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 VCC.
2. Features and benefits
• Wide supply voltage range from 2.0 to 6.0 V
• CMOS low power dissipation
• High noise immunity
• Latch-up performance exceeds 100 mA per JESD 78 Class II Level B
• CMOS input levels
• Synchronous reversible counting
• Asynchronous parallel load
• Count enable control for synchronous expansion
• Single up/down control input
• Complies with JEDEC standards:
• JESD8C (2.7 V to 3.6 V)
• JESD7A (2.0 V to 6.0 V)
• 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