Philips Semiconductors
Product specification
Presettable synchronous BCD decade
up/down counter
FEATURES
•
Synchronous reversible counting
•
Asynchronous parallel load
•
Count enable control for synchronous expansion
•
Single up/down control input
•
Output capability: standard
•
I
CC
category: MSI
GENERAL DESCRIPTION
The 74HC/HCT190 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/HCT190 are asynchronously presettable
up/down BCD decade counters. They contain 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 number. Information present on
the parallel data inputs (D
0
to D
3
) is loaded into the counter
and appears on the outputs when the parallel load (PL)
input is LOW. As indicated in the function table, 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.
74HC/HCT190
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 “9”
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 Figs 5 and 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 as indicated in the function
table. 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 Figs 5 and 6 does not apply.
December 1990
2
Analog electronics
京公网安备 11010802033920号
EEWORLD
