Datasheet

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SCLS352H − JULY 1997 − REVISED OCTOBER 2005
2
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
description/ordering information (continued)
The output pulse duration is programmed 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
. To obtain variable pulse durations, connect an external variable resistance between
R
ext
/C
ext
and V
CC
. 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.
Once triggered, the basic pulse duration can be extended by retriggering the gated low-level-active (A
) or
high-level-active (B) input. Pulse duration can be reduced by taking CLR
low. CLR input can be used to override
A
or B inputs. The input/output timing diagram illustrates pulse control by retriggering the inputs and early
clearing.
The variance in output pulse duration from device to device typically is less than ±0.5% for given external timing
components. An example of this distribution for the ’AHC123A is shown in Figure 10. Variations in output pulse
duration versus supply voltage and temperature are shown in Figure 6.
During power up, Q outputs are in the low state, and Q outputs are in the high state. The outputs are glitch free,
without applying a reset pulse.
For additional application information on multivibrators, see the application report Designing With the
SN74AHC123A and SN74AHCT123A, literature number SCLA014.
FUNCTION TABLE
(each multivibrator)
INPUTS
OUTPUTS
CLR A B Q Q
L X X L H
X HXL
†
H
†
X XLL
†
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.