schmitt-trigger action
Inputs accept voltages higher than V
CC
DC triggered from active HIGH or active LOW inputs
Retriggerable for very long pulses up to 100 % duty factor
Direct reset terminates output pulse
For 74AHC123A only: operates with CMOS input levels
For 74AHCT123A only: operates with TTL input levels
ESD protection:
HBM JESD22-A114E exceeds 2000 V
MM JESD22-A115-A exceeds 200 V
CDM JESD22-C101C exceeds 1000 V
Multiple package options
Specified from 40 C to +85 C and from 40 C to +125 C
74AHC123A; 74AHCT123A
Dual retriggerable monostable multivibrator with reset
Rev. 4 — 8 November 2011 Product data sheet
