74HCT123

The 74HCT123 is 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 74HCT123 is 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 Fig.6.

Once triggered, the basic output pulse of the 74HCT123 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 lso inhibits the triggering.

An internal connection of the 74HCT123 from nR_D to he input gates makes it possible to rigger the circuit by a positive-going signal at input nR_D as shown in the function table. Figures 7 and 8 illustrate pulse control by retriggeringand early reset. The basic output pulse width is essentially determined by the values of the external timing components R_EXT and C_EXT. For pulse widths, when C_EXT < 10 000 pF, see Fig.9.

When C_EXT > 10 000 pF of the 74HCT123, the typical output pulse width is defined as:tW = 0.45 ´ R_EXT ´ C_EXT (typ.),where: tW = pulse width in ns; R_EXT = external resistor in kW; C_EXT = external capacitor in pF.

Schmitt-trigger action of the 74HCT123 in the nA and nB inputs, makes the circuit highly tolerant to slower input rise and fall times.

The '123' is identical to the '423' but can be triggered via the reset input.