74F191DCQB NSC [National Semiconductor], 74F191DCQB Datasheet - Page 2

• Manufacturer Part Number: 74F191DCQB
• Description: Up/Down Binary Counter with Preset and Ripple Clock
• Manufacturer: NSC [National Semiconductor]

ure 1 each RC output is used as the clock input for the next
Unit Loading Fan Out
Functional Description
The ’F191 is a synchronous up down 4-bit binary counter It
contains four edge-triggered flip-flops with internal gating
and steering logic to provide individual preset count-up and
count-down operations
Each circuit has an asynchronous parallel load capability
permitting the counter to be preset to any desired number
When the Parallel Load (PL) input is LOW information pres-
ent on the Parallel Data inputs (P
counter and appears on the Q outputs This operation over-
rides the counting functions as indicated in the Mode Se-
lect Table
A HIGH signal on the CE input inhibits counting When CE is
LOW internal state changes are initiated synchronously by
the LOW-to-HIGH transition of the clock input The direction
of counting is determined by the U D input signal as indi-
cated in the Mode Select Table CE and U D can be
changed with the clock in either state provided only that the
recommended setup and hold times are observed
Two types of outputs are provided as overflow underflow
indicators The Terminal Count (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 then remain HIGH until a state change occurs
whether by counting or presetting or until U D is changed
The TC output should not be used as a clock signal be-
cause it is subject to decoding spikes
The TC signal is also used internally to enable the Ripple
Clock (RC) output The RC output is normally HIGH When
CE is LOW and TC is HIGH the RC output will go LOW
when the clock next goes LOW and will stay LOW until the
clock goes HIGH again This feature simplifies the design of
multistage counters as indicated in Figures 1 and 2 In Fig-
higher stage This configuration is particularly advantageous
when the clock source has a limited drive capability since it
drives only the first stage To prevent counting in all stages
it is only necessary to inhibit the first stage since a HIGH
signal on CE inhibits the RC output pulse as indicated in the
RC Truth Table A disadvantage of this configuration in
some applications is the timing skew between state chang-
es in the first and last stages This represents the cumula-
tive delay of the clock as it ripples through the preceding
stages
Pin Names
CE
CP
P
PL
U D
Q
RC
TC
0
0
–P
–Q
3
3
Count Enable Input (Active LOW)
Clock Pulse Input (Active Rising Edge)
Parallel Data Inputs
Asynchronous Parallel Load Input (Active LOW)
Up Down Count Control Input
Flip-Flop Outputs
Ripple Clock Output (Active LOW)
Terminal Count Output (Active HIGH)
Description
0
–P
3
) is loaded into the
HIGH LOW Output I
2
50 33 3
50 33 3
50 33 3
1 0 3 0
1 0 1 0
1 0 1 0
1 0 1 0
1 0 1 0
U L
A method of causing state changes to occur simultaneously
in all stages is shown in Figure 2 All clock inputs are driven
in parallel and the RC outputs propagate the carry borrow
signals in ripple fashion In this configuration the LOW state
duration of the clock must be long enough to allow the neg-
ative-going edge of the carry borrow signal to ripple through
to the last stage before the clock goes HIGH There is no
such restriction on the HIGH state duration of the clock
since the RC output of any device goes HIGH shortly after
its CP input goes HIGH
The configuration shown in Figure 3 avoids ripple delays
and their associated restrictions The CE input for a given
stage is formed by combining the TC signals from all the
preceding stages Note that in order to inhibit counting an
enable signal must be included in each carry gate The sim-
ple inhibit scheme of Figures 1 and 2 doesn’t apply be-
cause the TC output of a given stage is not affected by its
own CE
H
L
X
TC is generated internally
e
e
e
PL
H
H
H
L
e
e
LOW Voltage Level
HIGH Voltage Level
Immaterial
54F 74F
CE
H
X
L
LOW-to-HIGH Clock Transition
LOW Pulse
20 A
20 A
20 A
20 A
20 A
b
b
b
Input I
1 mA 20 mA
1 mA 20 mA
1 mA 20 mA
CE
H
L
L
X
Inputs
b
b
b
b
b
IH
OH
1 8 mA
0 6 mA
0 6 mA
0 6 mA
0 6 mA
Inputs
TC
U D
H
I
Mode Select Table
X
L
IL
I
H
X
X
L
OL
RC Truth Table
CP
X
X
CP
X
X
Count Up
Count Down
Preset (Asyn )
No Change (Hold)
Mode
Output
RC
H
H