1
Designing With the
SN74AHC123A and
SN74AHCT123A
SCLA014
October 1999
2
IMPORTANT NOTICE
Texas Instruments and its subsidiaries (TI) reserve the right to make changes to their products
or to discontinue any product or service without notice, and advise customers to obtain the latest
version of relevant information to verify, before placing orders, that information being relied on
is current and complete. All products are sold subject to the terms and conditions of sale supplied
at the time of order acknowledgement, including those pertaining to warranty, patent
infringement, and limitation of liability.
TI warrants performance of its semiconductor products to the specifications applicable at the
time of sale in accordance with TI’s standard warranty. Testing and other quality control
techniques are utilized to the extent TI deems necessary to support this warranty. Specific testing
of all parameters of each device is not necessarily performed, except those mandated by
government requirements.
CERTAIN APPLICATIONS USING SEMICONDUCTOR PRODUCTS MAY INVOLVE
POTENTIAL RISKS OF DEATH, PERSONAL INJURY, OR SEVERE PROPERTY OR
ENVIRONMENTAL DAMAGE (“CRITICAL APPLICATIONS”). TI SEMICONDUCTOR
PRODUCTS ARE NOT DESIGNED, AUTHORIZED, OR WARRANTED TO BE SUITABLE FOR
USE IN LIFE-SUPPORT DEVICES OR SYSTEMS OR OTHER CRITICAL APPLICATIONS.
INCLUSION OF TI PRODUCTS IN SUCH APPLICATIONS IS UNDERSTOOD TO BE FULLY
AT THE CUSTOMER’S RISK.
In order to minimize risks associated with the customer ’s applications, adequate design and
operating safeguards must be provided by the customer to minimize inherent or procedural
hazards.
TI assumes no liability for applications assistance or customer product design. TI does not
warrant or represent that any license, either express or implied, is granted under any patent right,
copyright, mask work right, or other intellectual property right of TI covering or relating to any
combination, machine, or process in which such semiconductor products or services might be
or are used. TI’s publication of information regarding any third party’s products or services does
not constitute TI’s approval, warranty or endorsement thereof.
Copyright 1999, Texas Instruments Incorporated
iii
Contents
Title Page
Abstract 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Features 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rules for Operation 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output Pulse Duration 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Calculations 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Retriggering Data 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Variation in Output Pulse Duration Due to Temperature and VCC Levels 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Special Considerations 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Setup Guidelines 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Distribution of Units 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Applications 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Delayed-Pulse Generator With Override 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Missing-Pulse Generator 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Low-Power Pulse Generator 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Negative- or Positive-Edge-Triggered One-Shot Multivibrator 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pulse-Duration Detector 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Frequency Discriminator 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusion 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgments 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix A 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
One-Shot Monostable Multivibrator 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
List of Illustrations
Figure Title Page
1 SN74AHC123A and SN74AHCT123A Logic Diagram for Each Multivibrator 2. . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 T iming Component Connections 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Output Pulse Duration vs External Timing Capacitance 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4 External Capacitance vs Multiplier Factor 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 Retrigger Pulse Duration 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6 Input/Output Requirements 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7 Variations in Output Pulse Duration for Various Temperatures and VCC Levels 7. . . . . . . . . . . . . . . . . . . . . . . . . . . .
8 Distribution of Units vs Output Pulse Duration 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9 Delayed-Pulse Generator With Override 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10 Missing-Pulse Detector 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11 Low-Power Pulse Generator 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12 Negative- or Positive-Edge-Triggered One-Shot Multivibrator 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13 Pulse-Duration Detector 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14 Frequency-Discriminator Circuit 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A–1 One-Shot Monostable Multivibrator and Function Block Diagram 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
iv
1
Abstract
This application report is designed to answer any questions that the user may have on the operation of the
SN74AHC/AHCT123A. It also covers the most frequently asked questions and includes detailed instructions on how to
calculate the external components required to make the device function correctly. Several circuits using this device also are
included to show the versatility of operations that can be performed using this part.
Introduction
The SN74AHC123A and SN74AHCT123A are dual, retriggerable, monostable multivibrators (see Appendix A) that have
similar functions. The SN74AHCT123A has TTL inputs and CMOS outputs. The SN74AHC123A has CMOS inputs and
outputs. These devices require external resistors and capacitors for proper operation, and the resulting RC time constant
determines the output pulse duration. These devices are capable of very-long-duration output pulses by retriggering the inputs
at appropriate times. An input clear can be used to decrease the output pulse duration. This application report discusses the
following points:
•Rules for operation, which include the output pulse duration, its calculation, and retriggering data
•Setup of the device in relation to its external components and the variation from unit to unit
•Applications
Features
Features of these devices are:
•Retriggerable
•Edge triggered from active-high or active-low logic inputs
•Inputs are TTL-voltage compatible for the SN74AHCT123A.
•VCC range of the SN74AHC123A is 2 V to 5.5 V.
•VCC range of the SN74AHCT123A is 4.5 V to 5.5 V.
•Clear (CLR) input overrides the other inputs (A and B).
•When inputs A and B have pulses applied to them, the signal that occurs first determines the pulse that triggers the
output.
•Three inputs (A, B, and CLR) have Schmitt triggers with suf ficient hysteresis to handle slow input transition rates
with jitter-free triggering at the output.
Figure 1 illustrates the logic diagram for both devices. Each multivibrator has two inputs, one that is active low and the other
that is active high, allowing leading- and trailing-edge triggering. The output pulse duration can be increased by retriggering
the input signal in use. The retrigger pulses at the input must occur after a certain period to be recognized and acted upon by
the device. If the input retrigger pulse follows the initial input pulse after 0.30 × the initial output pulse duration, the output
is retriggered. CLR terminates the output pulse at any time.
CLR
Cext
Rext/Cext
R
B
A
Q
Q
Figure 1. SN74AHC123A and SN74AHCT123A Logic Diagram for Each Multivibrator
2
Table 1. Function Table for the SN74AHC123A and SN74AHCT123A
INPUTS OUTPUTS
CLR A B Q Q
L X X L H
XHXL
†H†
XXLL
†H†
H L ↑
H
#
H
↑L H
†These outputs are based on the
assumption that the indicated
steady-state conditions at the A and B
inputs have been set up long enough to
complete any pulse started before the
setup.
3
Rules for Operation
Proper use depends on observing these rules of operation:
•Minimum value of external resistance (RT) is 250 Ω.
•External capacitance (CT) can have any value.
•Input voltage range is from 0 V to 5.5 V.
•SN74AHC123A VCC must be 2 V to 5.5 V, and SN74AHCT123A VCC must be 4.5 V to 5.5 V.
•SN74AHC123A and SN74AHCT123 TA can be between –40°C and 85°C.
•A switching diode on one side of the capacitor is not needed for the timing scheme.
•Required connections of RT and CT, for the proper operation of the devices, are shown in Figure 2. CT can be
grounded at the Cext terminal.
1CLR
1B
1A 1Q
1Q
1/2 ’AHC123A
or
1/2 ’AHCT123A
1
2
3
13
4
15 14
CT
RT
VCC
2CLR
2B
2A 2Q
2Q
1/2 ’AHC123A
or
1/2 ’AHCT123A
9
10
11
5
12
76
CT
RT
VCC
Rext/Cext Cext Rext/Cext Cext
Figure 2. Timing Component Connections
Output Pulse Duration
The output pulse duration (tw) for both devices is determined primarily by the values of CT and RT. The timing components
are connected as shown in Figure 2.
The definition of the output pulse duration is shown in equation 1.
tw
+
K
RT
CT
Where:
tw= pulse duration in ns
K = multiplier factor
RT= external timing resistance in kΩ
CT= external capacitance in pF
If:
CT is ≥1000 pF, K = 1.0
CT is <1000 pF, K can be determined from Figure 3
The minimum output pulse duration, using the minimum value of external resistance (250 Ω) and a minimum value of external
capacitance (open air), is approximately 290 ns.
(1)
4
11010
2103104105106107
1.00E+00
1.00E+01
1.00E+02
1.00E+03
1.00E+04
1.00E+05
1.00E+06
1.00E+07
1.00E+08
1.00E+09
RT = 1k Ω
RT = 5k Ω
RT = 10k Ω
RT = 80k Ω
RT = 150k Ω
RT = 200k Ω
VCC = 5V
TA = 25°C
Output Pulse Duration – ns
w
t–
CT – External Timing Capacitance – pF
(1 ns)
(1 µs)
(1 ms)
Example 3
Example 1
Example 2
Figure 3. Output Pulse Duration vs External Timing Capacitance
Calculations
Equation 1 and Figure 3 can be used to determine values for output pulse duration and external resistance and capacitance
values for the SN74AHC123A and SN74AHCT123A.
Equation 1 and Figures 3 and 4 can be used to solve the following problems.
TA = 25 °C
VCC = 5 V
For Capacitor Values of 0.001 µF, or Greater,
K = 1.0 (K is Independent of RT)
Multiplier Factor – K
CT– External Capacitor Value – µF
0.001
0.0001
0.00001 4.504.003.503.002.502.001.501.00
Figure 4. External Capacitance vs Multiplier Factor
5
•Pulse duration for a given external resistance and capacitance
For example, if an 80-kΩ resistor and a 100-nF capacitor are used, the pulse duration is obtained by following the
RT = 80-kΩ curve to the line indicating that CT = 100 nF. The y (vertical) coordinate of this point gives the value
of the pulse duration, which is approximately 8 × 106 ns (8 ms).
The pulse duration (tw) for an external resistance of 80 kΩ and a capacitor of 100 nF is:
tw
+
1.0
80 k
W
1
105pF
+
8
106ns
+
8ms
(K
+
1.0 because CTis greater than 1000 pF)
This value was obtained graphically in Figure 3.
•Required external resistance for a given pulse duration and external capacitance
If a pulse duration of 200 ns is desired and a timing capacitance of 100 pF is used, the resistance needed is found
where the horizontal line of 200 ns on the tw axis intersects 100 pF on the CT axis. This point may be along one of
the curves and, in this case, the point is slightly above the RT = 1-kΩ curve. So the required resistance is
approximately 1.3 kΩ.
The required external resistance (RT) that produces a pulse duration of 200 ns with a 100-pF external capacitor is:
RT
+
tw
K
CT
+
200 ns
1.5
100 pF
+
1.3 k
W
Because CT < 1000 pF , use Figure 4 to find the value of K. Follow the tw = 100-pF horizontal line from the vertical
axis to the curve, then drop a vertical line to the CT axis. The intersection on the CT axis gives the K value; in this
case, K = 1.5.
This value of RT is approximately equal to the value of RT as given in Figure 3.
•Required external capacitance for a certain pulse duration and external resistance
For example, if CT = 10 kΩ and a pulse duration of 1 × 106 ns is desired, the timing capacitance required can be
obtained by finding the point where tw = 1 × 106 ns (from the vertical axis) intersects the 10-kΩ curve. This point
is then dropped vertically, to cross the horizontal axis at CT = 1 × 105 pF (100 nF).
The external capacitance (CT) that produces an output pulse duration of 1 × 106 ns, with a timing resistance of 10 kΩ,
is:
CT
+
tw
K
RT
+
1
106ns
1.0
10 k
W
+
1
105pF
+
100 nF
The value of capacitance is unknown; therefore, the explanation following equation 1 cannot be used directly to find
a value for K. Figure 4 must be studied to determine a value for K. Using Figure 3, the maximum value for a 1000-pF
capacitor and 200 kΩ resistor is about 2 × 105 ns, which is much lower than the desired output pulse duration of 1
× 106 ns for this application. It can be concluded that the external capacitance is larger than 1000 pF and K = 1.0.
The value of CT here is the same as that in Figure 3.
(2)
(3)
(4)
6
Retriggering Data
The retrigger pulse duration is calculated as shown in Figure 5.
tMIR
tRT
tPLH tw
Input
Output
tRT = tw + tPLH = K × RT × CT + tPLH
Where:
tMIR = Minimum Input Retriggering Time
tPLH = Propagation Delay
tRT = Retrigger Time
tw= Output Pulse Duration Before Retriggering
Figure 5. Retrigger Pulse Duration
tMIR is the minimum time required after the initial signal before retriggering the input. After tMIR, the device retriggers the
output. Experimentally, it also can be shown that, to retrigger the output pulse, the two adjacent input signals should be tMIR
apart, where tMIR = 0.30 × tw.
The minimum value from the end of the input pulse to the beginning of the retriggered output should be approximately 15 ns
to ensure a retriggered output. This is illustrated in Figure 6.
Input
Output
tMRT= Minimum Time Between the End of the Second Input Pulse and the Beginning of the Retriggered Output
tMRT= 15 ns
tMRT
Figure 6. Input/Output Requirements
7
Variation in Output Pulse Duration Due to Temperature and VCC Levels
Figure 7 shows the percentage variation in the output pulse duration due to temperature and VCC of the devices. All points on
the graph are plotted relative to TA = 25°C and VCC = 5 V (which is assumed to be 0% variation). For example, according
to Figure 7, at a temperature of 40°C and a VCC level of 4 V , the value of the output pulse duration differs by 2% from the reading
at VCC = 5 V and TA = 25°C.
tw = 866 ns
VCC = 5 V
TA = 25°C
RT = 10 kΩ
CT = 50 pF
VCC = 2.5 V
VCC = 3 V
VCC = 3.5 V
VCC = 4 V
VCC = 5 V
VCC = 6 V
VCC = 7 V
TA – °C
Variation in Output Pulse Duration – %
14
12
10
8
6
4
2
0
–2
–4
–6
–60 –40 –20 0 20 40 60 80 100 120 140 160
Figure 7. Variations in Output Pulse Duration for Various Temperatures and VCC Levels
8
Special Considerations
Setup Guidelines1
Because the SN74AHC123A and SN74AHCT123A monostable multivibrators are half analog and half digital and are
inherently more sensitive to noise on the analog portion (timing leads) than standard digital circuits, they should not be located
near noise-producing sources or transient-carrying conductors. Liberal power-supply bypassing is recommended for greater
reliability and repeatability. Also, a monostable multivibrator should not be used as a solution for asynchronous systems;
synchronous design techniques always provide better performance. For time delays over 1.5 seconds or timing capacitors over
100 µF, it usually is better to use a free-running astable multivibrator and two inexpensive decade counters (such as a 7490A)
to generate the equivalent of a long-delay one-shot multivibrator. Astable oscillators made with monostable building blocks
have stabilities approaching 5 parts in 100, and should not be used if system timing is critical. Crystal oscillators provide better
stability.
In all one-shot multivibrator applications, follow these guidelines:
•Use good high-frequency 0.1-µF (ceramic disk) capacitors, located 1 to 2 inches from the monostable package, to
bypass VCC to ground.
•Keep timing components (RT, CT) close to the package and away from high-transient-voltage or current-carrying
conductors.
•Keep the Q output trace away from the CLR lead; when the one-shot multivibrator times out, the negative-going
edge may cause the CLR lead to be pulled down, restarting the cycle. If this happens, constantly high (Q = H, Q =
L) outputs with 50-ns low spikes occur at the repetition rate determined by RT and CT. If sufficient trace isolation
cannot be obtained, a 50-pF capacitor, bypassing the CLR lead to ground, usually eliminates the problem.
•Beware of using the diode or transistor protective arrangement when retriggerable operation is required; the second
output pulse may be shorter due to excess charge left on the capacitor . This may result in early timeout and apparent
failure of retriggerable operation. Use a capacitor that is able to withstand 1 V in reverse and meet the
leakage-current requirements of the particular one-shot multivibrator.
•Remember that the timing equation associated with each device has a prediction accuracy.
Distribution of Units
Figure 8 shows the variation in the output pulse duration for a random sample of both devices. The average pulse width of the
output is 856 ns, with a standard deviation of 3.5 ns. The median has the same value as the mean. There also is a high frequency
of finding units with an average output pulse duration. A unit with the median-value pulse duration was tested to obtain the
plots in Figures 3, 4, 7, and 8.
Output Pulse Duration – tw
Relative Frequency of Occurrence
Mean = 856 ns
Median = 856 ns
Std. Dev. = 3.5 ns
VCC = 5 V
TA = 25°C
CT = 50 pF
RT = 10 kΩ
+3 σ
Median
–3 σ99% of Units
Figure 8. Distribution of Units vs Output Pulse Duration
9
Applications
Delayed-Pulse Generator With Override1
In Figure 9, the value of the delay time depends on the values of RT1 and CT1 in the first one-shot multivibrator (OS1). The
second one-shot multivibrator (OS2) determines the output pulse duration that is defined by the values of RT2 and CT2. A
positive rising pulse into the override circuit can terminate the output pulse at any time.
1/2 ’AHC123A
or
1/2 ’AHCT123A
OS1
CT1
VCC
Output
Input
Override
Input
RT1
Q
Input
Output OS1
1
0
1CLR
1/6 ’LS04
1A
1B
1/2 ’AHC123A
or
1/2 ’AHCT123A
OS2
CT2
RT2
Q
1
0
1
0
Delayed OS2
2CLR
2A
2B
Figure 9. Delayed-Pulse Generator With Override
10
Missing-Pulse Generator1
The external resistance (RT1) and the external capacitance (CT1) determine the pulse duration of OS1. This pulse duration is
set to be greater than one-half of the incoming frequency. A transition from low to high logic on the incoming pulse sets the
output of OS1 to a high logic level. The output of OS1 remains high as long as the input pulse consistently switches from high
to low to high logic at regular intervals. This implies that the one-shot multivibrator is being retriggered. If there is a missing
pulse in the pulse train of the input, the output of OS1 falls to a low logic level and the output of OS2 rises to a high level (see
Figure 10).
Input
Output OS1
1
0
1
0
1
0
Output OS2
1/2 ’AHC123A
or
1/2 ’AHCT123A
OS1
CT1
VCC
Output
Input
RT1
Q
1CLR
1A
1B 1/2 ’AHC123A
or
1/2 ’AHCT123A
OS2
CT2
RT2
Q
2CLR
2A
2B
VCC VCC
Figure 10. Missing-Pulse Detector
11
Low-Power Pulse Generator1
In Figure 11, the first one-shot multivibrator (OS1) is responsible for the output frequency of the generator. The external
resistance (RT1) and the external capacitance (CT1) determine the frequency. The OS2 configuration gives rise to the output
pulse duration, which is determined by RT2 and CT2.
Output OS1
1
0
Output OS2
1/2 ’AHC123A
or
1/2 ’AHCT123A
OS1
CT1
VCC
Outputs
RT1
1CLR
1A
1B
1/2 ’AHC123A
or
1/2 ’AHCT123A
OS2
CT2
RT2
2CLR
2A
2B
VCC VCC
QQ
QQ
1
0
Duty cycle of output pulse
+
Rext
Cext
Rext1
Cext1
f
+
1
K
Rext1
Cext1 MHz
Where Rext is in kΩ and Cext is in pF.
See appropriate figure for values of K.
Figure 11. Low-Power Pulse Generator
12
Negative- or Positive-Edge-Triggered One-Shot Multivibrator1
The circuit in Figure 12 is arranged such that a negative-going input pulse causes a low-to-high-to-low pulse on OS1. A
positive-going input pulse causes a low-to-high-to-low pulse on OS2. The outputs of OS1 and OS2 are connected to an OR gate
that outputs a pulse when OS1 or OS2 switches. The circuit in Figure 12 also can be used as a frequency doubler.
1/2 ’AHC123A
or
1/2 ’AHCT123A
OS1
CT1
VCC
RT1
1CLR
1A
1B
Q
Q
1/2 ’AHC123A
or
1/2 ’AHCT123A
OS2
CT2
VCC
RT2
2CLR
2A
2B
Q
Q
Input
Output
1/4 74LS32
Figure 12. Negative- or Positive-Edge-Triggered One-Shot Multivibrator
13
Pulse-Duration Detector1
Figure 13 shows a circuit using the AHC/AHCT123A chip, which generates an output pulse (t3) if the trigger pulse duration (t2)
is wider than the programmed output pulse duration (tw = K × RT × CT). It functions as follows:
The A input of the AHC/AHCT123A is approximately VCC and the transistor Q1 usually is off. The Q output of the
AHC/AHCT123A normally is low and the output of Q2 is of f (the output normally is low because no pullup exists). A trigger
of duration t1 applied at the input is differentiated by the R1C1 combination and Q1 is turned on. The result of that momentary
condition at the base of Q1 is a negative-going pulse at point 1 (the A input of the AHC/AHCT123A), which triggers
AHC/AHCT123A. The AHC/AHCT123A remains on for the time tw = K × R T × CT, which is waveform t2. Q2 on the output
of the device is turned on for a time equal to t2 and, after this time, turns off. If the input pulse is still high after this, it appears
at the output. The circuit output pulse duration, t3, equals the input pulse duration, minus the pulse duration of the
AHC/AHCT123A.
1/2 ’AHC123A
or
1/2 ’AHCT123A
OS1
CT1
VCC
RT1
1CLR
1A
1B
Q
VCC
Output
10 kΩ
Q2
VCC
“A” or “1” 47 kΩ
Input
C1
0.001 µF
R1
10 kΩ
Q1
Input
t1
A or 1
’AHC123A or
’AHCT123A
Output
t2
VOH
Output
t3
0
1
Figure 13. Pulse-Duration Detector
14
Frequency Discriminator1
RT1 and CT1 in Figure 14 form a resistor-capacitor integration network that produces an output voltage proportional to the
frequency. This plot is linear and is valid over a limited range.
Input
Output
1
0
VOH
0
1/2 ’AHC123A
or
1/2 ’AHCT123A
OS1
CT1
VCC
Input
RT1
Q
1CLR
1A
1B
VCC
Output
C1
R1
Figure 14. Frequency-Discriminator Circuit
Conclusion
The SN74AHC123A and SN74AHCT123A function similarly. Both devices require external resistors and capacitors for
proper operation, and these timing units can be used to determine the output pulse duration. These devices can be retriggered
to create very long output pulses. If the input signal is triggered and the time length to the previous input signal is less than
0.30 × initial output pulse duration in seconds, the output duration remains unchanged. A clear input can be used at any time
to terminate the output pulse. The output pulse duration also varies according to the temperature of operation and the VCC of
the device.
There are several applications in which these dual retriggerable monostable multivibrators can be used. In all cases, it is
essential that the setup guidelines be followed to promote the safety and reliability of the devices.
Acknowledgments
The authors of this application report are Sujatha Garimella, Dennis Wade, Clif Dugan, and Gerry Balmer . The authors wish
to thank Arnie Ruiz, Joe Costa, Jim Stein, Dave Long, Imran Sohrab, Bryan Lammers, Mark Frimann, Keith Henderson,
Terence Ndofor, and Hari Garimella for their assistance with this application report.
References
1. Clif Dugan, Designing with the SN54/74LS123, Texas Instruments Incorporated, Dallas, TX, 1990.
2. Walter H. Buchsbaum, Sc.D., Encyclopedia of Integrated Circuits: A Practical Handbook of Essential Reference
Data, Prentice-Hall Incorporated, Englewood Cliffs, NJ, 1981.
Information from the following web sites also was used in this application report:
http://www.sh-gpl.ti.com/mirrors/
http://www-s.ti.com/sc/psheets/schs142/schs142.pdf
http://www-s.ti.com/sc/psheets/sdls043/sdls043.pdf
http://www-s.ti.com/sc/psheets/scls420a/scls420a.pdf
http://www-s.ti.com/sc/psheets/scls352b/scls352b.pdf
15
Appendix A
One-Shot Monostable Multivibrator†
Multivibrators are a form of flip-flop circuit in which an RC time constant is used to determine the rate of change of state
(toggling). In the monostable or one-shot multivibrator (MV), an external trigger signal starts the change of state of this MV,
and the external RC time constant determines the time required from the beginning to the end of this one-shot oscillation.
A basic monostable MV is shown in Figure A–1. The key elements are the two trigger inputs, the reset input, and the values
of the external RC time constant. The OR circuit that triggers the MV has a small circle on one of its inputs, indicating that
it can accept either a positive- or a negative-edge trigger. The edge-triggering ability is produced because this particular OR
circuit is combined with a Schmitt-trigger effect. In some ICs, this type of circuit has a hysteresis characteristic and is referred
to as a transmission gate.
The operation of the monostable MV requires that it first be reset so that the Q output is 0 and the Q output is 1. When either
a positive or a negative trigger signal is entered, Q immediately changes to 1 and Q to 0. After a period of time, determined
by the RC time constant, the MV returns to its original state, having generated one pulse. When the reset signal occurs, the
MV returns to the original state where Q is at 0. In retriggerable monostable MVs, any trigger signal that occurs during the
period when Q is at 1 prolongs the duration of the pulse beyond the time determined by the RC time constant.
The function block of a typical emitter-coupled logic (ECL) monostable MV is illustrated in Figure A–1, which shows some
of the various features that are available in monostable MV ICs. Trigger signals are applied to the trigger input, and the external
+enable or –enable signals determine whether the MV accepts positive- or negative-going edges. Internal Schmitt-trigger
circuits make the trigger input insensitive to rise and fall times. Although there is an external RC time constant, there also is
an input for external pulse-width control. With an external resistor, a control voltage can be used to vary the pulse width. When
a control current is used, the resistor is not required. In addition, this ECL IC has a special high-speed-trigger input that bypasses
the internal Schmitt-trigger circuits and permits a very rapid response.
Monostable MVs that can be triggered multiple times within a given time period to increase the pulse duration according to
a fixed ratio are available. Other monostable MVs include a preset feature that can be combined with retriggering to generate
specific-pulse waveforms.
†W alter H. Buchsbaum, Sc.D., wrote Appendix A and provided Figure A–1 (see
Encyclopedia of Integrated Circuits: A Practical Handbook
of Essential Reference Data).
16
ECL
Monostable
Multivibrator Q
Q
RC
Trigger
+Enable
–Enable
External Pulse Duration
High-Speed Trigger
VCC
Multivibrator
Q
Q
Trigger
Reset
VCC
RC
Figure A–1. One-Shot Monostable Multivibrator and Function Block Diagram
