8.1 Overview

This edge-triggered multivibrator features output pulse-duration control by three methods. In the first method, the A input is low, and the B input goes high. In the second method, the B input is high, and the A input goes low. In the third method, the A input is low, the B input is high, and the clear (CLR) input goes high.

The output pulse duration is programmable by selecting external resistance and capacitance values. The external timing capacitor must be connected between C_ext and R_ext/C_ext (positive) and an external resistor connected between R_ext/C_ext and V_CC. Connect an external variable resistance between R_ext/C_ext and V_CCto obtain variable pulse durations. The output pulse duration also can be reduced by taking CLR low.

Pulse triggering occurs at a particular voltage level and is not directly related to the transition time of the input pulse. The A, B, and CLR inputs have Schmitt triggers with sufficient hysteresis to handle slow input transition rates with jitter-free triggering at the outputs.

When triggered, the basic pulse duration can be extended by retriggering the gated low-level-active (A) or high-level-active (B) input. Pulse duration may be reduced by taking CLR low. The input-output timing diagram (Figure 1) shows pulse control by retriggering the inputs and early clearing.

The Q outputs are in the low state, and the Q outputs are in the high state during power up. The outputs are glitch free, without applying a reset pulse.

This device is fully specified for partial-power-down applications using I_off. The I_off circuitry disables the outputs, which prevents damaging current backflow through the device when it is powered down.

8.2 Functional Block Diagram

Figure 10. Logic Diagram, Each Multivibrator (Positive Logic)

8.3 Feature Description

The SN74LV123A operates over a wide supply range from 2 V to 5.5 V. The propagation delay has a maximum of 11 ns at 5-V supply. The typical output ground bounce is less than 0.8 V at 3.3-V supply and 25°C. The typical output V_OH undershoot is greater than 2.3 V at 3.3-V supply and 25°C.

These parts support mixed-mode voltage operation on all ports.

Schmitt-trigger circuitry on the A, B, and CLR inputs allow for slow input transition rates and noisy input signals.

This device can be configured for rising or falling edge triggering.

This device supports partial-power-down mode operation.

This device is retriggerable for very long output pulses up to 100% duty cycle.

The clear signal overrides an output pulse and terminates it early.

Glitch-free power-up reset on outputs.

8.4 Device Functional Modes

Table 1 shows the functional modes for each monostable multivibrator in the SN74LV123A-Q1.

Table 1. Function Table
(Each Multivibrator)

INPUTS			OUTPUTS
CLR	A	B	Q	Q
L	X	X	L	H
X	H	X	L(1)	H(1)
X	X	L	L(1)	H(1)
H	L	↑
H	↓	H
↑	L	H

(1) 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.