74HC191
Presettable synchronous 4-bit binary up/down counter
Rev. 4 — 5 October 2018
Product data sheet
1. General description
The 74HC191 is an asynchronously presettable 4-bit binary up/down counter. It contains four
master/slave flip-flops with internal gating and steering logic to provide asynchronous preset and
synchronous count-up and count-down operation. Asynchronous parallel load capability permits
the counter to be preset to any desired value. Information present on the parallel data inputs (D0
to D3) is loaded into the counter and appears on the outputs when the parallel load (PL) input is
LOW. This operation overrides the counting function. Counting is inhibited by a HIGH level on the
count enable (CE) input. When CE is LOW internal state changes are initiated synchronously by
the LOW-to-HIGH transition of the clock input. The up/down (U/D) input signal determines the
direction of counting as indicated in the function table. The CE input may go LOW when the clock is
in either state, however, the LOW-to-HIGH CE transition must occur only when the clock is HIGH.
Also, the U/D input should be changed only when either CE or CP is HIGH. Overflow/underflow
indications are provided by two types of outputs, the terminal count (TC) and ripple clock (RC).
The TC output is normally LOW and goes HIGH when a circuit reaches zero in the count-down
mode or reaches '15' in the count-up-mode. The TC output will remain HIGH until a state change
occurs, either by counting or presetting, or until U/D is changed. Do not use the TC output as a
clock signal because it is subject to decoding spikes. The TC signal is used internally to enable
the RC output. When TC is HIGH and CE is LOW, the RC output follows the clock pulse (CP). This
feature simplifies the design of multistage counters as shown in
Fig. 5
and
Fig. 6.
In
Fig. 5,
each
RC output is used as the clock input to the next higher stage. It is only necessary to inhibit the
first stage to prevent counting in all stages, since a HIGH on CE inhibits the RC output pulse. The
timing skew between state changes in the first and last stages is represented by the cumulative
delay of the clock as it ripples through the preceding stages. This can be a disadvantage of this
configuration in some applications.
Fig. 6
shows a method of causing state changes to occur
simultaneously in all stages. The RC outputs propagate the carry/borrow signals in ripple fashion
and all clock inputs are driven in parallel. In this configuration the duration of the clock LOW state
must be long enough to allow the negative-going edge of the carry/borrow signal to ripple through
to the last stage before the clock goes HIGH. Since the RC output of any package goes HIGH
shortly after its CP input goes HIGH there is no such restriction on the HIGH-state duration of the
clock. In
Fig. 7,
the configuration shown avoids ripple delays and their associated restrictions.
Combining the TC signals from all the preceding stages forms the CE input for a given stage. An
enable must be included in each carry gate in order to inhibit counting. The TC output of a given
stage it not affected by its own CE signal therefore the simple inhibit scheme of
Fig. 5
and
Fig. 6
does not apply. Inputs include clamp diodes. This enables the use of current limiting resistors to
interface inputs to voltages in excess of V
CC
.
2. Features and benefits
•
•
•
•
•
•
•
Complies with JEDEC standard no. 7A
CMOS input levels:
Synchronous reversible counting
Asynchronous parallel load
Count enable control for synchronous expansion
Single up/down control input
ESD protection:
•
HBM JESD22-A114F exceeds 2000 V
•
MM JESD22-A115-A exceeds 200 V
Specified from -40 °C to +85 °C and -40 °C to +125 °C
•
Nexperia
74HC191
Presettable synchronous 4-bit binary up/down counter
5.2. Pin description
Table 2. Pin description
Symbol
D0, D1, D2, D3
Q0, Q1, Q2, Q3
CE
U/D
GND
PL
TC
RC
CP
V
CC
Pin
15, 1, 10, 9
3, 2, 6, 7
4
5
8
11
12
13
14
16
Description
data input
flip-flop output
count enable input (active LOW)
up/down input
ground (0 V)
parallel load input (active LOW)
terminal count output
ripple clock output (active LOW)
clock input (LOW-to-HIGH, edge-
triggered)
supply voltage
6. Functional description
Table 3. Function table
H = HIGH voltage level; L = LOW voltage level; l = LOW voltage level one set-up time prior to the LOW-to-HIGH clock
transition; X = don’t care; ↑ = LOW-to-HIGH clock transition
Operating mode
parallel load
count up
count down
Hold (do nothing)
Input
PL
L
L
H
H
H
U/D
X
X
L
H
X
CE
X
X
l
l
H
CP
X
X
↑
↑
X
Dn
L
H
X
X
X
Output
Qn
L
H
count up
count down
no change
Table 4. TC and RC Function table
H = HIGH voltage level; L = LOW voltage level; X = don’t care;
HIGH clock transition.
Input
U/D
H
L
L
L
H
H
CE
H
H
L
H
H
L
X
X
CP
X
X
Terminal count state
Q0
H
H
H
L
L
L
Q1
H
H
H
L
L
L
Q2
H
H
H
L
L
L
Q3
H
H
H
L
L
L
L
H
H
H
= one LOW level pulse;
= TC goes LOW on a LOW-to-
Output
TC
L
H
RC
H
H
74HC191
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©
Nexperia B.V. 2018. All rights reserved
Product data sheet
Rev. 4 — 5 October 2018
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