Fully Programmable With Synchronous
Counting and Loading
SN74ALS867A and 4AS867 Have
Asynchronous Clear; SN74ALS869 and
4AS869 Have Synchronous Clear
Fully Independent Clock Circuit
Simplifies Use
Ripple-Carry Output for n-Bit Cascading
Package Options Include Plastic
Small-Outline (DW) Packages, Ceramic
Chip Carriers (FK), and Standard Plastic
(NT) and Ceramic (JT) 300-mil DIPs

description
These synchronous, presettable, 8-bit up/down
counters feature internal-carry look-ahead
circuitry for cascading in high-speed counting
applications. Synchronous operation is provided
by having all flip-flops clocked simultaneously so
that the outputs change coincidentally with each
other when so instructed by the count-enable
(ENP, ENT) inputs and internal gating. This mode
of operation eliminates the output counting spikes
normally associated with asynchronous (rippleclock)
counters. A buffered clock (CLK) input
triggers the eight flip-flops on the rising (positivegoing)
edge of the clock waveform.
These counters are fully programmable; they may
be preset to any number between 0 and 255. The
load-input circuitry allows parallel loading of the
cascaded counters. Because loading is
synchronous, selecting the load mode disables
the counter and causes the outputs to agree with
the data inputs after the next clock pulse.
The carry look-ahead circuitry provides for cascading counters for n-bit synchronous applications without
additional gating. Two count-enable (ENP and ENT) inputs and a ripple-carry (RCO) output are instrumental
in accomplishing this function. Both ENP and ENT must be low to count. The direction of the count is determined
by the levels of the select (S0, S1) inputs as shown in the function table. ENT is fed forward to enable RCO. RCO
thus enabled produces a low-level pulse while the count is zero (all outputs low) counting down or 255 counting
up (all outputs high). This low-level overflow-carry pulse can be used to enable successive cascaded stages.
Transitions at ENP and ENT are allowed regardless of the level of CLK. All inputs are diode clamped to minimize
transmission-line effects, thereby simplifying system design.
These counters feature a fully independent clock circuit. With the exception of the asynchronous clear on the
SN74ALS867A and 4AS867, changes at S0 and S1 that modify the operating mode have no effect on the Q
outputs until clocking occurs. For the 4AS867 and 4AS869, any time ENP and/or ENT is taken high, RCO either
goes or remains high. For the SN74ALS867A and SN74ALS869, any time ENT is taken high, RCO either goes
or remains high. The function of the counter (whether enabled, disabled, loading, or counting) is dictated solely
by the conditions meeting the stable setup and hold times.