M54HC423 datasheet(4/14 Pages) STMICROELECTRONICS

4 / 14 page

FUNCTIONAL DESCRIPTION (continued)

RE-TRIGGERED OPERATION

When a second trigger pulse follows the first its

effect will depend on the state of the multivibrator. If

the capacitor Cx is being charged the voltage level

of R/C external falls to Vrefl again and Q remains

high i.e. the retrigger pulse arrives in a time shorter

than the period Rx

⋅ Cx seconds, the capacitor

charging time constant. If the second trigger pulse

is very close to the initial trigger pulse it is ineffective

; i.e. the second trigger must arrive in the capacitor

discharge cycle to be ineffective; Hence the

minimum time for a second trigger to be effective

depends on VCC and Cx.

RESET OPERATION

CL is normally high. If CL is low, the trigger is not

effective because Q output goes low and trigger

control flip-flop is reset.

Also transistor Op is turned on and Cx is charged

quicky to VCC. This means if CL input goes low, the

IC becomes waiting state both in operating and non

operating state.

TRUTH TABLE

INPUTS

OUTPUTS

NOTE

OUTPUT ENABLE

INHIBIT

OUTPUT ENABLE

INHIBIT

X: Don’t Care Z: High Impedance

INPUT AND OUTPUT EQUIVALENT CIRCUIT

M54/M74HC423/423A

4/14

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