Philips Semiconductors
Product specification
Presettable synchronous BCD decade
up/down counter
FEATURES
•
Synchronous reversible counting
•
Asynchronous parallel load
•
Asynchronous reset
•
Expandable without external logic
•
Output capability: standard
•
I
CC
category: MSI
GENERAL DESCRIPTION
The 74HC/HCT192 are 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 74HC/HCT192 are synchronous BCD up/down
counters. Separate up/down clocks, CP
U
and CP
D
respectively, simplify operation. The outputs change state
synchronously with the LOW-to-HIGH transition of either
clock input. If the CP
U
clock is pulsed while CP
D
is held
HIGH, the device will count up. If the CP
D
clock is pulsed
while CP
U
is held HIGH, the device will count down. Only
one clock input can be held HIGH at any time, or
erroneous operation will result. The device can be cleared
at any time by the asynchronous master reset input (MR);
it may also be loaded in parallel by activating the
asynchronous parallel load input (PL).
The “192” contains four master-slave JK flip-flops with the
necessary steering logic to provide the asynchronous
reset, load, and synchronous count up and count down
functions.
Each flip-flop contains JK feedback from slave to master,
such that a LOW-to-HIGH transition on the CP
D
input will
decrease the count by one, while a similar transition on the
CP
U
input will advance the count by one.
74HC/HCT192
One clock should be held HIGH while counting with the
other, otherwise the circuit will either count by two’s or not
at all, depending on the state of the first flip-flop, which
cannot toggle as long as either clock input is LOW.
Applications requiring reversible operation must make the
reversing decision while the activating clock is HIGH to
avoid erroneous counts.
The terminal count up (TC
U
) and terminal count down
(TC
D
) outputs are normally HIGH. When the circuit has
reached the maximum count state of 9, the next
HIGH-to-LOW transition of CP
U
will cause TC
U
to go LOW.
TC
U
will stay LOW until CP
U
goes HIGH again, duplicating
the count up clock.
Likewise, the TC
D
output will go LOW when the circuit is in
the zero state and the CP
D
goes LOW. The terminal count
outputs can be used as the clock input signals to the next
higher order circuit in a multistage counter, since they
duplicate the clock waveforms. Multistage counters will not
be fully synchronous, since there is a slight delay time
difference added for each stage that is added.
The counter may be preset by the asynchronous parallel
load capability of the circuit. Information present on the
parallel data inputs (D
0
to D
3
) is loaded into the counter
and appears on the outputs (Q
0
to Q
3
) regardless of the
conditions of the clock inputs when the parallel load (PL)
input is LOW. A HIGH level on the master reset (MR) input
will disable the parallel load gates, override both clock
inputs and set all outputs (Q
0
to Q
3
) LOW. If one of the
clock inputs is LOW during and after a reset or load
operation, the next LOW-to-HIGH transition of that clock
will be interpreted as a legitimate signal and will be
counted.
December 1990
2
京公网安备 11010802033920号
Copyright © 2005-2026 EEWORLD.com.cn, Inc. All rights reserved
电子工程世界
