Good afternoon,

I was hoping someone could help me understand some of the terminology in this data sheet and how I translate that into a basic one-shot multivibrator that is retriggerable.


Page 11-13 are the description and application portion of the datasheet. My questions are

1)The pins A and B, and how you work with them to trigger the one-shot. My interpretation is that either A or B must be brought HIGH by a transient signal and the other must be LO(grounded) to trigger the one-shot.

2)the term "gated" with regards to retriggering the one-shot while it is still HI to extend the duty cycle by restarting the RC constant (how to retrigger the one-shot before the duty cycle ends).

Any help in understanding the involved circuitry would be greatly appreciated.

Thank you!

  • 1
    \$\begingroup\$ The reset must be high. If it is, and if A and B are high, then bringing A low will trigger. If A and B are low, bringing B high will trigger. Doing either of these while the output is active (in the middle of an output pulse) will extend the output pulse by the nominal pulse width. \$\endgroup\$ Commented Mar 7, 2017 at 22:28
  • \$\begingroup\$ Is the CLR pin what you are calling the "reset"? \$\endgroup\$
    – LevyDee
    Commented Mar 7, 2017 at 22:39
  • 1
    \$\begingroup\$ Yes. CLR stands for "clear". Also known as reset. Same function is called MR (master reset) on some counters and shift registers. \$\endgroup\$ Commented Mar 7, 2017 at 23:28
  • 1
    \$\begingroup\$ Also read Designing with the SN74LVC1G123 Monostable Multivibrator. \$\endgroup\$
    – CL.
    Commented Mar 8, 2017 at 10:02

1 Answer 1


The term "gated" simply means that their are two inputs, \$\small \overline{\text{A}}\$ and B, only one of which can be used at a given time -- one input acts as a enable (gate) for the other. If B is held high, then the '123 one-shot is triggered by a falling edge pulse on \$\small \overline{\text{A}}\$. If \$\small \overline{\text{A}}\$ is held low, then the '123 one-shot is triggered by a rising edge pulse on B.

In either case, the reset line \$\small \overline{\text{CLR}}\$ must be held high. If it goes low, then any output timing is prematurely reset.

While output timing is active, if the input \$\small \overline{\text{A}}\$ or B is triggered again, then the output is extended for a full-time period from that point, which is what is meant by being retriggerable.


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